AISC DG 37 - Hybrid Steel Frames With CLT [PDF]

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Design Guide 37

Hybrid Steel Frames with Wood Floors

Design Guide 37

Hybrid Steel Frames with Wood Floors David Barber, PEng Denis Blount John J. Hand, PE, SE, LEED AP Michelle Roelofs, PE Lauren Wingo, PE, LEED GA Jordan Woodson, PE Frances Yang, SE, LEED AP BD+C

American Institute of Steel Construction

© AISC 2022 by American Institute of Steel Construction All rights reserved. This book or any part thereof must not be reproduced in any form without the written permission of the publisher. The AISC logo is a registered trademark of AISC. The information presented in this publication has been prepared following recognized principles of design and construction. While it is believed to be accurate, this information should not be used or relied upon for any specific application without competent professional examination and verification of its accuracy, suitability and applicability by a licensed engineer or architect. The publication of this information is not a representation or warranty on the part of the American Institute of Steel Construction, its officers, agents, employees or committee members, or of any other person named herein, that this information is suitable for any general or particular use, or of freedom from infringement of any patent or patents. All representations or warranties, express or implied, other than as stated above, are specifically disclaimed. Anyone making use of the information presented in this publication assumes all liability arising from such use. Caution must be exercised when relying upon standards and guidelines developed by other bodies and incorporated by reference herein since such material may be modified or amended from time to time subsequent to the printing of this edition. The American Institute of Steel Construction bears no responsibility for such material other than to refer to it and incorporate it by reference at the time of the initial publication of this edition. Printed in the United States of America

Authors David Barber, P.Eng, is a Principal with Arup who specializes in the fire safety of mass-timber buildings. For more than 25 years, David has assisted with fire testing, developing new timber technologies, authoring fire safety design guides, working with timber product suppliers, and completing fire safety solutions for mid-rise and high-rise timber buildings. David leads a global team within Arup that works with researchers, architects, and developers in the fire safety of timber and hybrid structures. Denis Blount is an Associate Principal with Arup and leads the Acoustic, Audiovisual, and Theater Consulting team in Arup’s Seattle office. Denis is a leading advocate for mass-timber research and construction and has presented at numerous conferences on the topic. John J. Hand, P.E., S.E., LEED AP, is an Associate with Arup. His background includes the structural design of Arup’s first steel and CLT project in the Americas. Since then, he has continued to work on numerous steel, mass-timber, and hybrid masstimber building projects across the United States. Michelle Roelofs, P.E., is an Associate Principal with Arup. She has experience leading the structural design on a wide range of projects showcasing innovative use of materials. She is an advocate for the use of hybrid structures that leverage the strengths of different materials for highly efficient structures. Lauren Wingo, P.E., LEED GA, is a Senior Structural Engineer with Arup in Washington, D.C. Lauren is a regional expert in structural sustainability and applies this knowledge to her projects, focusing on reinvigorating existing building assets and utilizing low carbon structural materials. Jordan Woodson, P.E., is a Senior Engineer with Arup in Washington, D.C. Over the last decade, Jordan has worked in steel, timber, and concrete to provide innovative structural solutions for design-focused projects. Frances Yang, S.E., LEED AP BD+C, is a Structures and Sustainability Specialist in the San Francisco office of Arup and leads the Sustainable Materials Practice for the Americas region. She uses her structural and environmental background to drive down embodied carbon in the built environment through projects and external collaborations. She serves on the board of the Carbon Leadership Forum, co-chairs the ASCE/SEI SE 2050 Commitment Program, and had lead authorship of the ASCE Whole Building LCA: Reference Building Structure and Strategies handbook.

Acknowledgments The authors wish to thank Ben Loshin, an acoustic consultant in Arup’s Seattle office specializing in the acoustic design of masstimber structures, for his contributions. They also wish to thank the American Institute of Steel Construction and the following reviewers who provided valuable insight throughout the development of this Design Guide:

Eric Bolin Mark Braekevelt Jacinda Collins Don Davies Jim DeStefano

Cindi Duncan Lucas Epp Ted Hazledine Ben Johnson John Klein

Larry Kruth Tanya Luthi Margaret Matthew Paul Richardson Megan Stringer

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Preface This Design Guide provides guidance for the design of steel-framed structures with mass-timber floors. The Design Guide is intended for structural engineers with a background in steel design who may not have experience with mass-timber design. The goal of the Design Guide is to provide a multi-disciplinary review of the design considerations that impact the structural design of hybrid steel-framed structures with mass-timber floors, including fire, acoustics, and sustainability.

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Table of Contents ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

3.4

FIRE PERFORMANCE OF STEEL-TO-MASSTIMBER INTERFACE . . . . . . . . . . . . . . . . . 32 3.5 DETAILING . . . . . . . . . . . . . . . . . . . . . . . . 33

PURPOSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

CHAPTER 4 ACOUSTICS . . . . . . . . . . . . . . . . . . . . . 35 CHAPTER 1 MASS-TIMBER BACKGROUND INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . 1

4.1

1.1 1.2 1.3

4.2

INTRODUCTION TO MASS TIMBER . . . . . . . 1 TYPES OF MASS TIMBER . . . . . . . . . . . . . . 1 RELEVANT CODES AND STANDARDS . . . . . 2 1.3.1 2018 IBC . . . . . . . . . . . . . . . . . . . . . . 2 1.3.2 2021 IBC . . . . . . . . . . . . . . . . . . . . . . 2

4.3

BASICS OF ACOUSTICS IN MASS TIMBER . . . . . . . . . . . . . . . . . . . . . 35 4.1.1 Acoustic Design Metrics . . . . . . . . . . . 35 TYPICAL MASS-TIMBER FLOOR BUILDUPS . . . . . . . . . . . . . . . . . . . 40 ACOUSTIC TOPPING OPTIONS . . . . . . . . . 41

CHAPTER 5 SUSTAINABILITY . . . . . . . . . . . . . . . . 45 CHAPTER 2 INTRODUCTION TO HYBRID STEEL-TIMBER SYSTEMS . . . . . . . . . . . . . . . 5 2.1 2.2

2.3

2.4 2.5

5.1

BENEFITS OF HYBRID STEEL-TIMBER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . 5 CASE STUDIES . . . . . . . . . . . . . . . . . . . . . . 5 2.2.1 Rhode Island School of Design . . . . . . . . 6 2.2.2 901 E. 6th St. . . . . . . . . . . . . . . . . . . . 9 2.2.3 Houston Endowment Headquarters . . . . 10 HYBRID VS. TRADITIONAL MASS TIMBER . . . . . . . . . . . . . . . . . . . . . 13 2.3.1 Gravity Systems . . . . . . . . . . . . . . . . 17 2.3.2 Lateral Systems . . . . . . . . . . . . . . . . . 20 BASIC HYBRID SYSTEM . . . . . . . . . . . . . . 20 2.4.1 Primary Frame . . . . . . . . . . . . . . . . . 20 2.4.2 Example System . . . . . . . . . . . . . . . . 21 MECHANICAL SERVICES INTEGRATION . . 22 2.5.1 Horizontal Distribution of Services . . . . 22 2.5.2 Vertical Distribution of Services . . . . . . 22

5.2

5.3

5.4

CHAPTER 6 STRUCTURAL DESIGN . . . . . . . . . . . 53 6.1

CHAPTER 3 FIRE DESIGN . . . . . . . . . . . . . . . . . . . . 27 3.1 3.2

3.3

BASICS OF EMBODIED CARBON . . . . . . . . 45 5.1.1 Introduction to Embodied Carbon . . . . . 45 5.1.2 Carbon Storage and Biogenic Carbon . . . 45 COMPARATIVE LIFE-CYCLE ASSESSMENT OF HYBRID STEEL-TIMBER . . . . . . . . . . . 46 5.2.1 Introduction to Life-Cycle Assessment . . 46 5.2.2 Comparative Life-Cycle Assessment Results . . . . . . . . . . . . . . 47 PRODUCT SUSTAINABILITY CERTIFICATIONS . . . . . . . . . . . . . . . . . . . 50 5.3.1 Environmental Product Declarations . . . 50 5.3.2 Recycled Content, Recyclability, and Circularity of Steel . . . . . . . . . . . . . . . 50 5.3.3 Sustainable Wood Product Certifications . . . . . . . . . . . . . . . . . . 51 SUSTAINABILITY CONCLUSION . . . . . . . . 52

BASICS OF FIRE PERFORMANCE . . . . . . . 27 3.1.1 Mass Timber . . . . . . . . . . . . . . . . . . . 27 3.1.2 Structural Steel . . . . . . . . . . . . . . . . . 27 CODE CONSIDERATIONS . . . . . . . . . . . . . 27 3.2.1 Type III, IV-HT, V . . . . . . . . . . . . . . . 28 3.2.2 Type IV-A, -B, -C . . . . . . . . . . . . . . . 30 3.2.3 Podium Construction . . . . . . . . . . . . . 30 3.2.4 Tall Buildings—Alternative Code Approaches . . . . . . . . . . . . . . . . . . . . 30 FIRE PROTECTION OPTIONS FOR HYBRID SYSTEMS . . . . . . . . . . . . . . . . . . 30 3.3.1 CLT Fire Resistance . . . . . . . . . . . . . . 30 3.3.2 Structural Steel Fire Resistance . . . . . . . 32

6.2

6.3

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TYPICAL FLOOR PLATE DESIGN . . . . . . . . 53 6.1.1 Steel Member Design . . . . . . . . . . . . . 53 Design Example 6.1—Noncomposite Hybrid Steel Beam with CLT Panel . . . . . . . . . . . 54 6.1.2 Timber Panel Design . . . . . . . . . . . . . 58 Design Example 6.2—Timber Panel Design . . . . 59 Design Example 6.3—Fire Resistance Rating of 5-Ply CLT Panel . . . . . . . . . . . . . . . . . . 64 LATERAL SYSTEM DESIGN . . . . . . . . . . . . 65 6.2.1 Steel Lateral Force-Resisting Systems . . 65 6.2.2 Diaphragms . . . . . . . . . . . . . . . . . . . 65 Design Example 6.4—CLT Diaphragm Design . . 71 COMPOSITE SYSTEMS . . . . . . . . . . . . . . . 76 6.3.1 Composite vs. Noncomposite Behavior . 76 Design Example 6.5—Composite Hybrid Steel Beam with CLT Panel . . . . . . . . . . . . . . . . 79

6.4 VIBRATION . . . . . . . . . . . . . . . . . . . . . . . . 88 6.4.1 Principles of Vibration Design . . . . . . . 88 6.4.2 Methods of Analysis . . . . . . . . . . . . . . 88 6.5 MASS TIMBER-TO-STEEL CONNECTION TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 6.5.1 Fastener Types . . . . . . . . . . . . . . . . . . 89 6.5.2 Shop Fabrication . . . . . . . . . . . . . . . . 91 6.6 DETAILING CONSIDERATIONS . . . . . . . . . 91 6.6.1 Steel Moment Connections . . . . . . . . . 91 6.6.2 Steel Beam Camber . . . . . . . . . . . . . . 91 6.6.3 Timber Panel Penetrations . . . . . . . . . . 91 6.6.4 Integrity and Redundancy . . . . . . . . . . 93 6.6.5 Façade Supports . . . . . . . . . . . . . . . . 93 6.6.6 Timber Shrinkage and Swelling . . . . . . 93 CHAPTER 7 CONSTRUCTABILITY . . . . . . . . . . . . 97 7.1 PROCUREMENT . . . . . . . . . . . . . . . . . . . . 97 7.2 ERECTION . . . . . . . . . . . . . . . . . . . . . . . . 97 7.3 TOLERANCES . . . . . . . . . . . . . . . . . . . . . . 98 7.4 TIMBER PROTECTION . . . . . . . . . . . . . . . . 98 7.5 FIRE RISK DURING CONSTRUCTION . . . . . 99 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

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Abbreviations The following abbreviations appear in this Design Guide. The abbreviations are written out where they first appear within a section. ACI (American Concrete Institute) AESS (architecturally exposed structural steel) AHJ (authority having jurisdiction) AISC (American Institute of Steel Construction) ANSI (American National Standards Institute) APA (APA—The Engineered Wood Association) ASCE (American Society of Civil Engineers) ASD (allowable strength design) ASTC (apparent sound transmission class) ASTM (American Society for Testing and Materials) AV (audiovisual) AWC (American Wood Council) BIM (building information modeling) BOF (basic oxygen furnace) CG (center of gravity) CLT (cross-laminated timber) CWC (Canadian Wood Council) DF (douglas fir) DLT (dowel-laminated timber) EAF (electric arc furnace) EPD (environmental product declaration) FRR (fire resistance rating) FSC (Forest Stewardship Council) FSTC (field sound transmission class) GLT (glued-laminated timber) GWP (global warming potential) HSS (hollow structural section) IBC (International Building Code) ICC (International Code Council) IE (impact estimator) IIC (impact insulation class) ILFI (International Living Future Institute) IPD (integrated project delivery) ISO (International Organization for Standardization) LCA (life-cycle assessment) LEED (Leadership in Energy and Environmental Design) LFRS (lateral force-resisting system) LRFD (load and resistance factor design)

LSL (laminated strand lumber) LVL (laminated veneer lumber) LWC (lightweight concrete) MC (moisture content) MEP (mechanical, electrical, plumbing) MPP (mass plywood panels) MSR (machine stress rated) NDS (National Design Specification for Wood Construction) NFPA (National Fire Protection Association) NIC (noise isolation class) NLT (nail-laminated timber) NNIC (normalized noise isolation class) NWC (normal weight concrete) OSB (oriented-strand board) PCR (product category rules) PNA (plastic neutral axis) PSL (parallel strand lumber) PT (post-tensioned) PV (photovoltaic) RF (reduction factor) RISD (Rhode Island School of Design) SCL (structural composite lumber) SDL (superimposed dead load) SDPWS (Special Design Provisions for Wind and Seismic) SEI (Structural Engineering Institute) SFRM (sprayed fire-resistant materials) SOM (Skidmore, Owings, & Merrill) SPF (spruce-pine-fir) STC (sound transmission class) SYP (southern yellow pine) TCC (timber concrete composite) UL (Underwriters Laboratories) USDA (U.S. Department of Agriculture) WBLCA (whole building life-cycle assessment) WSP (wood structural panels)

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Purpose This Design Guide is written for structural engineers who are designing hybrid steel-framed structures with mass-timber floors. It is intended for engineers who are experienced in structural steel design but not necessarily experienced in the design of mass timber or mass-timber hybrid structures. While this Design Guide is intended primarily for structural engineers, the design of hybrid steel-framed structures with mass-timber floors requires a holistic approach that considers fire performance, acoustic considerations, and an understanding of sustainability principles. Background information and high-level advice for these topics are covered in the Design Guide to provide the structural engineer with resources during early design stages to make informed design decisions. This intent of this Design Guide is to provide a comprehensive view of the design issues associated with hybrid steel-frame structures with mass-timber floors in the United States. The intent is not to replicate detailed information that has been published in other code and industry documents. Detailed information regarding mass-timber design can be found in the following suggested resources:

• National Design Specification (NDS) for Wood Construction (AWC, 2018) • Calculating the Fire Resistance of Wood Members and Assemblies (AWC, 2021a) • Mass Timber Design Manual (Think Wood, 2021) • U.S. Mass Timber Floor Vibration Design Guide (WoodWorks, 2021b) • Mass Timber Buildings and the IBC (AWC/ICC, 2020) • CLT Handbook (FPInnovations, 2013) • Standard for Performance-Rated Cross-Laminated Timber (APA, 2019) • Nail Laminated Timber U.S. Design & Construction Guide (BSLC, 2017)

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Chapter 1 Mass-Timber Background Information 1.1

INTRODUCTION TO MASS TIMBER

Timber is a common construction material in the United States. The two most common forms of timber construction are light-frame timber and mass timber. Light-frame timber is a construction typology that utilizes dimension lumber and small section engineered wood products (e.g., engineered joists) that are closely spaced together and typically sheathed with structural wood panels (e.g., plywood) and gypsum board. This construction type is typical in singlefamily and low-rise multifamily residential construction. Mass timber differs from light-frame timber framing in that the products are typically panelized and engineered. Mass timber is typically factory made from smaller sawn or peeled wood members. The members are typically a minimum of 4 in. in thickness, making the structural performance and fire performance different than light-frame construction. 1.2

TYPES OF MASS TIMBER

There are several mass-timber products available. WoodWorks, an initiative of the Wood Product Council, defines some of the various mass-timber products as follows: Cross-laminated timber (CLT)—CLT consists of layers of dimension lumber (typically three, five, or seven plies) oriented at right angles to one another and then glued to form structural panels with exceptional strength, dimensional stability, and rigidity. CLT can be used for walls, floors, and roofs—as a stand-alone system or with other structural products (e.g., post and beam)—and is often left exposed on the interior of buildings. Because of the cross-lamination, CLT offers two-way span capabilities. Glued-laminated timber (glulam or, when used as panels, GLT)—Glulam is composed of individual wood laminations (dimension lumber), selected and positioned based on their performance characteristics and bonded together with durable, moisture-resistant adhesives. These adhesives are applied to the wide face of each lamination. Glulam has excellent strength and stiffness properties and is available in a range of appearance grades. It is typically used as beams and columns, but it can also be used in the plank orientation for floor or roof decking. It can also be curved and bent, lending itself to the creation of unique structural forms. Nail-laminated timber (NLT)—NLT is created from individual dimension lumber members (2×4, 2×6, etc.), stacked

on edge and fastened with nails or screws to create a larger structural panel. Commonly used in floors and roofs, it offers the potential for a variety of textured appearances in exposed applications. Like glulam, NLT lends itself to the creation of unique forms, and wood structural panels (WSP) can be added to provide a structural diaphragm. Dowel-laminated timber (DLT)—Common in Europe and gaining popularity in the United States, DLT panels are made from softwood lumber boards (2×4, 2×6, etc.) stacked like the boards of NLT but friction-fit with hardwood dowels. The dowels hold each board side-by-side, while the friction fit adds dimensional stability. Structural composite lumber (SCL)—SCL is a family of wood products created by layering dried and graded wood veneers, strands, or flakes with moisture-resistant adhesive into blocks of material that are subsequently resawn into specified sizes. Two SCL products—laminated veneer lumber (LVL) and laminated strand lumber (LSL)—are relevant to the mass-timber category because they can be manufactured as panels in sizes up to 8 ft wide with varying thicknesses and lengths. Parallel strand lumber (PSL) columns are also commonly used in conjunction with other masstimber products. Mass plywood panels (MPP) are another mass-timber product that can be used as an alternative to CLT. In this product, each 1 in. lamella that is used to construct the panel is constructed of nine layers of 9 in. veneer. MPP are commonly available from 2 to 12  in. thick. MPP is classified as cross-laminated timber (CLT) under the ANSI/APA PRG 320 certification (APA, 2019). Timber cassette systems can also be used as floor panels to achieve longer spans with reduced material usage. Timber cassettes typically consist of timber ribs with thin horizontal layers top and bottom. Thermal and acoustic layers can be incorporated into the ribs to create a holistic design solution. The availability of timber cassette systems is limited in the United States, partially due to the challenge of meeting U.S. fire code limitations with nonsprinklered air cavities. Several manufacturers exist in Europe (e.g., Lignatur, Kerto-Ripa, Kielsteg), and domestic products may become available as the timber market matures in the United States. The terms heavy timber and mass timber are sometimes used interchangeably. For the purpose of this document, heavy timber is used to describe historic construction types

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 1

utilizing solid sawn timber elements, while mass timber is used to describe the modern engineered timber products summarized in this section. 1.3

RELEVANT CODES AND STANDARDS

Within the United States, each state adopts one or more model building codes, and all 50 states adopt the International Building Code (ICC, 2021a), hereafter referred to as the IBC. The most current version was released in 2021, with different states adopting earlier versions based on their code adoption review cycle. Some states also adopt the National Fire Protection Association Life Safety Code (NFPA, 2021a), which will be referred to as NFPA 101 in this document. Many states further amend the model codes to provide the basis for construction compliance. Other codes and standards impact construction, fire-protection systems, maintenance, and firefighting operations, including the International Fire Code (ICC, 2021b). IBC incorporates by reference the following structural design standards: AISC Specification for Structural Steel Buildings (AISC, 2016c), hereafter referred to as the AISC Specification, and National Design Specification (NDS) for Wood Construction (AWC, 2018), hereafter referred to as NDS. These standards govern steel and timber design, respectively. Design provisions for the use of CLT as a slab were added to the NDS in the 2015 version (AWC, 2015). In addition to the U.S. codes and standards, several international standards can be useful resources for mass-timber design, including CSA O86, Engineering Design in Wood (CWC, 2014), and Eurocode 5: Design of Timber Structures (CEN, 2004).

1.3.1 2018 IBC Allowable construction types are defined in IBC, Section 602 (ICC, 2018). Mass timber is permitted in the following construction types: • Type III (Ordinary Construction) • Type IV (Heavy Timber) • Type V (Wood-Frame Construction) Types III and IV allow up to six floors and 85  ft for B (office) uses, and five floors and 85 ft for R (residential) uses. Type V allows up to four floors for B and R uses and 70 ft. For all requirements and limitations related to the construction type, refer to the relevant version of the IBC. 1.3.2 2021 IBC The 2021 IBC (ICC, 2021a) introduces three new construction types, Types IV-A, IV-B, and IV-C, that permit mass timber. Type IV from the 2018 IBC is now referred to as Type IV-HT. Type IV-C is similar to Type IV-HT (Type IV under the 2018 IBC) allowing mass timber up to 85 ft with a greater number of floors and floor area compared with Type  IV-HT. Type IV-B allows mass timber up to 12 floors and 180 ft in height, for R, B, and A (assembly) uses, with a 2-hour fire-rated primary structure. Type IV-A allows mass timber up to 18 floors and 270  ft, with a 3-hour fire-rated primary structure and 2 hours for floors. For all requirements and limitations related to the construction type, refer to the version of the IBC relevant to your project. Figure 1-1 illustrates the mass-timber construction types in the 2018 and 2021 IBC. Code considerations for hybrid steel and mass-timber construction are discussed in detail in Section 3.2.

2 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Fig. 1-1.  Comparison of 2018 and 2021 IBC mass-timber construction types.

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Chapter 2 Introduction to Hybrid Steel-Timber Systems 2.1

BENEFITS OF HYBRID STEEL-TIMBER SYSTEMS

Modern hybrid steel and mass-timber systems utilize the strengths of both materials to achieve highly efficient and attractive structural systems. There are many benefits to using hybrid steel-timber systems, including: • Aesthetics. A shift toward biophilic design (design connected with nature) has spurred the interest in mass-timber structural elements used as an architectural finish. Hybrid systems showcase the beauty of mass timber along with architecturally exposed steel that requires little additional architectural finish. • Sustainability. Most structural steel in the United States is recycled, and the recycling rate exceeds any other construction material. Mass timber serves as a low carbon structural material option that can reduce the use of high carbon materials within structural systems. Hybrid systems can achieve low embodied carbon by optimizing steel and timber for their unique strengths to minimize overall structural material usage. • Long beam spans. Many building typologies require typical beam spans of 30 to 45 ft to accommodate established leasing and programmatic requirements. Masstimber post and beam construction is more efficient with beam spans less than 20 to 25 ft and require deep beams to accommodate longer beam spans. Steel is better suited to these market-driven long spans and can reduce the overall building height as compared to timber post and beam construction. • Reduced column size. Timber columns are often required to have a large cross section in order to meet fire and strength requirements. Steel columns can offer a significantly smaller profile, therefore providing more usable floor area compared to timber columns. • Mechanical, electrical, and plumbing (MEP) services coordination. The coordination of MEP services in a mass-timber building is a challenge, particularly on projects where column spacing exceeds 20 ft. Glulam beams are difficult to penetrate or notch without significant reduction in beam capacity, although modest pipe penetrations can typically be accommodated. Steel beams are easy to penetrate and reinforce as needed to accommodate MEP services.

• Vibration. Mass-timber structures are lightweight and can encounter vibration issues for longer spans or in program areas sensitive to vibration. Steel floor framing provides a stiffer and tested solution to mitigate vibration risks for certain building typologies. • Lightweight. Hybrid systems are one of the most lightweight structural systems available for large scale projects. The reduced weight of this system as compared to other conventional construction systems allows for reduced foundations and makes hybrid construction an ideal solution for vertical additions to existing buildings. • Speed of construction. Both steel and mass timber are fabricated off site with all connections and penetrations prefabricated. This prefabrication allows for easy and fast installation on site, reducing overall project schedules. • Prefabrication. In addition to schedule benefits, offsite prefabrication allows for high levels of quality control with minimal construction waste. • Tall building capability. As the availability and popularity of mass timber grows in the United States, clients will want to build taller buildings utilizing the material. Structural timber has limitations in its strength and stiffness that will limit the height of mass-timber vertical and lateral systems. Steel superstructures are proven and reliable systems for tall buildings. Although not currently available as a construction type within U.S. building code framework, the combination of a steel superstructure and mass-timber floor panels is a reliable and repeatable system for tall buildings. 2.2

CASE STUDIES

As panelized mass-timber products such as CLT have become increasingly popular in the United States, a new era of hybrid steel and timber typologies have emerged. Some examples of modern hybrid systems are as follows: • Steel-framed structures with mass-timber floors (focus of this document) • Mass-timber post and beam construction with steel lateral force-resisting system (LFRS) • Steel columns with point-supported mass-timber floors • Glulam columns with steel floor framing and mass-timber floors • Steel and timber trusses supporting mass-timber floors

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All the typologies mentioned utilize steel in the timber connections. Additionally, many mass-timber buildings utilize extensive secondary steel systems to support the façade (where steel primary beams are not already present at the façade). Custom hybrid systems utilizing asymmetrical steel shapes are also available in the U.S. market. It is possible to specify an asymmetric system similar to the system developed by AISC and Skidmore, Owings, & Merrill, Ltd. (SOM) in AISC Steel & Timber Research for High-Rise Residential Buildings (SOM, 2017). Proprietary hybrid systems also exist in the U.S. market. There are several built examples of hybrid systems that showcase some of the possible combinations of structural steel and mass timber. The Beverly Regional Airport in Massachusetts, shown in Figure 2-1, opened in 2016 and is an early project that used steel columns and a steel LFRS supporting a mass-timber roof structure. The Stamford Media Village, shown in Figure 2-2, is an early example of the hybrid steel-frame structure with masstimber floors utilized as part of an adaptive reuse project. The existing cast-in-place concrete building was vertically extended by adding three floors of hybrid steel-frame and CLT floor construction. The 20 ft × 40 ft column bays did not lend themselves to a pure mass-timber solution because very deep glulam beams would be needed to span 40 ft. The solution utilized an architecturally exposed structural steel (AESS) frame with CLT floor panels. The lightweight nature of hybrid structures makes them an ideal candidate for adaptive reuse where it is advantageous to minimize loads on an existing structure. While the scope of this document is limited to steelframed structures with mass-timber floors, many topics are relevant to other types of hybrid steel-timber construction described in this section. The following examples showcase

case studies of hybrid systems with steel columns, steel beams, steel LFRS, and mass-timber floors. 2.2.1 Rhode Island School of Design The Rhode Island School of Design (RISD) North Hall, shown in Figure 2-3, is one of the first examples of modern hybrid steel-timber projects in the United States. The sixstory residence hall was delivered using an integrated project delivery (IPD) method that fosters early collaboration between the owner, design team, and contractor. Some of the key project partners are as follows: • Owner: Rhode Island School of Design • Architect: NADAAA • Structural engineer: Odeh Engineers • Contractor: Shawmut • CLT supplier: Nordic • Steel fabricator: Ocean Steel • Steel erector: HB Welding The project team considered multiple structural options for the project, including all mass-timber post and beam, steel frame plus CLT slab hybrid, and steel frame plus precast panels. Working within the IPD format, the collaborators were able to evaluate each scheme against various metrics, including speed, sustainability, cost, durability, and aesthetics. The hybrid solution was ultimately selected by the project team because of the following advantages over the other structural systems: • The pull planning identified that that the structure could be erected in 6 weeks, which gave the system a speed advantage over the other systems. The project was ultimately

Fig. 2-1.  Beverly Regional Airport (photo courtesy of William Horne). 6 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

erected in 2.5 weeks, outperforming the original estimate. There was also time savings in the shop drawing process as compared to precast planks. • The CLT panels have reduced embodied carbon compared to the proposed precast planks. This provided a clear sustainability advantage over the precast plank system. • The CLT panels could be exposed at the soffit, which provided an aesthetic advantage to the hybrid system (Figure 2-4). The ability to remove the ceiling in many areas also contributed positively to the sustainability aspect by reducing the amount of finish material on the project. The hybrid system was estimated to have a slightly higher cost premium (approximately 10%) but the advantages of the system outweighed the slight increase in structural cost. The project is classified as Type III-B construction on top of a Type I-A podium construction. There are no explicit fire protection requirements for the primary structure in Type

III-B construction which allowed the steel framing and the CLT to be exposed. The structural system consists of steel beams spanning in one direction that are typically 12  in. deep, supporting 6d-in. Nordic 5-ply CLT panels (X-LAM). The panels are E1 grade as defined by the Standard for Performance-Rated Cross-Laminated Timber (APA, 2019) with machine stress rated (MSR) lumber in the primary direction and sprucepine-fir (SPF) No. 3 stud grade in the secondary direction. The CLT panels are connected to the steel beams with selftapping screws through the top flange of the beams. The connections are noncomposite but provide restraint to the top flange of the steel beams. For acoustic performance, 2  in. gypsum concrete topping and 1 in. acoustic mat are provided on top of the CLT panels. Figure 2-5 shows the typical floor system used in the project. The LFRS is reinforced masonry walls. The project utilized the CLT slab as the diaphragm for the project. In early stages of the design, an additional layer

Fig. 2-2.  Stamford Media Village (photo courtesy of DeStefano & Chamberlain). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 7

Fig. 2-3.  Exterior view of RISD North Hall (photo courtesy of John Horner).

  Fig. 2-4.  Interior views of RISD North Hall showing exposed CLT soffit and steel beams (photos courtesy of John Horner).

8 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

of plywood was included to act as the diaphragm following a prescriptive code approach. The CLT supplier, Nordic, provided data on their CLT spline connections that allowed Odeh Engineers to utilize the CLT panel as the diaphragm and eliminate the plywood sheathing, resulting in a cost and material savings for the project. The diaphragm was designed as a semi-rigid diaphragm. The project team engaged WoodWorks throughout the project for advice on this innovative system. One suggestion from WoodWorks was to provide CLT panels that were the full width of the building to minimize the number of panels. Each panel is approximately 20 psf and weighs less than 5 tons total, which meant that they could be easily erected by a small crane. This is compared to precast concrete planks that weigh approximately 75 to 80  psf and would require significantly more crane picks to install the same square footage. The steel columns are three levels tall and include only one splice due to the height of the building. The columns were erected with one level of steel framing. The CLT panels had precut edges and corners that allowed the panels to

be placed around the columns as shown in Figure 2-6. The remaining floor beams and CLT panels were erected floor by floor. The CLT panels were shipped with little camber and tight tolerances, allowing for quick erection and alignment of horizontal joints. The panel splices were installed using a nailed splice detail that was quick to execute in the field. 2.2.2 901 E. 6th St. The 901 E. 6th St. project is a five-story, 130,000 ft2 core and shell office tailored for creative office tenants located in Austin, Texas (Figure 2-7). Completed in 2018, it is one of the first examples of hybrid steel frame with mass-timber floors in the United States. Key design team members include: • Owner: Endeavour • Architect: Thoughtbarn/Delineate Studio LLC • Structural engineer: Leap!Structures • Fire engineer: Arup • Contractor: DCA Construction • CLT panel manufacturer: Structurlam

Fig. 2-5.  Typical RISD floor system (image credit: Odeh Engineers, Inc.). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 9

The design team was inspired by the building typology of turn of the century warehouses that expressed the structural system. The hybrid steel-frame structure with CLT floors matched this intended design aesthetic. Leaving the timber structure exposed created a warm palette that was intended to be memorable and desirable for creative office tenants. The project team credits this choice in material with supporting faster leasing and higher lease rates. The leasing requirements of this office project demanded 45 ft × 30 ft structural column bays, which is traditional for many office floor plates. Post and beam glulam construction was considered but would have required very deep glulam beams to adequately achieve the spans. The hybrid steelframe system was chosen to utilize the strength and flexibility of the steel structure. This system allowed for shallower beam profiles that could be penetrated to accommodate MEP systems. The LFRS for the project is steel braced frames surrounding the interior service cores. The 5-ply, 6d  in. CLT panels span approximately 9  ft between steel beams. The CLT panels were SPF with a douglas fir (DF) bottom layer. The panels were planed rather than sanded which provided a more rustic appearance for a reduced cost. The hybrid structural system can be seen in Figure 2-8. Given the project height and area, the building is Type IIIA, which requires a 1-hour fire-resistance rating (FRR) for the primary structural frame and floors and a 2-hour FRR for

the shafts. The project was designed and built under the 2012 IBC (ICC, 2012), which did not recognize CLT as a building material. The project team worked with the city of Austin using existing test data and analytical approaches to justify the 1-hour FRR of the CLT panels as a floor system. The project team utilized conventional steel and gypsum wall shaft construction to achieve a 2-hour FRR for the shaft. The exposed steel was protected with intumescent paint. The design team engaged CLT suppliers at the early stages of design to ensure success utilizing this innovative structural system. The supplier was engaged in a design-assist process, working with the design team to design panel layouts and connections for the project. The design team and fabrication team utilized building information modeling (BIM) and prefabrication on this project, but early trade coordination of the MEP systems could have allowed for further leveraging of the prefabrication abilities of mass timber and steel. The floor assembly consisted of 3 in. concrete topping on a 4  in. acoustic mat on a nominal 7  in., 5-ply CLT panel to achieve the acoustic requirements of the project, as illustrated in Figure 2-9. 2.2.3 Houston Endowment Headquarters The Houston Endowment Headquarters shown in Figure 2-10 in downtown Houston, Texas, leverages the benefits of steel framing and CLT floor panels for a cost-effective

Fig. 2-6.  CLT panels precut around columns (photo courtesy of Odeh Engineers, Inc.). 10 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Fig. 2-7.  901 E. 6th St. exterior view (photo courtesy of Delineate Studio and Casey Dunn).

Fig. 2-8.  CLT panels supported on steel beams (photo courtesy of David Barber).

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 11

Fig. 2-9.  Typical floor assembly (credit: Delineate studio).

Fig. 2-10.  Houston Endowment Headquarters frontal elevation rendering (photo courtesy of Kevin Daly Architects with Productora, 2021).

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and flexible structural design. The building is a 40,000  ft2 two-story office above parking to house the headquarters for The Endowment, one of the largest philanthropic organizations in the city. The project is situated near downtown Houston off Buffalo Bayou in Spotts Park. A signature steel photovoltaic (PV) canopy covers the primary building mass and integrates the project into the surrounding landscape and city. Key design team members include: • Owner: Houston Endowment • Architect: Kevin Daly Architects (kdA) with Productora • Local representative: Kirksey Architecture • Structural engineer: Arup • Contractor: W.S. Bellows Construction Corporation • CLT supplier: Nordic • Steel fabricator: Myrex Industries During the conceptual design phase, the project was originally designed as a concrete structure. Concrete was selected at first due to the ease of integrating an embedded radiant cooling system in the flooring—key to the sustainability goals of the project to minimize energy consumption. However, when exploring this initial concrete structural scheme, several challenges arose that resulted in significant structural costs. Drivers for the structural costs include the tower crane mobilization costs, concrete formwork shoring costs, and a long construction schedule. Finally, there was still a significant steel scope in the PV canopy that had to be integrated into the concrete structure. Another project challenge was the difficult site and soil conditions. The site was previously home to a YMCA that was demolished. However, the existing foundations of the previous building were left in the ground. Portions of the old building were also filled in with poor fill that could not be used for bearing. As the initial concrete structural scheme was relatively heavy, this resulted in large foundations that increased the conflicts with the existing foundations, more excavation, and costly fill replacement. Due to the challenges encountered with the initial concrete framing scheme, the design team quickly explored a range of other structural systems that could accomplish the architectural goals and reduce the structural costs. While not used in the region previously, a hybrid steel and CLT structural scheme was selected for the following benefits: • Reduced cost and schedule. The steel framing and CLT eliminated the shoring and tower crane mobilization costs. The steel and CLT framing could be installed much quicker—effectively as a prefabricated structure—and use smaller mobile cranes. The number of trades on site was also reduced and provided much simpler interfacing with the canopy structure. In all, the structural cost came down approximately by half from the original concrete scheme.

• More flexibility. The steel framing could easily accommodate the architectural programmatic goals with atriums, stairs, cantilevers, and a signature canopy much more simply and with less structural depth. Architectural renderings show these features in Figures 2-11 and 2-12. • Strong sustainability. The CLT panels replaced a significant amount of concrete in the upper levels, reducing the carbon footprint of the structure by almost 50%. The CLT also resulted in a lighter building, reducing the concrete in the foundations. Thermal breaks could also be easily integrated into the project at the envelope allowing for reduced energy. The structural frame is composed of steel framing bearing on shallow foundations. The first level of steel framing covers a partial-basement parking level. This first floor decking is composite concrete atop metal deck due to the exterior exposure condition, acoustic considerations, and large in-plane forces from the basement walls. The second level and roof levels are exposed steel framing and CLT decking. The 3-ply CLT decking spans approximately 10 ft, and the maximum column bays are approximately 30 ft × 30 ft. The typical floor system is shown in Figure 2-13, and the typical floor plan is shown in Figure 2-14. The PV canopy is composed of hollow structural steel (HSS) framing with infill aluminum louvers. Atria connect the two levels with signature steel plate stairs. Conference rooms on the east and west sides of the building cantilever up to 15 ft from the column lines. Moment frames are the primary lateral system that distribute the lateral forces to the foundation system and provide the most flexibility with the office and parking planning. The CLT levels utilize the CLT decking as the diaphragm for lateral stability. The shear forces for CLT splines were documented on the framing planes, as illustrated in Figure 2-15, so the spline connections could be designed to the CLT fabricator’s preference. Light-gauge steel strapping atop the CLT was used for tension chord and collector splices at panel joints. For significant lateral load transfers or chord forces, the steel framing members and connections were sized for the axial loads. Thermal breaks are located at most conditions where steel framing transitions from an interior to exterior condition. The project is classified as Type III-B and therefore no fireproofing was required, allowing the steel to be fully exposed. The exposed steel and CLT are visible in the construction photo in Figure 2-16. 2.3

HYBRID VS. TRADITIONAL MASS TIMBER

The use of mass timber has limitations on the gravity and lateral systems due to the strength and stiffness of the material, code limitations, vibration requirements, and market factors. The use of steel alleviates the limitations of mass

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 13

Fig. 2-11.  Houston Endowment Headquarters side rendering (photo courtesy of Kevin Daly Architects with Productora, 2021).

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Fig. 2-12.  Houston Endowment Headquarters interior rendering (photo courtesy of Kevin Daly Architects with Productora).

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 15

Steel framing

Finished floor

Raised floor system (1'-8" tall)

2" Gypcrete slab

Sound mat (⅜")

3-Ply CLT (4⅛")

Steel framing

Fig. 2-13.  Hybrid steel and CLT floor build-up (image credit: Kevin Daly Architects with Productora).

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timber, allowing for an efficient structural system that can meet many market demands.

• Program requirements for the desired use of the building (e.g., 40 ft office spans)

2.3.1 Gravity Systems

• Compatible grids for multiuse buildings (e.g., matching the building grid to below grade parking to minimize the need for transfers)

Traditional mass-timber systems are often limited by the typical span-to-depth ratio of glulam beams requiring greater depth than comparable composite steel framing. In typical mass-timber construction, this often results in the need for higher floor-to-floor heights, more onerous coordination with services, and/or tighter structural grids than composite steel systems to accommodate this deeper structure. Hybrid steel and mass-timber systems offer more flexibility in design because steel can achieve longer spans with less depth than comparable glulam elements. An efficient hybrid steel and mass-timber column grid is driven by a number of factors, including:

• Depth available for structure and systems • Type of mass-timber floor system • Fire protection requirements In a conventional hybrid steel-timber system, the design and framing approach for steel is analogous to conventional composite steel framing, with the same primary drivers dictating the final column arrangement. The main differentiator between the two systems is the replacement of concrete on metal deck for a mass-timber slab.

Fig. 2-14.  Level 2 steel and CLT framing partial plan (image credit: Arup). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 17

Timber slabs come in a variety of assemblies and depths and offer different constructability impacts and span capabilities. While there are a number of products currently available on the market, summarized in Section  1.2, this Design Guide will use a conventional CLT slab for simplified calculations and discussion as the representative system for other mass-timber alternatives. In an efficient system, the slab thickness should be minimized and will drive secondary beam spacing, and as a result, the primary spacing of the column grid. A 5-ply CLT panel can typically span 14 to 17 ft while meeting the strength and serviceability requirements of traditional office, academic, or residential loading. Thinner 3-ply CLT panels can typically span up to 12  ft under the same loading and offer a good alternative to conventional concrete over metal deck if evaluating design options. These thinner panels, however, are not as commonly used due to their limited capacity for unreinforced slab openings and reduced resistance to fire. Larger 7-ply panels can span

beyond the 5-ply panel, but usually result in a less efficient system due to the large volume of timber required for their construction. The table shown in Figure  2-17 is provided by WoodWorks (WoodWorks, 2019b) and shows example ranges of floor spans for various mass-timber panel systems. Projectspecific loading, support conditions, fire-resistance requirements, vibration requirements, manufacturer specifics, and design criteria need to be considered for each project, but these span ranges can provide useful guidance in concept design. Other mass-timber slab options are continuing to be explored that offer increased spans. These efforts are largely focused on potential composite action of the CLT and a concrete topping, the steel beam and CLT slab, or the steel beam and concrete topping layer. There are also efforts to optimize timber fiber orientation and fiber type to increase performance of mass-timber panels.

Fig. 2-15.  Level 2 CLT diaphragm shear diagram (image credit: Arup). 18 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

2.3.1.1 Comparison to Market-Driven Column Grids Many project typologies require minimum column spacing requirements to achieve column-free areas that are expected in the market. Many office buildings require 40  ft clear lease spans that drives the column spacing. Typical glulam beam depths exceed those of similar noncomposite steel beams, and thus, market-driven grids are more challenging

to achieve in an all-timber solution. Efficient glulam beam spans range from 20 to 30 ft, and although spans up to 40 ft are achievable, the depth of beam can be prohibitive to multi­ story construction. Vibration often controls design of masstimber buildings at these longer spans. Utilizing steel beams can help mitigate vibration issues for market-driven spans. Further, the detailing requirements to penetrate glulam beams can force the majority of the mechanical, electrical,

Fig. 2-16.  Construction photo (image credit: Arup).

Panel

Example Floor Span Ranges

3-ply CLT (48 in. thick)

Up to 12 ft

5-ply CLT (6d in. thick)

14 to 17 ft

7-ply CLT (9s in. thick)

17 to 21 ft

2×4 NLT

Up to 12 ft

2×6 NLT

10 to 17 ft

2×8 NLT

14 to 21ft

5 in. MPP

10 to 15 ft

Fig. 2-17.  Example mass-timber floor panel span ranges (WoodWorks, 2019b). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 19

and other technical services to live in a plane below that of the structure—which can require all-timber buildings to have even greater floor-to-floor heights to compensate. Many building typologies also aim to maximize leasable floor area. Glulam column dimensions are typically larger in cross section than an equivalent steel section due to fire and strength requirements. The use of steel columns in a hybrid system will yield more usable floor area than an all-timber building. Hybrid steel-framed structures with timber floors can be efficiently constructed using the same market-driven structural grids as conventional composite steel structures; however, additional aspects of the design must be considered. First, many hybrid solutions are noncomposite and therefore the steel beams are typically deeper and have increased tonnage compared to similar composite steel schemes. For larger grids, this may impact ceiling heights or increase the floor-to-floor heights required to achieve design goals. However, because there is often a desire to keep the timber soffits exposed, the deeper steel sections may be acceptable if they can be aligned with partitions or higher ceilings are capitalized on between the beams. This is particularly relevant to office typologies where large, column-free bays are desired for leasing opportunities. Engineering solutions that still achieve composite steel beams in conjunction with concrete topping slabs or timber slabs, if done thoughtfully, will often result in both cost and carbon reductions on the project. In other building typologies with more permanent de­mising (partition) walls and smaller grids, such as residential construction, the loss of composite action may not be a major factor, and the steel beams can be factored into the proposed partition construction. Despite the naturally tighter grids in residential construction, hybrid systems are still worth pursuing in these typologies due to the ability to easily penetrate steel beams and provide service distribution throughout the floorplate. Hybrid steel-framed systems provide further benefit in that the primary structural frame is code-approved for tall buildings, and a code modification for wood floors may be more palatable to local authorities. 2.3.2 Lateral Systems Both conventional mass-timber buildings and hybrid steeltimber buildings often utilize a conventional concrete or steel lateral system in lieu of a mass-timber lateral system. There is a significant amount of ongoing research on the use of CLT as shear walls under seismic loading. CLT shear walls are not yet codified by IBC, but they have been added as a LFRS in ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, here­after referred to as ASCE/SEI 7 (ASCE, 2022). It is expected that CLT shear walls will be included in the 2024 version of IBC. CLT shear wall provisions are included in the 2021 Special Design Provisions for Wind and Seismic (SDPWS)

(AWC, 2021b). Should CLT walls be the desired lateral system, approval from the local authority having jurisdiction (AHJ) for a code modification would be required. These constraints, coupled with stringent fire requirements limiting the exposure of the mass-timber walls, typically drive the selection of codified steel or concrete lateral systems. For hybrid steel-framed structures with mass-timber floors, it is recommended to utilize steel braced frames or moment frames that comply with the seismic requirements prescribed by ASCE/SEI 7. A steel lateral frame eliminates the need for an additional trade to construct the lateral system; allows for simplified gravity beam-to-lateral frame connections; and can capitalize on the faster, stick-built construction afforded by the gravity construction. The timber slab or concrete topping may be utilized as the diaphragm; these options are discussed in Section 6.2.2. 2.4

BASIC HYBRID SYSTEM

2.4.1 Primary Frame As described in previous sections, the composition of a steel-framed structure with mass-timber floors is highly analogous to a conventional steel framed structure with concrete on metal deck. In a basic hybrid system, steel primary and secondary beams span between columns providing support for mass-timber slabs. Basic floor construction would consist of a 3-ply or 5-ply CLT slab spanning 10 to 15  ft between secondary beams. These spans may vary based on the required grids needed for the building program, design loads, fire rating requirements, serviceability requirements, and selected finishes; however, the spans provided here are valid rules-of-thumb for schematic design consideration of conventional office and residential loading. Note that optimizing beam spacing to minimize CLT slab thickness will generally result in the most effective solution. Secondary beams typically span the longer bay directions to primary beams in the short directions. This can offer more consistent beam sizing between primary and secondary elements, allowing for a more uniform elevation for bottom of steel for coordination. In an office layout, this long bay dimension runs perpendicular to the façade, providing large column-free leasable spans. For the purpose of this document, CLT will be used as the default timber floor material because of its dominance in the market; however, it should be noted that many different mass-timber products may be considered that offer project specific benefits over CLT. These include products such as NLT or DLT that could potentially span further than CLT with less depth and use mechanical lamination in lieu of adhesives for panel build-up. MPP can offer greater flexibility in dimensions and ability to span in two directions. Other considerations may include preassembled timber panel and glulam beam cassettes that function more analogously to

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ribbed slabs. These may help alleviate the number of pieces connected and assembled on site, as well as offer longer spans between secondary elements. 2.4.2 Example System Within this section, an example hybrid steel-framed structure with mass-timber floors is compared to a composite steel building with similar design constraints. Both systems aim to meet the following basis for design: • The proposed floor plate is modeled after a standard 100 ft × 240 ft floor plan. This is divided into 40 ft × 30 ft column bays, with a 20 ft internal bay (see Figure 2-18). In this scheme, a central core is assumed to house vertical transportation, services, and the building’s primary LFRS. • Conventional office loading: 80  psf live load allowance with 20  psf superimposed dead load (SDL) allowance for ceilings, services, and finishes. The 80  psf live load allowance provides flexible office fit-out for partitions and corridors. The self-weight of the steel, CLT panel, and concrete topping are considered in the dead load. • Steel beams supporting the slabs are restricted to L/ 360 deflection under live loads and L/ 240 total deflection. • Façade loading is 300 plf along the perimeter, allowing for a 20 psf façade weight over a 15 ft floor-to-floor height. • Elements supporting the façade are limited to a maximum deflection of L/ 500 under live loads and L/ 360 total.

• No consideration has been made for live-load reduction of beam elements. The steel beams in the hybrid system are considered noncomposite, but the CLT slab provides restraint to the top flange of the beam. 2.4.2.1 Composite Steel Framing Example In the typical 30 ft × 40 ft bay: • The composite steel frame features a 34 in. lightweight concrete topping over a 2 in. metal deck. • The metal deck spans 10 ft between W18×35 secondary beams. • Secondary beams span 40 ft to W21×50 primary beams at the building interior, with W21×44 beams at the façade. • Primary beams span 30 ft to columns. The floor framing yields approximately 5.3  psf in steel tonnage. This tonnage does not include connections, nor does it account for any additional complexity around framed openings or project-specific loads or requirements. The total depth of structure (including slab over metal deck but excluding fire protection or finishes) is approximately 264  in. Beams are consistently cambered throughout the frame, consistent with industry standard guidance and a maximum of 2 in. camber. The primary framing shown consists of 94 steel beams.

Fig. 2-18.  Example noncomposite steel-framed structure with mass-timber floors. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 21

2.4.2.2 Hybrid Steel Frame with Mass-Timber Floor Example In the typical 30 ft × 40 ft bay: • The hybrid frame features 6d in. 5-ply CLT panel topped with a 1 in. acoustic mat and 3 in. normal weight concrete topping. • The CLT floors span 15  ft between W27×84 secondary beams. • Secondary beams span 40 ft to W27×84 primary beams at the building interior. • Primary beams span 30 ft to columns. The floor framing yields approximately 8.5  psf in steel tonnage. This tonnage does not include connections, nor does it account for any additional complexity around framed openings or project-specific loads or requirements. The total depth of structure (including CLT slab, acoustic membrane, and topping but excluding fire protection or finishes) is approximately 37d in. Beams are assumed to have no camber to allow for easier coordination and installation of the CLT panels. The primary framing shown consists of 73 steel beams. The primary difference in the steel design between the typical composite steel framing and the hybrid steel frame is that the steel beams are designed as noncomposite in the hybrid system. Many of the built examples of hybrid systems utilize noncomposite construction for ease of construction and simplicity of connection detailing. Material efficiencies can be gained by introducing composite action to the steel members, which is discussed in Section 6.3. Although the hybrid system can result in higher steel tonnage than a conventional slab on metal deck system, the advantages outlined in Section 2.1 often outweigh the cost penalty of additional steel tonnage. Additionally, the overall piece count is significantly lower in the hybrid system, which can result in cost savings not reflected in the pure tonnage. The effect of noncomposite action is less significant for smaller column bays that may be suitable for multifamily residential building typologies. For these small bay typologies, the impact in steel tonnage and structural depth is minimal. Projects that may otherwise utilize a girder-slab system with precast hollow-core planks can achieve significant sustainability, aesthetic, building weight, and construction speed benefits when the precast hollow-core planks are substituted with mass-timber floor panels.

2.5

MECHANICAL SERVICES INTEGRATION

2.5.1 Horizontal Distribution of Services In hybrid steel-timber construction, it is often a design aspiration to leave the underside of the timber slabs exposed, putting significant onus on the design team to ensure the exposed services within the ceiling plenum appear rational, organized, and do not distract from the overall architectural aesthetic. Timber and hybrid timber systems are often deeper than conventional composite steel or concrete systems, which adds additional complexity in coordination. Care must be given early in the design process to understand the structural layout and beam depth, technical service routing, and desired finish ceiling heights. Early in the design process it is useful to consider how routing of these systems may cross primary framing lines—either via beam penetrations, revised routing, raised floor systems, or minimized beam depths. An example of services distributed within a mass-timber structure is shown in Figure 2-19. Unlike traditional full mass-timber construction, hybrid systems offer more flexibility with systems coordination; steel beams can be castellated or penetrated to allow for services to occupy the same elevation within a ceiling plenum. Figures 2-20 and 2-21 show an example of castellated beams at 6 Orsman Road by Waugh Thistleton Architects. 2.5.2 Vertical Distribution of Services Requirements for vertical penetrations will vary greatly depending on the planned use of the building. Like most construction typologies, it is best to consolidate penetrations to centralized risers to limit the quantity of penetrations in the floor plate. Like composite slabs, the size and location of a vertical penetration will dictate the quantity of reinforcing needed in a wood floor. Unlike these more conventional systems, reinforcing may impact design goals of the ceiling plenum below. In addition, one-way mass-timber systems have limited ability to span in two directions and will often require additional beam framing to pick up an unsupported slab edge for larger openings. CLT and MPP have the ability to span in two directions and can accommodate moderate openings without additional reinforcement. Refer to Chapter 6 for further discussion on openings in mass-timber slabs.

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Fig. 2-19.  Example services distribution in a full mass-timber structure (image credit: Arup).

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Fig. 2-20.  Example of castellated beams with CLT floor panel at 6 Orsman Road (photo courtesy of Waugh Thistleton Architects).

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Fig. 2-21.  Example of castellated beams with CLT floor panel at 6 Orsman Road (photo courtesy of Waugh Thistleton Architects).

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Chapter 3 Fire Design A hybrid building may require the primary structure to be fire rated in accordance with the locally enforced building code due to the use, height, or area. A number of members and assemblies will contribute to the fire resistance for the hybrid structure, including: • Structural steel columns • Structural steel beams

3.1.2 Structural Steel

• Mass-timber floor assembly While each of the three major structural elements that make up a hybrid building can be treated somewhat separately for fire resistance, the steel beam-to-mass-timber floor interface and connectivity needs to be addressed. Though not covered in this Design Guide, other fire-rated building elements such as exterior walls, internal fire barriers, and partitions will also be required. 3.1

structure is required to provide an FRR that meets the IBC construction type. The FRR is achieved by increasing the thickness of the member and is referred to as the “wood cover,” as this additional mass timber chars in fire (see Figure 3-1). CLT fire performance can also be enhanced, where required, by lining with fire-resisting gypsum board.

BASICS OF FIRE PERFORMANCE

Fire endurance of a building element or assembly is measured in time and is referred to as the fire resistance rating (FRR). Proving fire endurance is a measure of how a building element or assembly resists applied loads, prevents the passage of heat and flames, and is able to resist temperature rise on the nonfire side. To achieve an FRR, a structural element or assembly is required to pass the criteria of ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials (ASTM, 2020), or ANSI/UL 263, Fire Tests of Building Construction and Materials (UL, 2020), which apply a standardized heating regime to a loaded building element to determine the fire rating. 3.1.1 Mass Timber

To achieve an FRR of 1 or 2 hours, structural steel members utilize the application of additional protection, which include sprayed fire-resistant materials (SFRM), noncombustible fire-resistive boards, or intumescent paints. Intumescent paint is the ideal fireproofing solution for the hybrid steel-timber system to showcase the beauty of the exposed steel and timber. An example of intumescent paint utilized in a hybrid project is shown in Figure 3-2. Other options can include concrete encasement, enclosure with masonry, or specialized flexible wraps. Each protective solution provides an insulative barrier to the structural steel to prevent increasing temperatures due to fire exposure, with the steel member relying on the additional protection to retain strength. The steel protective product chosen to be used is dependent on final desired appearance, constructability, and cost. Fire-rated solutions for steel members are available from individual suppliers that provide specifications on application, thickness, and maintenance. Guidance on structural steel protection is available from AISC Design Guide 19, Fire Resistance of Structural Steel Framing (Ruddy et al., 2003). Generic protection solutions are also available for specification from the IBC for certain types of products, such as SFRM. ANSI/UL 263 also lists fire-rated solutions. 3.2

Timber as a structural material is combustible. While combustible, the inherent fire resistance provided by mass-timber building elements such as glulam and CLT is distinctly different from the minimal fire resistance of light-frame wood members. Mass-timber elements such as CLT have very good fire resistance due to the depth of timber, as similar member types will char at a predictable rate when exposed to ASTM E119 or ANSI/UL 263, allowing design for long­er duration of structural performance in fire. For example, many mass-timber elements do not need additional fire protection to achieve an FRR of up to 2 hours. Where mass timber is exposed and not protected or encapsulated by noncombustible boards, the mass-timber primary

CODE CONSIDERATIONS

Timber construction is referred to as combustible construction in the IBC. Concrete and steel construction is referred to as noncombustible construction. Hybrid timber-steel construction utilizes both combustible and noncombustible structural elements, and therefore, a hybrid building will be determined as a construction type based on the timber (combustible) construction. Within the IBC, timber can be utilized within Types III, IV, and V construction, and hence these are the construction types applicable for hybrid buildings. More specifically, based on the 2021 IBC, construction Types III, IV-HT, IV-C, and V are limited to low- and medium-rise buildings. Types IV-A and IV-B are high-rise mass-timber construction.

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The IBC states limitations on building floor area, number of stories, and overall height, based on differing types of construction, occupancy use, fire protection systems installed, and the FRR to be achieved. For low-rise buildings (up to four floors), the use of light-frame and mass timber is fairly unrestricted and can be included within exterior walls. For buildings with an occupied floor above 75 ft (defined as high rise), the IBC requires an increased level of fire protection and structural performance. Fire protection and structural performance are increased for high-rise buildings, with a minimum FRR of 2 hours for floors and 2 or 3 hours for columns. The primary structure is required to have sufficient resistance to survive full burnout of a fire in the highly unlikely scenario where the sprinklers fail and the fire department has limited intervention [see AWC/ICC (2020) for further background]. Compared to a low- or mid-rise building, there is a significant increase in expected structural performance for all high-rise buildings in fire. Above 85 ft building height (to the roof), mass timber is only permitted with Type IV-A or IV-B construction. Guidance documents on the height and areas permitted in the IBC for timber construction is freely available from the ICC, AWC, and WoodWorks.

3.2.1 Type III, IV-HT, and V Types III, IV-HT, and V are construction types that allow combustible structures and can be utilized for hybrid construction following the 2018 IBC and earlier versions. These construction types are limited to low- and mid-rise buildings. The following limitations should be considered at the early stages of a project when studying overall building massing and choosing construction types: • Type III allows combustible construction for internal load-bearing and nonload-bearing elements. Exterior wall assemblies may be fire-retarded treated timber, provided they meet a 2-hour FRR, otherwise external walls must be noncombustible. This is a construction type suitable for hybrid construction. • Type IV-HT is a method of construction based on timber members having minimum dimensions, which provides an inherent FRR. Type IV-HT construction also permits exterior wall assembles to be CLT or fire-retarded timber. If load bearing, they must provide a 2-hour FRR, otherwise they must be noncombustible. Because IV-HT construction is primarily based on the structure being timber, this construction type is not well suited for hybrid

Fig. 3-1.  CLT panel after 1-hour fire test in accordance with ASTM E119 (photo courtesy of David Barber). 28 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

construction (where structural steel is used it must achieve an FRR, which is nominally 1 hour, though this is not specifically stated within the IBC). • Type V construction permits any materials used by the IBC and permits wall assemblies to be combustible but is limited to buildings of four floors or less with very restrictive floor area. This is a construction type suitable for hybrid construction. • Types III, IV-HT, and V are permitted by the IBC to have all the load-bearing mass-timber (or heavy timber) structure fully exposed, where the structure has been proven to provide the required FRR for Types III and V and the minimum dimensions for Type IV-HT. Thus, CLT used for floors can be exposed provided it meets the FRR requirements. • Type III and IV-HT can be constructed to a maximum building height of 85  ft and up to six floors in height. The number of floors can be increased where a podium

is introduced, which allows for additional floors to be included, with the building height still limited to 85 ft. For buildings of Type III or V construction, the requirements for interior finishes must also be met. Interior finishes for mass timber must meet minimum requirements for flammability and smoke development based on fire testing to ASTM E84, Standard Test Methods for Surface Burning Characteristics of Building Materials (ASTM, 2021c), which provides a classification of how flames will spread on a material and the smoke that develops. Most engineered timber products have been tested to this standard and have classifications that allow them to be used as an interior finish in most building applications. A CLT supplier can provide a test report stating their compliance with ASTM E84. Careful consideration must be given where stair and elevator shafts are higher than four floors because these are required to be 2-hour rated. Given that the timber structure only requires a 1-hour rating or less, the stair and elevator

Fig. 3-2.  Intumescent paint (photo courtesy of Megan Stringer). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 29

shafts must be independent of the timber structure. If they are not, then the timber structure rating will need to increase to 2 hours. 3.2.2 Type IV-A, -B, and -C The 2021 IBC has introduced three new construction types, including high-rise mass timber. All three construction types require automatic sprinkler protection in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems (NFPA, 2021c), throughout. The three new construction types are summarized in the following, with more information and background available from WoodWorks and the American Wood Council (AWC). 3.2.2.1 Type IV-C Type IV-C permits mass-timber construction up to 85 ft in height, with residential uses allowed up to eight floors and office uses up to nine floors. Assembly uses are limited to six floors. All mass timber can be exposed. Type IV-C also allows the use of mass timber for shafts. The required FRR is 2 hours for all primary structure (IBC defines primary structure as the columns, beams, girders, trusses, floors, and roof members that have direct connection to the columns). Combustible concealed spaces must be protected with fire-rated gypsum board. Hybrid construction can utilize Type IV-C construction to enable mass-timber floors to be exposed on the underside. 3.2.2.2 Type IV-B Type IV-B permits mass-timber construction up to 12 floors and 180 ft for residential, office, and assembly uses. Shafts can also be constructed from mass timber, though must be fully protected with fire-rated gypsum board. The FRR is 2 hours for all primary structures. There are limitations on how much mass-timber structure can be exposed, with the underside of floors limited to 20%, or where vertical elements or walls are present, these can be exposed up to 40%. The protection to be provided is two layers of s in. Type X board, which also contributes to the floor assembly fire resistance. The topside of the CLT floor must also be covered with a minimum 1  in. noncombustible covering. As with Type IV-C, combustible concealed spaces must be protected with fire-rated gypsum board. Hybrid construction can also utilize Type IV-B construction and the entire 20% exposed mass-timber area can be made up of the mass-timber floor panels. In most mass-timber buildings, the 20% exposed area has to incorporate the masstimber beams supporting the mass-timber floor panels, with the beam area included within the 20%. A hybrid building can have more mass-timber floor panel exposed area because the steel beams are noncombustible.

3.2.2.3 Type IV-A Type IV-A permits mass-timber construction up to 18 floors and 270 ft for residential, office, and assembly uses. Shafts must be constructed of noncombustible construction. The required FRR for the primary structure is 3  hours, with 2 hours for floors. All mass-timber elements are to be protected with three layers of s in. Type X board, which also contributes to the assembly fire resistance. The topside of the CLT floor must also be covered with a minimum 1 in. noncombustible covering. As with Type IV-C and -B, combustible concealed spaces must be protected with fire-rated gypsum board. Hybrid construction can also utilize Type IV-A construction. Both structural steel and the CLT can use fire-rated gypsum board as a continuous FRR solution. 3.2.3 Podium Construction Use of IBC, Section 510, allows an increased number of floors beyond the limits for Type III, IV, or V construction. The lower one or two stories of the podium are formed from Type I-A construction, with a 3-hour fire-rated horizontal separation provided to the upper floors of Type III, IV, or V. The limiting height for the building is the height limit for the Type III, IV, or V construction. A podium can also be used with Type IV-A, -B, or -C construction. 3.2.4 Tall Buildings—Alternative Code Approaches If a building is to be designed that does not meet IBC requirements, an application based on “alternative materials, design and methods of construction and equipment” can be submitted for discussion and potential approval by the AHJ. The alternative materials and methods approach should only be undertaken after an initial discussion with the AHJ and local fire department has occurred to assess the likelihood of approval where appropriate technical justification, testing, and analysis can be provided. Examples where an alternative materials and methods application may be beneficial to a project include protection of combustible concealed spaces, use of differing noncombustible protection products, proving the fire resistance of an innovative CLT-to-steel beam connection, or larger areas of exposed mass timber. 3.3

FIRE PROTECTION OPTIONS FOR HYBRID SYSTEMS

3.3.1 CLT Fire Resistance CLT performance in fire has been very well studied, with the char rate being dependent on ply thickness, number of plies, and type of adhesive. There has been significant fire testing in Europe, Canada, and more recently, in the United States. The outcomes from fire testing have been consistent,

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showing that CLT will char in a predictable manner, and fire resistance can be calculated based on the number and thickness of plies that each panel is made up of and the type of adhesive used. Fire-resistance tests performed to ASTM E119 in the United States have shown wall fire ratings of more than 3 hours and floor fire ratings in excess of 2 hours with the CLT elements loaded. Figure 3-3 shows the resulting char layers after a fire test of a CLT panel. CLT adhesives have been the subject of ongoing research related to performance in fire and improvement of CLT performance, with the North American manufacturing standard ANSI/APA PRG 320 (APA, 2019) updated to only use adhesives that were shown to prevent issues of bond-line integrity (often called CLT delamination). Since 2018, all CLT suppliers are required to use adhesives that meet ANSI/APA PRG 320 to provide an improved resistance to fire. For a building permit application, the fire resistance of the CLT needs to be proven. To assist architects, engineers, and contractors, the IBC has several methods for fire resistance compliance through fire testing or calculation. All North American CLT suppliers have had their CLT panels fire tested to meet ASTM E119, demonstrating a 1‑hour or 2‑hour FRR. A fire test report is available from each supplier to support the use of their products as load-bearing floors, which needs to be reviewed for applicability for a project in support of a building permit application. As an alternative, due to the number of fire tests to ASTM E119 that have been completed, and similar standard fire testing globally, there are calculation methods to determine the fire resistance of CLT (see NDS Chapter 16). A comprehensive review of fire testing and data is provided in the U.S. Department of Agriculture (USDA) and U.S. Forest Service guide, CLT Handbook (U.S. Edition), or AWC’s Calculating the

Fire Resistance of Wood Members and Assemblies (AWC, 2021a). WoodWorks maintains an online inventory of 1‑hour and 2‑hour fire test reports from a number of CLT suppliers to assist designers. 3.3.1.1 Timber Panel-to-Panel Connections Connections for CLT panels are proprietary to each manufacturer based on their unique panel lay-up and testing completed. When a CLT panel is fire tested to ASTM E119, the panel-to-panel connections are also fire tested and as a result, each CLT supplier has a connection type that is approved for use with their 1‑hour or 2‑hour rated panel. The specifications for this connection must be followed in construction to provide a compliant fire-resistance rated floor system. Figure 3-4 shows typical panel-to-panel CLT connections after a fire test. 3.3.1.2 Concealed Spaces Mass-timber buildings that utilize CLT as the primary floor elements have no concealed spaces within the primary structure due to the solid timber construction of CLT. Where concealed spaces for building utilities such as pipes, cables, and ducts are formed with CLT, a combustible concealed space is created which can be an issue for potential fire spread. Concealed spaces need to be protected and any exposed CLT covered with noncombustible protection. 3.3.1.3 Penetrations “Through penetrations” occur in all buildings for plumbing, electrical cables, telecommunications, heating, and cooling. For penetrations in CLT floors, the installation of a collar,

Fig. 3-3.  CLT panel with char layer after fire test (photo courtesy of David Barber). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 31

fire mastic, or fire damper that has been previously tested for use in a CLT wall or floor penetration is specified that meets ASTM E119 or ASTM E84. Currently there are a limited number of suppliers of these products available in the United States, and CLT penetration fire sealing needs to be addressed on a project-by-project basis. 3.3.1.4 Considerations for Alternative Mass-Timber Floor Materials Where other mass-timber panel floor assemblies are being used, such as nail NLT or DLT, they are required to achieve a proven FRR through testing to ASTM E119 or ANSI/ UL 263. The FRR for the floor assembly, whether by fire test or through engineering calculation, must include all joints between panels, including the gaps included for shrinkage or expansion. 3.3.2 Structural Steel Fire Resistance For hybrid construction where the steel structure is typically exposed along with the mass timber, intumescent paint is most commonly used when fire protection is required. Intumescent paint protects the underlying steel by charring and expanding when exposed to fire to insulate the member. The paints are proprietary and will react when exposed to heat of about 480°F and can expand in thickness by up to 100 times. The thickness of paint will be dependent on the member size and the hourly rating required. The paint can be factory applied or site applied. Another option for steel fire protection is to use fire-rated gypsum board and “box” around the steel columns and beams to provide the fire resistance. As noted previously, if Type IV-A construction is being used, the fire-rated gypsum that is required to protect the CLT for 2 hours can also be utilized to protect the steel beams and columns, noting that the primary structure requires 3 hours. Where the steel is concealed, spray-on fire resistive materials can also be used.

3.4

FIRE PERFORMANCE OF STEEL-TOMASS-TIMBER INTERFACE

Hybrid buildings can be designed with the CLT floor acting as a lateral restraint to the top flange of the steel beam to prevent beam lateral-torsional buckling, a method consistent with the design of steel structures with concrete over metal deck slabs. Regularly spaced screws connect the top flange of the beam to the underside of the CLT floor to provide the lateral restraint, where required. If the steel beams are designed with the CLT floor providing restraint, then restraint under fire exposure needs to be assessed. Of concern is when the steel structure is exposed to fire, heat is transferred into the CLT via the top flange of the beam and the regularly spaced screws. Fire protection such as intumescent paint forms an insulating layer to prevent steel beam failure, with the protective layer reducing heat transfer into the steel beam. But the fire protection does not prevent the beam from heating up. ASTM E119, for example, permits steel beams to pass a fire-resistance test provided the average temperature of the beam remains below 1,100°F. As the top flange and the screws heat up through thermal transfer, the CLT also increases in temperature. Timber strength reduces rapidly at temperatures above 300°F, as does timber density, which is an important factor for screw resistance. Thus, screws providing lateral restraint need to be checked for shear resistance in timber of decreasing strength. Thermal modeling and structural evaluation have shown that failure of a steel beam can occur by lateral-torsional buckling due to a lack of top-flange restraint before the required 1-hour or 2-hour fire resistance period is reached for the floor. The screws connected to the CLT can lose shear capacity as both the steel beam and CLT lose strength under exposure to an ASTM E119 fire. Heat transfer into the CLT occurs through the top flange and the screws, resulting in the timber weakening as fire exposure time increases. Solutions to prevent this issue can include designing steel beams that do not need top flange lateral restraint under fire

  Fig. 3-4.  Fire test of typical timber panel-to-panel connection (photos courtesy of David Barber). 32 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

conditions or increasing the thickness of fire protection to the steel beam to keep the screws and top flange below a temperature of 325°F for the fire resistance period [connection acceptance criteria from AWC TR-10 (AWS, 2021a)]. A performance-based approach can also be utilized to assess the required beam-to-CLT connectivity. 3.5 DETAILING There are a number of areas where additional design detailing is needed to address the fire resistance of the primary structure: • Where fire-rated gypsum board is used to protect the steel, the interface and transition from the fire-rated gypsum to the CLT needs to be detailed carefully, following proprietary fire sealing solutions. The CLT will reduce in depth under fire exposure, whereas the gypsum board will remain relatively fixed, therefore potentially creating a gap in the fire protection. • Where CLT is protected with fire-rated gypsum board required by the IBC, such as for Type IV-B buildings, the interface between the layers of gypsum board and the steel beam needs to be considered carefully. This is particularly important where the steel beam includes intumescent paint as the fire-resistive protection. Where two

differing forms of fire-resistive protection are interfacing, an overlap is normally required. • Where the top flange of the steel beam does not require intumescent paint because the top flange will not have space for the paint to intumesce, being screw-fixed to the CLT. Any charring that may occur at the interface between the edge of the top flange and the CLT would be protected by the surrounding expanding intumescent paint. • Where mass-timber panels are cut to be placed around the steel columns, a fire sealing solution needs to be installed to provide continuity of the floor integrity and insulation. Given the project-specific nature of this fire seal, this will need to be detailed carefully at the column interface. A concrete topping over the CLT can also be used to seal this gap. • If a curtain wall is installed on the building, a fire seal is required between the edge of the CLT floor and the back of the curtain wall, commonly referred to as a perimeter fire seal. There are no fire-tested perimeter seal solutions for buildings with CLT floors (at the time of writing), to seal against a curtain wall. Project-specific solutions and a negotiated approval may be required.

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34 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Chapter 4 Acoustics 4.1

BASICS OF ACOUSTICS IN MASS TIMBER

The acoustic design of mass-timber buildings, whether hybrid steel or all wood, presents a complex set of challenges and opportunities. The design considerations include acoustics, fire, and structural seismic issues, all of which have interrelated design variables and should be evaluated holistically at the start of design to determine an optimum solution. For example: 1. The structural design for the mass-timber floor thickness has associated acoustic and fire performance implications. 2.

The acoustic design approach of adding mass has structural and fire implications.

3. The code fire requirements for protective coverings have acoustic implications. This section will provide an overview of the primary acoustic issues in buildings with mass-timber floors, including sound isolation, flanking noise, and the importance of acoustic testing, both in the lab and in-situ. The fundamentals of acoustics are well documented in other references and not intended to be summarized here. However, the discussion that follows assumes a general understanding of how sound levels are measured, principally that the decibel scale is a logarithmic representation of sound. A change of 10 decibels (dB) anywhere on the scale corresponds to a 10-fold change in physical sound intensity, or one order of magnitude. (Normal hearing individuals can hear across many orders of magnitude). Subjectively, a 10 dB increase is typically perceived as twice as loud. For more in-depth information on this topic, see Design Guide 30, Sound Isolation and Noise Control in Steel Buildings (Markham and Unger, 2015). Note that the sound separation differences between steel column and steel beam supported mass-timber floors compared to wood column and wood beam supported masstimber floors are typically inconsequential given that the primary acoustic design elements are contained within the floor-ceiling assembly and demising walls, and not the supporting structure.

4.1.1 Acoustic Design Metrics 4.1.1.1 Acoustic Separation While not unique to mass timber, sound separation between interior spaces is a key acoustic design consideration for buildings with mass-timber floors given the relatively lightweight floor structures when compared to concrete buildings. Sound separation between spaces includes both airborne sound separation (horizontally and vertically) as well as impact sound isolation between vertically stacked spaces. Airborne sound separation is the measure of sound that is transmitted through the air into adjacent spaces via a sound isolating partition. Airborne sound separation can be measured in an acoustics laboratory in accordance with ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements (ASTM, 2016b). The results of the ASTM E90 test include transmission loss values reported in decibels in each of the 3 octave band frequencies ranging from 125 Hz up to 4 kHz. The measured transmission loss values are used to determine the Sound Transmission Class rating (STC) by comparing the measured transmission loss decibel values to the standard STC curve defined in ASTM E413, Classification for Rating Sound Insulation (ASTM, 2016a). Impact sound transmission is the measure of sound that is imparted into the adjacent space below from a “tapping machine” (a precision impact noise source with an array of metal hammers, each of a specific weight that are mechanically dropped from a specific height to induce a standard quantum of impact sound energy into a floor-ceiling assembly). Impact sound separation can be measured in an acoustics laboratory in accordance with ASTM E492, Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine (ASTM, 2016c). The results of the ASTM E492 test include measured airborne noise levels in the receiving room reported in decibels in each of the 3 octave band frequencies ranging from 100 Hz up to 3,150 Hz. The measured transmission loss values are used to determine the Impact Insulation Class (IIC) by comparing the measured tapping machine noise levels in the receiving space to a standard curve defined in ASTM E989, Standard Classification

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for Determination of Single-Number Metrics for Impact Noise (ASTM, 2021a). Airborne (STC) and impact sound (IIC) are illustrated in Figure 4-1. Standardized rating systems, such as STC and IIC, provide a convenient means of comparing the sound isolating performance of different constructions, but they should be used carefully with the following limitations in mind: 1. Both the STC and IIC ratings are single number ratings based on a distillation of acoustic performance across a spectrum of frequencies. Therefore, different partitions can have similar (or the same) STC/IIC rating while also having significantly different performance across the frequency spectrum. 2.

The STC and IIC ratings only consider the middle 60% or so of the audible spectrum. (We can hear approximately two full octaves below 125 Hz and more than

two full octaves above 4 kHz). Our subjective experience of sound includes frequencies well above and below the range of consideration of the single number ratings. 3. Both the STC and IIC ratings represent lab-tested performance under ideal conditions (i.e., without the multiple flanking paths present in the field). Performance achieved in the built environment is typically 5 to 8 points below lab results, and possibly more, depending on flanking paths. Flanking sound transmission refers to sound transfer paths “around the demising partition” that can degrade acoustic performance if not properly detailed to mitigate the sound transmission. Example flanking paths include gaps in weather­stripping around doors, unsealed gaps where sheets

Fig. 4-1.  Visual representation of airborne (STC) and impact sound (IIC) separation. 36 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

of gypsum wallboard meet each other or surrounding constructions, building services penetrations, and ridged connections between walls and floors. The net result is that our subjective impression can vary significantly for partitions of similar or equivalent ratings depending on a variety of factors including: 1. The 3  octave band performance. STC and IIC ratings are based on deviations from a standard spectrum, and these deviations, or relative deficiencies, may occur anywhere in the frequency range of STC or IIC consideration. 2.

The performance above and below the frequency range of STC and IIC consideration. The presence of lowand high-frequency sound can influence our subjective impression of the acoustic environment. Consider modern home audio systems produce low-frequency sound down to 40 Hz, where energy is readily transmitted through buildings. Conversely, our hearing systems are well attuned to high-frequency sounds such as the “clicky” sounds produced by hard soled shoes or the cries of an infant.

3. A number of environmental factors, such as the sound source (e.g., home theatre system, high-heeled shoes, etc.) and the ambient sound environment (e.g., how much background sound is present to mask noise ingress from the adjacent space). For these reasons, designers and project teams are encouraged to “experience” the acoustic performance of assemblies under consideration, where possible, in order to make informed cost-benefit design decisions. The subjective experience of the design options under consideration may be achieved either via virtual simulation created by a qualified acoustic consultant or through real-world benchmarks. In addition to the laboratory STC and IIC ratings, there exists a variety of acoustic rating systems to characterize performance in the field, such as the field sound transmission class (FSTC), the apparent sound transmission class (ASTC), the noise isolation class (NIC), and the normalized noise isolation class (NNIC). Field measurements are of particular importance for the development of the mass-timber building industry given that flanking paths may be included in the measurement data. When we experience sound transmission in the built environment, we encounter the direct sound (along the shortest path to the sound source through a separating wall or floor barrier), plus the various flanking paths that may be present, and other environmental factors. Thus, the field performance of the final constructed assembly carries a high importance due to its link to our subjective impression. Given the relative novelty of mass timber and timber-steel hybrid construction and the relatively low number of these building types completed in the United States to date, field measurements

are exceedingly rare at this time, despite the abundance of laboratory test data. Best practice for flanking path mitigation may continue to develop as insights are gained from the study of completed projects. The current best practice for flanking path mitigation is as follows: 1. Identify in plan and section flanking risk areas where acoustically critical sound isolating partitions meet other constructions (e.g., where a residential unit’s demising walls meet the façade, where a unit’s demising walls meet the mass-timber floor or ceiling assembly, where building services penetrations exist, etc.) 2. Develop draft architectural details that show in plan and section how the sound isolating constructions meet or interface and determine if there are opportunities for sound to “flank around” the sound isolating construction via sound paths that are significantly weaker than the demising construction. 3. If there exists a sound path that is significantly weaker than the demising construction (e.g., less mass, smaller airspace, undamped cavities or voids, unsealed intersections or interfaces between materials), then revise the intersection detail to add sealants, add mass, and/or add damping to voids. Unfortunately, there is no one-size-fits-all solution for flanking path mitigation—a professional acoustic consultant should be engaged to determine if there are flanking path risks and possible solutions. In-situ and laboratory testing can be performed to determine project-specific requirements and performance, as illustrated in Figure 4-2. 4.1.1.2 Room Acoustics Of important distinction from sound separation (STC, IIC) is the concept of room acoustics and reverberation. When a sound is emitted within an enclosed room, the sound waves propagate from the source and interact with the enclosing surfaces (e.g., walls, ceiling, floor). Some of the sound is transmitted through the separating construction (as shown in Figure 4-3); however, a significant portion of the sound energy is reflected off the surface and back into the room. A listener within the room will hear a mixture of the initial direct sound from the source, and reflections from the various surfaces spread out over time. The material properties of the various room surfaces, as well as room dimensions and volume, influence how much sound persists in the room after the initial sound is experienced. A visual representation of sound reflection is shown in Figure 4-4. Reverberation is quantified by measuring the time it takes for an emitted sound to diminish 60  dB below its original level. This is called the decay time or reverberation time (represented in Figure  4-5). While the reverberation time of a space can influence the amount and character of the

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(a) Laboratory testing (airborne and impact)

 

(b) Impact field testing

(c) Airborne field testing

Fig. 4-2.  Acoustic field-testing photos (photos courtesy of Arup).

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Fig. 4-3.  Visual representation of sound reflection, absorption, and transmission at a partition.

Fig. 4-4.  Visual representation of sound reflection in a room.

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 39

transmitted sound observed in the receiving space, room finishes do not have a significant impact on the design or performance of sound separation constructions. Therefore, project teams should not typically rely on sound absorbing finishes as a means to improve sound separation performance unless otherwise directed by a qualified acoustic consultant. 4.1.1.3 Acoustic Design Concepts The conceptual strategies for improving the acoustic performance of mass-timber floor assemblies include: • Add mass. The acoustic principle known as the “mass law” states that for every doubling of mass, 6 dB of additional airborne sound isolation is achieved. Adding mass to a mass-timber floor-ceiling assembly can improve the airborne sound isolation (STC). Many benchmark CLT floor assemblies incorporate a layer of dense construction (e.g., concrete, gypsum concrete, etc.) on top of the CLT to provide additional mass to the timber structure. The acoustic benefits of a heavier floor should be considered against the increased demands on the structure (i.e., more steel to support the added weight), dense material and associated labor costs, and any changes in how the structure behaves under force impulse. Consider that additional mass lowers the natural frequency of the floor, and lower frequency resonances may be more likely to be felt as vibration or interfere with sensitive equipment in a laboratory or medical space, for example. • Add airspace or decoupling. Note that the mass law as simply stated in the previous point applies to “singleleaf ” partitions. Airborne sound separation may also be improved by creating a “mass, airspace, mass” partition, or “double-leaf ” partition. The concept is analogous to a standard gypsum stud wall (as opposed to a solid masonry wall). From a first principles perspective, double-leaf partitions behave differently than single-leaf above a certain

frequency and can provide more sound attenuation with less overall mass, allowing a reduction in the weight of the structure. Considering the carbon intensive nature of standard dense construction, an additional sustainability benefit arises from the strategic use of air cavities. With any multi-leaf assembly, however, there can be acoustic weaknesses in parts of the audible spectrum. A qualified acoustic consultant should be involved to help designers and stakeholders understand the nuances of decoupled mass assemblies. • Add resilience. Integration of resilient layers within a sound separating assembly can improve the impact isolation performance (IIC), sometimes with a knock-on improvement to the airborne sound separation as well. Resilience can be introduced to a floor-ceiling assembly via an underlayment or pad or other isolator between the CLT and massive topping. The impact isolation performance typically improves with thickness of the resilient element, but also varies by material type. Resilient or soft floor finishes (such as carpet) can also significantly improve the impact sound isolation performance. 4.2

TYPICAL MASS-TIMBER FLOOR BUILDUPS

Note the following observations from comparing the STC and IIC data from multiple 5-ply CLT manufacturers: 1. Not all 5-ply assemblies have the same thickness, wood species, or density, which means acoustic performance will vary between panels. Not all 5-ply CLT panels perform the same and need to be evaluated on a case-bycase basis. 2. For a generic 6-in.-thick, 5-ply SPF CLT panel, an airborne sound separation performance of approximately STC 40 (+/− a few STC points) and an impact sound isolation performance of IIC in the mid-20s can be achieved with the CLT panel alone.

Fig. 4-5.  Visual representation of sound energy decay over time. 40 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

3. For many residential and office projects, the acoustic performance of the CLT panel alone is not sufficient. For reference, IBC Section 1206.2 and Section 1206.3 (ICC, 2018) note STC and IIC 50 are the minimum required performance ratings for separating construction between dwelling units, which is an order of magnitude or more above the performance of the bare panels noted previously. Thus, even a modest level of sound separation requires additional mass and resilience. Given this information, the typical design solution to achieve the required STC and IIC performance is to include an acoustic topping, as shown in Figure 4-6. 4.3

ACOUSTIC TOPPING OPTIONS

The optimum acoustic design considers the architectural, structural, and fire code requirements. Experts from the various disciplines should highlight the design requirements and possible design solutions that benefit the other disciplines, resulting in an efficient, cost-effective solution. The multidisciplinary factors to consider are shown in Figure 4-7. An example outcome of this approach could be a gypsum lining that can provide an acoustic benefit as well, particularly when attached via resilient connections and/or with an airspace. Furthermore, an acoustic mass may introduce benefits or drawbacks to the structural design, depending on the location or the type of material used for an acoustic mass. For example, the range of acoustic toppings with varying material properties (i.e., concrete versus gypsum concrete) may differ in their suitability as a substrate for anchoring

partitions to the floor structure. Unfortunately, there is no single approach suitable for all project types, and typically the primary design requirements are driven by the combination of code requirements (seismic and fire) and the owner’s project goals. Figure  4-8 shows STC and IIC results for numerous assembly options with a 5-ply CLT panel. Many benchmark CLT floor assemblies incorporate a cast-in-place concrete or gypsum concrete topping to provide additional mass in a cost-effective thin profile. However, other project considerations may make a dry topping such as cement board, plywood, oriented-strand board (OSB), precast concrete, or other sheet goods a desirable option. Benefits of a dry topping include speed because there is no cure time, offsite “prefab-ability” because dry toppings can be prefabricated with the mass-timber elements off site and then transported to site, and avoidance of potential moisture issues associated with pouring concrete or gypsum. Sheet goods also provide a means of achieving lighter or thinner toppings (where acoustically acceptable) that are more stable and crack resistant than the equivalent-mass cast-in-place topping. Acoustic drawbacks of dry toppings result from their typically lower density than concrete. Where the acoustic design requires a significant topping mass, additional thickness is needed to maintain the same surface mass. Further drawbacks include the risks of trapping water if applied too early, seams or gaps between boards that may result in acoustically weak areas, and/or additional labor may be required to lay out multiple layers of sheet goods instead of pouring gypsum or concrete over an entire floor area.

Fig. 4-6.  Typical mass-timber floor panel acoustic assembly. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 41

Multi-Disciplinary Design IBC building types Fire resistance ratings Char and encapsulation Concealed spaces Fire testing Approvals

Acoustics

Fire

Structural

Airborne sound isolation Impact sound isolation Flanking transmission Connection detailing Acoustic lab testing Acoustic field testing

Timber slab type/thickness Structural system • Post and beam • Point Supported • Rib deck • Timber/steel, Timber/concrete • Modular Spans Exposed/concealed connections Weather protection

Fig. 4-7.  Conceptual diagram of the interrelated design considerations for acoustics, fire, and structural designs.

42 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Fig. 4-8.  STC and IIC ratings for floor assemblies (Sabourin, 2015).

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 43

44 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Chapter 5 Sustainability 5.1

BASICS OF EMBODIED CARBON

5.1.1 Introduction to Embodied Carbon Embodied carbon refers to the total greenhouse gas emissions associated with the manufacturing, transportation, installation, maintenance, refurbishment, and disposal of building materials. Embodied carbon is measured in global warming potential (GWP) and includes many gaseous compounds that are normalized to carbon dioxide equivalents. Typically, these emissions can be considered on a basis of cradle-togate (from resource extraction through leaving the factory gate), cradle-to-site (adding transportation and construction site activities) or cradle-to-grave (from resource extraction through eventual disposal at end-of-life) scope. Cradle-tosite embodied carbon is also called “upfront embodied carbon” because the carbon emissions have occurred before the building opens and becomes operational (WGBC, 2021). The embodied carbon from building materials is estimated to be 11% of total global carbon emissions (CLF, 2021). Current growth rates indicate a continued need for large amounts of new building construction. As building operating

efficiency improves, the embodied carbon associated with building materials becomes a larger percentage of the whole life carbon emissions of a building. 5.1.2 Carbon Storage and Biogenic Carbon Interest in wood as a structural material is increasing because woody biomass is comprised of nearly 50% carbon, which was formed through the process of absorbing CO2 from the atmosphere through photosynthesis during growth of the tree. This CO2 is often said to be sequestered or stored in the wood product and is one form of biogenic carbon, along with the root structures and soil of the forest. However, this stored carbon will eventually be released as CO2 or other forms of greenhouse gases again when the woody biomass eventually burns or decomposes. For structural wood components, most of this does not occur until the end of life of the building in which they have served, as shown in Figure 5-1. The carbon form, timing, and other factors make the accounting of biogenic carbon associated with wood products complicated. In attempts to simplify the accounting, two popular approaches have emerged among the design

Fig. 5-1.  Life-cycle carbon emissions of a wood product (image credit: Arup). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 45

community. In the first, one assumes the sequestered carbon is entirely released as CO2 eventually, in which case the net effect is neutral. This assumption is based on another assumption—that the harvesting activities at the woodsupplying forest do not reduce its natural carbon stores over the long-term. Justification for that claim is typically derived by requiring robust sustainable forest management certification, such as through the Forest Stewardship Council (FSC) (FSC, 2019). The proponents of this scenario argue for exclusion of sequestered biogenic carbon in embodied carbon calculations. Including all the assumptions of the first scenario, the second approach goes further and argues that given the tendency for structural elements in buildings to last several decades, it is worth counting the sequestered carbon as long-term carbon storage. In this approach, it is assumed that some biogenic carbon remains stored at endof-life, giving a net biogenic carbon balance, because the carbon storage lasts past a short-term deadline for limiting greenhouse gas emissions. Essentially this approach shows how using wood instead of products that do not store carbon can “buy us time” in the few decades that climate scientists say the world has left to change the trajectory of global carbon emissions. In addition to the biogenic carbon related to the biomass of wood products themselves, there is additional biogenic carbon that also arises in the growth cycle of forests and the harvesting and manufacture of wood products. When forests grow, they turn atmospheric carbon into woody biomass, but when forests are cut and milled into lumber, much of that carbon is emitted back into the air. When a lumber mill burns wood waste for energy to kiln-dry the wood, carbon dioxide is emitted. When a forest is disturbed, as happens in logging when woody debris is left to decompose on the forest floor, carbon dioxide is also released. Life-cycle assessment (LCA) standards have allowed exclusion of emissions from biomass fuels if the wood qualifies as coming from “sustainably managed forests” as defined by ISO 21930 (ISO, 2017). The intent is to ensure a sufficient number of new trees are replanted to maintain an above-ground carbon balance against what was harvested and burned, in order to claim net neutrality of this particular use of biomass. Essentially, this definition of “sustainably managed forests” assumes that if the total area of forest land stays unchanged, the amount of carbon lost to logging (or other operations) is equaled by gains in growing forest areas. This is controversial, however, because there are a wide variety of forest management practices and not all ensure that the carbon emitted in disturbance of the forest will be replaced in the forest. Even on a large geographical scale, carbon storage has not diminished in North America in the last several decades, and measurements have shown that actual forest carbon flux varies enormously between forests under different management regimes. Despite all the caveats mentioned previously and acknow­ ledging there is no clear consistency in biogenic carbon

accounting, the reporting of biogenic carbon in the following example takes the approach of ISO 14067 (ISO, 2018): CO2 that is temporarily “stored” as carbon in the wood components of the assemblies compared in the following, as well as other types of biogenic carbon, is reported separately from the nonbiogenic embodied carbon of the assemblies. Mass timber is a wood building product that follows this same life cycle. By converting wood into a building product, carbon can be stored within mass timber before ultimately being released at end of life. Because mass timber is fabricated using small-diameter trees with shorter rotations, storing carbon within a mass-timber product requires more rotations of trees to replace those that have been harvested. This carbon storage benefit relies on forests being replenished to maintain the carbon cycle. Mass-timber products used in the United States are primarily from the United States, Canada, and Europe, which are all regions with an annual net increase in forest production on average. However, to ensure mass-timber products are sourced from sustainably managed forests, the best practice for engineers is to specify a robust sustainable forest management certification, such as from the FSC. 5.2

COMPARATIVE LIFE-CYCLE ASSESSMENT OF HYBRID STEEL-TIMBER

5.2.1 Introduction to Life-Cycle Assessment Whole building life-cycle assessment (WBLCA) is an accounting process for determining the total embodied environmental impacts associated with a building within a determined scope. Comparative WBLCA is incorporated into many green building rating systems as a credit option. WBLCA is a cradle-to-grave assessment, governed by the overarching ISO 14040, ISO 14041, ISO 14042, ISO 14043, and ISO 14044 (ISO, 2006a, 1998, 2000a, 2000b, 2006b) standards for LCA, as well as BS EN 15978 (CEN, 2011), which is more applicable to LCA for constructed assets. WBLCA is often limited to the scope of structure and enclosure because this is the scope required for the related Leader­ ship in Energy and Environmental Design (LEED) credit, although additional elements such as interiors and equipment should be included to more accurately account for a whole building. There are several commercially available whole building LCA tools within the United States, each with their own methodologies, assumptions, and approaches. Assessments can be conducted with comparison to a reference design to demonstrate embodied carbon reduction potential. For such comparisons to be realistic and meaningful, material quantities used in WBLCA should reflect actual engineered designs as opposed to theoretical sets of inputs that attempt to capture ranges in possible quantities without reference to engineering judgement.

46 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

The simplified LCA comparison does not consider the broader impacts on the building, but it is worth considering the further reductions in embodied carbon that a hybrid system can afford a project depending on the project specifics including the following: • Reduced or removed ceilings due to exposed wood ceilings

Framing: steel secondary beams spaced at 10 ft on center spanning to steel primary beams Cementitious spray fireproofing on underside of metal deck and around steel beams • Hybrid option 2 in. normal weight concrete slab over 5-ply CLT

• Reduced foundation structure due to reduced building weight (about 5% lighter than the steel system, 50% lighter than the concrete system)

Acoustic rubber mat

• Reduced vertical and lateral system structure due to reduced building weight

Intumescent paint on steel beams

5.2.2 Comparative Life-Cycle Assessment Results To estimate the embodied carbon reduction potential of steeltimber hybrid construction, a comparative LCA has been performed of 30 ft × 30 ft structural bay options using the same structural design criteria for each. The study compares the steel-timber hybrid to two common floor construction types in North America—prestressed concrete and composite concrete slab on steel deck. The three schemes are shown in Figure 5-2 with a description for each further detailed in the following. • Baseline prestressed concrete option 8 in. two-way slab with 6,000 psi normal weight concrete (NWC) Steel prestressing strands and mild reinforcement • Baseline composite steel option 34 in. lightweight concrete (LWC) slab over 18-gauge metal deck



Framing: steel secondary beams spaced at 15 ft on center spanning to steel primary beams For the purposes of this comparison, the scope is limited to the horizontal structure inside a typical interior bay. Items that would not alter the comparison, such as columns and architectural finishes, are not included. Structural elements beyond the scope of a single interior bay, such as lateral members and foundations, are also not included. The results of this study are not meant to be extrapolated to a whole building, but by focusing on the largest elements typically made with steel-timber hybrid framing, an order-ofmagnitude comparison between the systems can be made. To uphold functional equivalency between the schemes considered, a topping slab and acoustic mat are considered within the scope of the steel-timber hybrid scheme, whereas neither are necessary for the concrete and steel schemes. For fireproofing, intumescent paint has been used on steel beams for the hybrid scheme while cementitious spray fireproofing is used on the steel beams and underside of the metal deck in the composite steel scheme. Connections are accounted for within the scope of each scheme by increasing steel tonnage by 5% for the hybrid

(a) Post-tensioned concrete

(b) Composite deck plus steel

(c) CLT deck plus steel

Fig. 5-2.  Plan and section views of the composite and hybrid schemes used for the embodied carbon comparison. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 47

Table 5-1.  Summary of Material Quantities Used for Embodied Carbon Comparison Description

Pre-Tensioned Concrete

Units

8 in. NWC benchmark U.S., 6,000 psi Prestressing tendons

cyd

Composite Steel

Hybrid CLT plus Steel

–

– –

22.2 990

–

2 in. NWC benchmark U.S., 3,000 psi

cyd

lb

–

–

34 in. over 2 in. LWC benchmark U.S., 3,000 psi

cyd

–

11.8

– 516

5.60

CLT

3

ft

–

–

2 in. 18 ga. metal deck

lb

–

3220

Reinforcement

lb

1,170

565

227

Structural steel

lb

–

4,080

4,750

Connections

lb

–

408

237

Spray fire resistive material

2

ft

–

562

–

Intumescent paint

gal

–

–

2

–

–

Acoustic mat

ft

–

10.0 900

Table 5-2.  Embodied Carbon Results from Athena IE and Tally Total Global Warming Potential (a.k.a. Embodied Carbon) Athena IE Scheme Post-tensioned concrete (PT)

Tally

CO2e (kg)

CO2e/m2 (kg)

CO2e (kg)

CO2e/m2 (kg)

10,700

128

12,500

150

Composite steel

9,870

118

11,200

134

Hybrid CLT plus steel without biogenic carbon

7,610

91.0

11,100

133

Hybrid CLT plus steel with biogenic carbon

1,950

23.0

3,630

44.0

Note: CO2e is the carbon dioxide equivalent, which is the number of metric tons of CO2 emissions with the same global warming potential as one metric ton of another greenhouse gas.

scheme and 10% for the steel scheme. The latter is 5% higher to account for steel headed stud anchors. Regional average concrete mixes are assumed. The building life expectancy is assumed to be 75 years. A summary of the material quantities for each option is given in Table 5-1. The comparison was run in two different LCA tools, Athena Impact Estimator (IE) for Buildings (Athena, 2020) and Tally® (Tally, 2020), because of differences anticipated due to incongruencies in assumptions, data sources, and reporting of biogenic carbon. Where Athena IE reports biogenic carbon in module D for environmental impacts beyond the building boundary, Tally reports sequestered carbon in stage A1 if the user elects to see output including biogenic carbon. Athena IE includes biogenic carbon arising from the wood product biomass at end-of-life, but Tally only displays this if inclusion of biogenic carbon is selected. Both tools’ approaches inherently begin from the “carbon neutral” assumptions discussed in Section 5.1.2 that, at a minimum, require robust sustainable forest management certification, such as from the FSC.

Table 5-2 shows the global warming potential (GWP) of the LCA tools—Athena IE and Tally. In both comparisons, post-tensioned (PT) concrete is higher in embodied carbon than the hybrid system. Using Athena, the concrete and steel framing systems results are nearly the same and exceed the hybrid system by about 25% when excluding biogenic carbon emissions. In Tally, instead of the steel and concrete systems coming close to each other, the steel and hybrid systems are closer in total embodied carbon (excluding biogenic), and concrete is higher than the two by about 10%. In contrast, when biogenic carbon is included, the difference in Athena IE amounts to an 80% decrease in embodied carbon, and in Tally the reduction is near 70%. Both Athena and Tally assume that no maintenance is required for CLT over the lifespan of this building. The variation in output from using two different tools illustrates the inconsistency that arises because of the discrepancies built into the tools, especially their end-of-life assumptions.

48 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Moreover, the same construction materials could not always be found in both tools, so sometimes a proxy material had to be used instead. Figure 5-3 visually compares the results from Athena IE to Tally. The striking difference between the hybrid scheme output from Athena IE to Tally are due to many variables, including the following: • Drastically different end-of-life assumptions for biogenic carbon between the two tools, which makes wood appear much lower in GWP in Athena IE than in Tally.

• The steel tonnage in the hybrid scheme exceeds the steel in the composite scheme because the hybrid option does not assume composite action between the steel and the concrete topping or the CLT slab. Leveraging composite action would increase efficiency and reduce the embodied carbon impact of the hybrid scheme. However, composite systems for timber are not often seen due to additional time and labor that would be needed to install the steel headed stud anchors, so these were not examined in the comparative LCA study (SOM, 2017).

• Higher GWP factors for 6,000 psi concrete, rebar, CLT, and spray-applied fireproofing in Tally, making all options appear worse.

• The volume of concrete used in a composite system exceeds that used in a hybrid system, which counteracts the additional steel tonnage in embodied carbon.

• Higher GWP factor for steel shapes in Athena IE than Tally.

• The concrete used in the composite steel system is higher in embodied carbon because of its mix type. It is more common to use lightweight concrete in the composite system to obtain the required fire rating, whereas normal weight concrete topping is typically preferred in a hybrid system for its acoustic damping. Lightweight concrete has higher embodied carbon by volume than normal weight because it often needs more cement to compensate for its lower strength aggregate. Thus, the hybrid system offers lower GWP than the composite system through changes in concrete volume and mix type.

• Assumption of net neutrality of biogenic carbon related to biomass fuels in Athena IE, whereas Tally does not assume net neutrality when the “include biogenic carbon” option is selected. The output demonstrates that GWP values for CLT vary significantly across tools so much that an embodied carbon comparison excluding biogenic carbon makes it difficult to conclude that a hybrid system is lower or higher in embodied carbon than conventional systems, unless biogenic carbon is included. When biogenic carbon is included, the steel-timber hybrid is clearly lower in GWP using both tools. This phenomenon has been observed in other studies of mass-timber systems as well (SEI/SE2050, 2021; TDI, 2019). Despite these differences, some observations are common to the output from both tools:

PT Concrete

Composite Steel

Hybrid CLT + Hybrid CLT + Steel Without Steel With Biogenic Carbon Biogenic Carbon

• There are options to use noncementitious materials over the CLT deck to achieve equivalent acoustic performance which would likely lead to even lower embodied carbon of the hybrid assembly, such as gypsum backer board or a variety of composite wood products. These dry toppings are also appealing for avoidance of mixing wet and dry

PT Concrete

Hybrid CLT + Hybrid CLT + Composite Steel Without Steel With Steel Biogenic Carbon Biogenic Carbon

200 acousc mat

150

intumescent

CO2e/m2 (kg)

SFRM conn

100

steel rebar metal deck

50

CLT LWC 3 ksi

NWC 3 ksi

0

PT strand NWC 6 ksi

-50

Tally

Athena Impact Esmator

Fig. 5-3.  LCA of baseline and hybrid framing options. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 49

trades, but currently their cost and necessary depth keep cementitious toppings highest in popularity. • In the post-tensioned concrete scheme, it is likely that the GWP of a 6,000 psi mix is under-represented. Prestressing typically requires high early strength such that the cement content, and thus the GWP, is higher than an average mix of the same strength. This study used the industry average 6,000 psi mix to be conservative in the amount of embodied carbon reduction a hybrid system could offer compared to a PT concrete system. • For all schemes, using a very low cement concrete mix would help lower embodied carbon. This study used the national industry average, but if different mix proportions are more common for the project location, they should be used instead. • Although not as notable as the structural materials, the tendency for intumescent paint to be used on the steel beams in a hybrid system instead of cementitious spray fireproofing helps to slightly lower the embodied carbon. In any cradle-to-grave LCA, assumptions are made about the building’s end-of-life. Although alternative end-of-life scenarios were not considered in this LCA, it is worth considering other uses for the components of hybrid systems after their first use in a building. It is feasible for a hybrid system to be built considering the ability to disassemble and reuse elements of the structure at the building’s end-of-life. Utilizing all bolted connections and replacing wet topping with dry panelized toppings can facilitate easy deconstruction of a hybrid building, which may be appropriate for temporary, deployable, or short design-life structures. 5.3

PRODUCT SUSTAINABILITY CERTIFICATIONS

5.3.1 Environmental Product Declarations LCA tools for buildings, particularly Athena IE and Tally, are best for understanding impacts of materials generically, informing design direction, and helping the project team identify “hotspots” to focus their carbon reduction efforts. To evaluate specific products, an environmental product declaration (EPD) can be more helpful. In contrast to an LCA for an entire building or assembly, an EPD is a transparency report that documents the life-cycle impacts of a single product. The life-cycle impacts reported in an EPD include GWP, along with other measurable environmental impacts. EPD are developed using life-cycle assessment and are based on product category rules (PCR) that apply to each category of products. EPD can be either product-specific or industry-wide. Several industry groups, including AISC, AWC, and the Canadian Wood Council (CWC), have industry-wide Type III (third-party verified) EPD available. PCR

typically stipulate that the industry-wide LCA used to produce the EPD statistically represents the entire industry, not just those producers that participated. In this way, even if there are some producers that did not participate in LCA, the industry-wide LCA/EPD still represents the entire industry. Suppliers may also have product specific EPD available. While AWC and CWC do not have an industry-wide EPD for CLT, several CLT suppliers have product-specific EPD. For steel products, several suppliers have mill-specific EPD available. [Supplier-specific CLT and mill-specific steel EPD can be found at www.buildingtransparency.org (Building Transparency, 2021)]. PCR are necessary to use EPD for environmental comparisons across products fulfilling the same function within a single product category. However, there may be disparities between the LCA behind different EPD that make the EPD incomparable, even if the LCA stay within the bounds prescribed by the PCR. PCR tend to expire every 3 to 5 years, and with each update the rules are often revised to make subsequent EPD more comparable. This cycle for renewal is often more frequent than the age of data in WBLCA tools, which could make EPD a better reflection of current practice. Concurrently, once an LCA is completed for an EPD, WBLCA tool developers can use information from the LCA to update the data for those materials in the LCA tools for buildings, if given access to the LCA reports. The relationship between PCR, EPD, LCA, and WBLCA is shown graphically in Figure 5-4. Most notably, EPD are useful for understanding the basic materials and processes involved in producing a product, the energy intensity and fuels used, and additional environmental information. If the EPD are comparable, which often requires that the LCA was conducted by the same professional, under the same PCR and version, and for the same manufacturer(s), then they can also give a sense of the relative differences in environmental impacts between products. Comparing EPD from different years for the same product may also indicate how much manufacturers have improved the environmental profiles of their products over time. 5.3.2 Recycled Content, Recyclability, and Circularity of Steel The embodied carbon of steel is influenced primarily by its recycled content. Steel is typically produced in one of two ways around the world: by smelting and refining iron ore to produce virgin steel or by melting down recycled steel to be reshaped and repurposed. The basic oxygen furnace (BOF) is used to produce the vast majority of the world’s virgin steel, while electric arc furnaces (EAF) are used to melt down primarily recycled steel. Structural steel produced in the United States is comprised of 93% recycled steel scrap, on average (AISC, 2021).

50 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

100% of domestically produced hot-rolled sections are made via EAF, ensuring a high recycled content. Structural plate and HSS may be produced via BOF or EAF methods, which can be confirmed by the mill documentation. Specifying structural steel shapes that originate from EAF mills (e.g., using hot-rolled shapes instead of HSS when their total weight is near equal) is one way to ensure a higher rate of recycled content. In addition to high recycled content, steel is the only structural material with the ability to be recycled back into new steel products with no loss of its physical properties. Nearly 100% of all structural steel is recycled back into new steel products. This unique attribute means steel can be recycled infinitely, exemplifying a “closed-loop” type of system promoted by the circular economy. Furthermore, a hybrid system introduces the potential to reuse the steel beams and columns if the CLT and topping slab can be removed without damaging the steel members, and if steel elements use bolted connections instead of welds. It is technically conceivable to reuse the steel (and CLT), an opportunity that transcends recycling in the circular economy hierarchy. Both recycling and reuse are not only good for reducing embodied carbon; they are strategies that help conserve natural resources, including raw materials, water, and the fuels to produce energy needed to recycle steel. 5.3.3 Sustainable Wood Product Certifications For forest products, sustainability encompasses much more than carbon emissions and sequestration. Although forest lands are a key part of mitigating climate change, forests do more than act as carbon storage. Forests provide essential ecosystem services critical to the health of our planet,

such as clean water, clean air, and protection of biodiversity. When forests become a source of economic productivity, stewardship is needed to ensure the forest is managed sustainably. Three types of instruments help to characterize and link forest management practices at the landscape scale to the wood products used for buildings. Sustainable forest management is the “stewardship and use of forests and forest lands in a way, and at a rate, that maintains their biodiversity, productivity, regeneration capacity, vitality, and their potential to fulfill, now and in the future, relevant ecological, economic, and social functions, at local, national, and global levels.” Certification of sustainable forest management adds credibility through third party verification by trained auditors against specific and publicly disclosed criteria. Criteria often consider protections on watersheds, habitat, biodiversity, soil health, fair labor, indigenous people’s rights, and other issues in addition to all applicable laws and regulations. While several forest management certifications exist (APA, 2021), the Forest Stewardship Council program is the only one recognized by the standard credits in the LEED rating system (USGBC, 2019) and the International Living Future Institute (ILFI) Living Building Challenge (ILFI, 2019) and zero carbon (ILFI, 2020) certifications. While a forest certification by itself is insufficient to quantitatively derive amounts of carbon storage in a given forest, according to ISO 21930 (ISO, 2017), forest certification can be a qualifier for carbon neutrality assumptions. As noted at the beginning of this chapter, the ISO definition of sustainable forest management is essentially that new trees are planted in place of the harvested stands, making it a prerequisite to claim carbon neutrality for the biomass contained

Fig. 5-4.  Relationship among PCR, EPD, LCA, and WBLCA. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 51

in wood products. ISO does not require consideration of broader ecosystem impacts, although if there is no indication that the trees were replanted, then one has to take a penalty for the biogenic emissions from both types of biomass. Until better quantification methods arise, ISO sets the latest guidelines on biogenic carbon, and this principle appears in many other carbon accounting standards (BSI, 2011; ISO, 2018). Figure 5-5 shows the system boundary limits of EPD and LCA tools. These boundary limits necessitate forest management certifications and chains of custody as evidence of broader benefits of sustainable forest management practices. Most wood products have a complex supply chain with multiple participants along the chain. To link sustainable forest management to the final wood product that is sold to a consumer, sustainable wood certification programs require a chain-of-custody component. Chain of custody refers to the entire path of certified products from forests through the supply chain, linking the forest to the end consumer. Chain-ofcustody certifications also require anyone taking ownership of the forest product along the supply chain to be certified in the same system, ensuring that the chain is not broken. Most, but not all, sustainable forest management certification programs also offer chain-of-custody certifications. Lastly, “responsible fiber sourcing” refers to uncertified wood that is not illegally harvested. Smaller, family-owned forests for which certification can become a prohibitive cost often participate in responsible fiber sourcing programs, such as the FSC Controlled Wood certification programs, instead. Under ASTM D7612 (ASTM, 2021b), which distinguishes

three tiers of fiber sourcing, this type of certification is the second tier. While these programs help verify the legitimacy of wood sources, these should not be confused or equated with sustainable forest management certification (the third and highest tier). 5.4

SUSTAINABILITY CONCLUSION

The limited LCA analysis presented in this chapter demonstrates the shortcomings of attempting to simplify the complexity of biogenic carbon into a single calculation methodology. While output from both LCA tools used show that if including biogenic carbon, the hybrid system results in lower overall GWP, the striking difference in output from two of the building industry’s most reputable LCA tools shows that using LCA to compare options may not give as consistent an answer as desired. Between the conventional composite steel and hybrid options, project teams should be careful not to base their decision on LCA results alone, especially without looking at the impact on the entire building, where hybrid systems may help save on foundations, lateral systems, and architectural finishes. The decision to use a hybrid steel-timber system should be driven by all the benefits identified throughout this Design Guide and the value of those opportunities to the project. Moreover, when using wood products, verifiability of sustainable forest management that encompasses ecosystem and societal impacts beyond carbon, and is tied to sourcing of the wood components, is essential for calling the designed system “sustainable.”

Fig. 5-5.  System boundary limits of EPD and LCA tools. 52 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Chapter 6 Structural Design 6.1

TYPICAL FLOOR PLATE DESIGN

6.1.1 Steel Member Design  Steel framing is the primary structural system in a hybrid steel-framed structure with mass-timber floors. Steel columns and beams support mass-timber panels that span between secondary steel beam framing. While the CLT design performance is specified by the structural engineer, to create an efficient layout, the engineer needs to take into consideration that each manufacturer may have slightly different panel lengths and build-ups; engaging a manufacturer early in the design process can be advantageous to optimize spacing and to minimize CLT panel waste. Composite behavior of the timber and CLT panel is generally ignored in strength design, although procedures are available in the U.S. edition of the CLT Handbook and Eurocode 5, Annex B (CEN, 2004), to explicitly design composite CLT and concrete slabs. Steel members are designed in accordance with the AISC Specification for Structural Steel Buildings (AISC, 2016c), hereafter referred to as the AISC Specification, taking into account the bracing provided by the timber panels. Mass-timber panels are directly attached to the steel members with self-tapping screws installed through the bottom of the top flange at a regular spacing. CLT panels have significant in-plane lateral stiffness and strength and are generally treated as providing continuous top-flange lateral bracing against lateral-torsional buckling in beam flexure. For lower levels of axial loads, CLT panels can also provide continuous weak-axis beam buckling bracing. However, engineering judgement should be used where large flexural or axial loads are present, such as transfer girders, trusses, and lateral load-resisting frames. To rely on the CLT panels for stability for these critical conditions, the engineer should use AISC Specification Appendix 6, Member Stability Bracing, to confirm if sufficient bracing by the CLT can be relied upon. Connection slip at timber connections needs to be considered in any stiffness checks. An example of a connection to provide column weak-axis bracing is shown in Figure 6-1.

Fig. 6-1.  Example of column weak-axis bracing, RISD (photo courtesy of Odeh Engineers, Inc.). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 53

The effects of panel continuity should also be considered in the steel framing. The first interior secondary support beam will generally see a higher load than at end-panel conditions, as CLT panels are not typically designed to transfer any moment at the panel spline connection. NLT and DLT panels are considered similarly to CLT panels in steel member design. However, NLT and DLT panels can have significantly less strength and stiffness in the weak direction of the panels and often require small gaps for expansion. Therefore, the restraint provided to steel elements in the weak direction by NLT and DLT panels should be carefully considered. Vibration can often control the design of steel and mass-timber panel floor systems due to the reduced mass. See the vibration discussion in Section 6.4 for an in-depth discussion of this topic. There are also important detailing considerations for the steel framing, including camber requirements, that are discussed in Section 6.6. Design Example 6.1—Noncomposite Hybrid Steel Beam with CLT Panel The typical hybrid solution utilizes noncomposite steel beam design with CLT panels providing lateral restraint. This design example will illustrate the design of the steel beam in this type of system. An example of composite design is shown in Design Example 6.5. Given: Using the example hybrid system described in Section 2.4.2, depicted in Figure 6-2, check the primary beam for strength and serviceability. The trial secondary beam size is an ASTM A992/A992M W27×84. The beam configuration is summarized in the following: Primary beam span = 30 ft Secondary beam span, exterior bays = 40 ft Secondary beam span, middle bay = 20 ft Secondary beam tributary width = 15 ft

Fig. 6-2.  Example noncomposite steel-framed structure with mass-timber floors. 54 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Loads: Dead load (DL) = 37.5 psf (3 in. NWC topping) + 20 psf (6d in. CLT panel) = 57.5 psf Superimposed dead load (SDL) = 20 psf Live load (LL) = 80 psf Self-weight (SW) = 84 lb/ft (for assumed secondary trial beam W27×84) Serviceability limits: l 360 ( 30 ft )(12 in./ft ) = 360 = 1.00 in.

Δ LL =

l 240 ( 30 ft )(12 in./ft ) = 240 = 1.50 in.

Δ TL =

Solution: From AISC Steel Construction Manual (AISC, 2017), hereafter referred to as the AISC Manual, Tables 1-1 and 2-4, the material and geometric properties for the trial W27×84 beam are as follows: Fy = 50 ksi Ix = 2,850 in.4 Zx = 244 in.3 d = 26.7 in. tw = 0.460 in. h/tw = 52.7 The distributed loads on the secondary beams are calculated as follows: wD = ( Tributary Width) ( SDL + DL ) + SW = (15 ft ) ( 20 psf + 57.5 psf ) + 84 lb/ft = 1,250 lb/ft wL = ( Tributary Width ) ( LL ) = (15 ft ) (80 psf ) = 1,200 lb/ft From ASCE/SEI 7-22, Chapter 2 (ASCE, 2022), the following load combinations will govern for gravity design cases: LRFD

ASD wa,s = D + L = wD + wL

wu,s = 1.2D + 1.6L = 1.2wD + 1.6wL = 1.2 (1,250 lb/ft ) + 1.6 (1,200 lb/ft )

= 1,250 lb/ft + 1,200 lb/ft

= 3,420 lb/ft

= 2,450 lb/ft

The loads on the primary beam are a combination of the distributed load from the self-weight of the beam and point loads from the secondary beams (40 ft and 20 ft long). The assumed self-weight of the beam is 84 lb/ft. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 55

LRFD

ASD

wu,p = 1.2D + 1.6L

⎛ 40 ft + 20 ft ⎞ ⎝ ⎠ 2 ( 2,450 lb/ft )( 30 ft ) = 1,000 lb/kip = 73.5 kips

Pa,p = wa,s

= 1.2wD + 1.6wL 1.2 (84 lb/ft ) + 1.6 ( 0 ) 1,000 lb/kip = 0.101 kip/ft =

⎛ 40 ft + 20 ft ⎞ ⎝ ⎠ 2 ( 3,420 lb/ft )( 30 ft ) = 1,000 lb/kip = 103 kips

Pu,p = wu,s

Determine the shear and moment demands using beam equations for a simply supported beam with a uniformly distributed load and a concentrated point load at midspan. LRFD 2

Pu,p l wu,p l + 8 4 (103 kips) ( 30 ft ) ( 0.101 kip/ft ) ( 30 ft )2 = + 4 8 = 784 kip-ft

Mu =

Pu,p + wu,p l 2 103 kips + ( 0.101 kip/ft )( 30 ft ) = 2 = 53.0 kips

Vu =

ASD 2

Pa,p l wa,p l + 8 4 ( 73.5 kips) ( 30 ft ) ( 0.0840 kip/ft )( 30 ft )2 = + 4 8 = 561 kip-ft

Ma =

Pa,p + wa,p l 2 73.5 kips + ( 0.0840 kip/ft ) ( 30 ft ) = 2 = 38.0 kips

Va =

The nominal shear strength, Vn, is determined using AISC Specification Equation G2-1: Vn = 0.6Fy AwCv1 Aw = dt w = ( 26.7 in.) ( 0.460 in.) = 12.3 in.2 Check the h/tw ratio: h = 52.7 tw 2.24

E 29,000 ksi = 2.24 Fy 50 ksi = 54.0

56 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

(Spec. Eq. G2-1)

Because

E h < 2.24 : Fy tw

ϕv = 1.00 Ωv = 1.50 Cv1 = 1.0

(Spec. Eq. G2-2)

Vn = 0.6 ( 50 ksi ) (12.3 in. ) (1.0 ) 2

= 369 kips LRFD

ASD

ϕ vVn = 1.00 ( 369 kips ) = 369 kips > 53.0 kips

o.k.

Vn 369 kips = Ωv 1.50 = 246 kips > 38.0 kips

o.k.

The nominal flexural strength, Mn, is the lower of the values obtained according to the limit states of yielding and lateral-torsional buckling. The beam flange in compression is fully braced by the CLT deck; therefore, lateral-torsional buckling does not apply. The nominal flexural strength of the beam for the limit state of yielding is: Mn = Mp = Fy Z x =

(Spec. Eq. F2-1)

( 50 ksi ) ( 244 in.3 )

12 in./ft = 1,020 kip-ft

 LRFD

ASD

ϕb Mn = 0.90 (1,020 kip-ft ) = 918 kip-ft > 784 kip-ft

o.k.

Mn 1,020 kip-ft = Ωb 1.67 = 611 kip-ft > 561 kip-ft

o.k.

Check deflections, ignoring camber: PD = wD l =

(1,250 lb/ft )( 30 ft )

1,000 lb/kip = 37.5 kips PL = wL l =

(1,200 lb/ft )( 30 ft )

1,000 lb/kip = 36.0 kips Δ LL = =

PL l 3 48EI ( 36.0 kips) ( 30 ft )3 (12 in./ft )3 48 ( 29,000 ksi ) ( 2,850 in.4 )

= 0.423 in. < 1.00 in.

o.k.

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 57

Δ TL =

( PD + PL ) l 3 5wD l 4 48EI

+

384EI ⎛ 0.0840 kip/ft ⎞ ( 30 ft )4 (12 in./ft )4 12 in./ft ⎠ 384 ( 29,000 ksi ) ( 2,850 in.4 )

( 37.5 kips + 36.0 kips)( 30 ft )3 (12 in./ft )3 + 5 ⎝ = 48 ( 29,000 ksi ) ( 2,850 in.4 )

= 0.883 in. < 1.50 in.

o.k.

In practice, designers will likely utilize software packages to design the steel floor framing. It is important to modify the following parameters in the software package to account for hybrid design: • Noncomposite beams assumed (unless explicitly detailed for composite action). • Dead loads account for CLT panel and topping (coordinate with acoustic consultant to ensure sufficient topping assumption, it is recommended to assume minimum 3 in. NWC topping for conceptual design). • Either do not allow for camber or take a maximum camber of w in. if panel fit-up is closely considered (see Section 6.6.2 for an in-depth discussion of this topic). 6.1.2 Timber Panel Design  6.1.2.1 Mass-Timber Panel Types Virtually any mass-timber panel type could be used in steel hybrid construction. However, the most common mass-timber panel types currently used are CLT, NLT, and DLT panels. All mass-timber panels need to meet the strength and serviceability requirements specified in the NDS (AWC, 2018). Additionally, special care needs to be taken for vibration and fire design, which are discussed separately in this Design Guide. CLT Panels Due to the cross-laminations, CLT has significantly higher in-plane strength and stiffness and is the most common panel type used in hybrid construction. The most typical panel sizes used at this time are 3-ply and 5-ply panels, but 7-ply and 9-ply panels are also available if required. CLT panels can be used as diaphragms in most applications, including seismic applications using ANSI/AWC SDPWS, Special Design Provisions for Wind and Seismic (SDPWS) (AWC, 2021b). Steel secondary beams supporting the CLT should be coordinated with the panel layout to minimize panel waste. Panel sizing and layout have a significant impact on piece count and should be coordinated with the secondary framing to minimize waste. Each manufacturer has slightly different dimensions for panels, and this should be considered when selecting panels and designing the structure. In general, however, panel sizes range from 8 ft to 10 ft widths and are 40 ft to 60 ft in length depending on shipping method and transport restrictions. CLT panels are sized for strength and serviceability according to NDS Chapter 10. CLT panels are typically designed as oneway spanning elements in the strong-axis direction of the panel only. While CLT panels do have some capacity in the weak-axis direction of the panel, this behavior is not generally relied upon because moment continuity is not easily provided at panel spline connections. The weak-axis bending capacity of CLT panels is generally only utilized at small two-way cantilever conditions and for rationalizing unreinforced penetrations. One notable exception to this is point supported CLT floors that rely on two-way behavior; however, point supported floors are not considered in this Design Guide. The rolling shear of the cross-laminations in the CLT panels plays a significant role in out-of-plane bending, shear, and deformation behavior. ANSI/APA PRG 320 (APA, 2019) specifies the minimum effective design properties that consider the effects of rolling shear. Manufacturers test their panels to meet the minimum properties outlined in ANSI/APA PRG 320, which are in turn applied to the NDS Chapter 10 provisions. As with all-timber construction, care must also be taken to account for creep, shrinkage, wet-service, and high-temperature conditions. Reference the U.S. CLT Handbook for a detailed discussion of sizing CLT panels according to the NDS.

58 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

NLT and DLT Panels NLT and DLT panels are also sized for strength and serviceability according to the NDS. NLT and DLT panels are composed of dimensional lumber attached side-by-side in their strong axis and designed to span in only one direction. NLT and DLT panels are not generally considered to have diaphragm continuity on their own; additional plywood or other structural topping is used for the diaphragm. The bracing provided to steel members with NLT and DLT panels has to be carefully considered for any significant spans and loads. Reference the Nail-Laminated Timber U.S. Design and Construction Guide (BSLC, 2017) or a detailed discussion of sizing NLT panels according to the NDS. 6.1.2.2 Material Grades and Species Allowable grades of CLT panels are specified in ANSI/APA PRG 320. Each manufacturer specifies the species and grades used for their panels. The most common wood species currently used are SPF, DF, or southern yellow pine (SYP). For CLT panels, many manufacturers have an option to use a higher architectural grade on the bottom exposed lamination layer. This exposed lamination layer could have a different species than the other laminations. Most panel manufacturers are able to provide samples of various grades of finish to assist the architect in the selection of the finish grade and species. Design Example 6.2—Timber Panel Design Using the example hybrid system described in Section 2.4.2, this design example will check the CLT panel for strength and serviceability. This design example follows the design provisions outlined in NDS Chapter 10. Using the nomenclature from ANSI/APA PRG 320, the CLT is loaded in the flatwise direction. The panel is designed to span one way, therefore the design example is checking capacity in the major-axis or major-strength direction as shown in Figure 6-3. Given: The 5-ply CLT panel is 30 ft long supported at each end and the midpoint by a steel frame. The properties of the CLT panel are as follows: Grade (ANSI/APA PRG 320) = E1 Lamination thickness = 1a in. Number of plies =5 Total thickness = 6d in. Specific gravity, ρ = 0.35 From ANSI/APA PRG 320, Table A2—Major Strength Direction CLT Panel Properties: (FbS)eff,f,0 = 10,400 lb-ft/ft of width (EI)eff,f,0 = 440 × 106 lb-in.2/ft of width (GA)eff,f,0 = 0.92 × 106 lb/ft of width Vs,0 = 1,970 lb/ft of width

Fig. 6-3.  5-ply panel in flatwise, major-strength direction (APA, 2019). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 59

Note: In practice, the designer should review potential CLT suppliers appropriate to the project location and review available panel types and properties. Panel grades, lamination thicknesses, widths, and lengths vary by manufacturer. For uniform loading, pinned ends: Ks = 11.5

(NDS Table 10.4.1.1)

Loading: LL = 80 psf SDL = 20 psf DL = 40 psf (topping slab and finish) SW = 20 psf Solution: Convert the area load to a line load per foot width of CLT panel: LL = 80 psf (1 ft) = 80.0 lb/ft SDL = 20 psf (1 ft) = 20.0 lb/ft DL = 40 psf (1 ft) = 40.0 lb/ft SW = 20 psf (1 ft) = 20.0 lb/ft Determine the distributed line load per foot width of the CLT panel: LRFD

ASD

wu = 1.2D + 1.6L

wa = D + L

= 1.2 ( 20.0 lb/ft + 40.0 lb/ft + 20.0 lb/ft ) + 1.6 (80.0 lb/ft )

= ( 20.0 lb/ft + 40.0 lb/ft + 20.0 lb/ft ) + (80.0 lb/ft )

= 224 lb/ft

= 160 lb/ft

Assume a simply supported, continuous two-span condition. Using the beam equations in AISC Manual Table 3-22c, determine the critical moment and shear for the CLT panel per foot width. Midspan moment = 0.07wl2 Middle support moment = 0.125wl2 Shear, end bay = awl Shear, center bay = swl LRFD Midspan moment

Middle support moment

Shear, end bay

Shear, center bay

= 0.07wl

ASD 2

Midspan moment

= 0.07wl 2

= 0.07 ( 224 lb/ft ) (15 ft )2

= 0.07 (160 lb/ft ) (15 ft )2

= 3,530 lb-ft/ft

= 2,520 lb-ft/ft

= 0.125wl

2

Middle support moment

= 0.125wl 2

= 0.125 ( 224 lb/ft ) (15 ft )2

= 0.125 (160 lb/ft ) (15 ft )2

= 6,300 lb-ft/ft

= 4,500 lb-ft/ft

= awl

Shear, end bay

= awl

= a ( 224 lb/ft ) (15 ft )

= a (160 lb/ft ) (15 ft )

= 1,260 lb/ft

= 900 lb/ft

= swl

Shear, center bay

= swl

= s ( 224 lb/ft ) (15 ft )

= s (160 lb/ft ) (15 ft )

= 2,100 lb/ft

= 1,500 lb/ft

60 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Therefore, the controlling required flexural strength is 6,300 lb-ft/ft for LRFD and 4,500 lb-ft/ft for ASD. The critical required shear strength is 2,100 lb/ft for LRFD and 1,500 lb/ft for ASD. The CLT design capacity can be calculated using the 2015 version of NDS or later (CLT was introduced into NDS in the 2015 revision of the standard). Panel properties can be obtained from ANSI/APA PRG 320, from individual panel manufacturers, or can be calculated explicitly for custom layups using the shear analogy method described in CLT Handbook Section 3.3. For this example, the ASD reference design capacities are used from ANSI/APA PRG 320, Table A2. The ASD reference design values are modified by the adjustment factors in NDS Chapter 10. NDS Table 10.3.1 is reproduced in Figure 6-4. Note: Based on NDS Equation 3.4-1, NDS uses Fs(Ib/ Q)eff to indicate flatwise panel shear. Panel manufacturers often use Vs to indicate shear capacities. The terms planar shear, out-of-plane shear, or rolling shear are also used to describe flatwise panel shear. Determine the adjustment factors: CD = 1.0 (for occupancies containing live load, see NDS Table 2.3.2) CL = 1.0 (depth of bending member does not exceed its breadth, see NDS Section 3.3.3) CM = 1.0 (moisture content less than 16%, as in most covered structures, see NDS Section 10.3.3) Ct = 1.0 (structural members will not experience sustained exposure to elevated temperatures up to 100°F; see NDS Table 2.3.3) Factors specific to LRFD: Format conversion factor, KF = 2.54 for bending, 2.00 for shear in CLT, from Figure 6-4 Resistance factor, ϕ = 0.85 for bending, 0.75 for shear Time effect factor, λ = 0.8 for live load combination when L is based on occupancy (NDS Table N3)

Fig. 6-4.  NDS Table 10.3.1 (courtesy of the American Wood Council, Leesburg, Va.) (AWC, 2018). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 61

Calculate the bending capacity of the CLT panel, based on NDS Section 10.3, as follows: LRFD

ASD

( Fb S )′eff, f 0 = ( Fb S )eff , f 0 C M CtC L K F ϕλ = (10,400 lb-ft/ft ) (1.0 ) (1.0 ) (1.0 ) ( 2.54 ) ( 0.85) ( 0.8 ) = 18,000 lb-ft/ft > 6,300 lb-ft/ft

(Fb S )′eff, f 0 = ( Fb S )eff, f 0 CDC M CtC L

o.k.

= (10,400 lb-ft/ft ) (1.0 ) (1.0 ) (1.0 ) (1.0 ) = 10,400 lb-ft/ft > 4,500 lb-ft/ft

o.k.

Calculate the shear capacity of the panel: LRFD

ASD

Vs,0 ′ = Vs,0C M Ct K F ϕ

Vs,0 ′ = Vs,0CM Ct

= (1,970 lb/ft ) (1.0 ) (1.0 ) ( 2.0 ) ( 0.75)

= (1,970 lb/ft ) (1.0 ) (1.0 )

= 2,960 lb-ft/ft > 2,100 lb/ft

= 1,970 lb-ft/ft > 1,500 lb/ft

o.k.

o.k.

Calculate the deflection of the CLT panel with the following assumptions: Kcr = 2.0 for wood panels in dry service condition (MC < 16%) ΔLT = immediate deflection due to long-term component of design load (e.g., Δmax,dead) ΔST = immediate deflection due to short-term component of design load (e.g., Δmax,live) For the simply supported, two-span beam condition with uniformly distributed load on both spans, the maximum deflection is: Δ max =

wD l 4 185EI

The deflection is calculated using the applicable adjustment factors, CM and Ct, and an adjusted value of EI as follows: Δ max =

wD l 4

185 ( EI )app, f,0 C M Ct

The reduced stiffness property, (EI)app,f,0, adjusts (EI)eff,f,0 for shear deformation. The shear deformation adjustment factors are provided in NDS. From the CLT Handbook, (EI)app,f,0 is calculated using Equation 5:

( EI )app, f,0 = 1+ = 1+

( EI )eff, f,0 K s ( EI )eff , f ,0

(CLT Handbook Chapter 3, Eq. 5)

GAeff L2 440 × 106 lb-ft/ft (11.5)( 440 × 106 lb-ft/ft )

(0.92 × 106 lb/ft ) (180 in.)2

= 376 × 106 lb-in.2 /ft



And then the maximum dead and live load deflections can be calculated as: Δ max, dead =

(80.0 lb/ft )(15 ft )4 (12 in./ft )3 185 (376 × 106 lb-in.2 ) (1.0 ) (1.0 )

= 0.101 in.

62 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Δ max,live =

(80.0 lb/ft )(15 ft )4 (12 in./ft )3 185( 376 × 106 lb-in.2 ) (1.0) (1.0 )

= 0.101 in. From NDS Section 3.5: Δ max,longterm = K cr ( Δ max,dead ) + Δ max,live = 2.0 ( 0.101 in.) + 0.101 in. = 0.303 in. = L 594 deflection

o.k.

Note: This design example does not consider patterned live loading, which would need to be checked as applicable. Calculate the maximum span for vibration for the 5-ply panel following the procedure outlined in the CLT Handbook:

( EI )0.293 app, f,0 l≤ 0.122 12.05 (ρA ) 1

(CLT Handbook Chapter 7, Eq. 4) 0.293

( 376 × 106 lb-in.2 ) ≤ 0.122 12.05 [( 0.35) ( 6 d in.) (12 in.)] 1

≤ 17.9 ft

o.k.



6.1.2.3 Fire Design of CLT Panel If the building structure is required to be fire rated, the timber panels can be designed for fire conditions. NDS Chapter 16 outlines how to design exposed timber members for fire conditions. As noted in Chapter 3, mass timber provides fire resistance through the charring of the outer layer of timber material. This reduction in section capacity needs to be checked against appropriate load combinations for the fire load case. CLT (or equivalent alternative mass-timber floor) panel thicknesses generally need to be increased in thickness to accommodate the panel section loss due to charring. First the effective char depth must be considered, which is a function of the char rate and required fire endurance. NDS Section 16.2.1.3 is used to determine the char depth and account for the laminations in CLT panels. NDS addresses reductions of strength and stiffness of wood directly adjacent to the char layer by accelerating the char rate by 20% (WoodWorks, 2019a). CLT panels primarily rely on the outer laminations for flexural strength in the panel strong axis. Once the outer lamination is lost due to charring in fire conditions, the flexural strength of the panel is significantly reduced because only the laminations oriented in the direction of the span being considered are being used. Lower-grade timber material may also be used in the inner laminations, but this variation in strength needs to be considered in the design. The load combination used under fire conditions is generally taken as the service dead plus live load. ASD is used for fire design to NDS provisions. Because a fire condition is treated as a very rare and short-time duration event, there is a conversion for allowable design stress to average ultimate strength adjustment factor summarized in NDS Table 16.2.2, shown in Figure 6-5. Alternatively, ASCE/SEI 7-22, Section 2.5, provides a load combination for extreme events in LRFD design. AWC TR-10 explains the fire design of timber elements in great detail. The char depth can be calculated using the NDS Chapter 16 method. The effective residual cross section of the CLT floor is calculated from: hfire = h − achar where h = initial cross-section depth, in. hfire = effective cross-section depth, in.

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 63

From NDS Equations 16.2-4 and 16.2-3, the char depth, achar, is calculated as: achar = 1.2 ⎡n lam hlam + β η ( t − nlamt gi ) ⎣

0.813

⎤ ⎦

(from NDS Eq. 16.2-4)

where hlam = lamination thickness, in. nlam = number of laminations charred (rounded to lowest integer) t = exposure time, hr tgi = time for char front to reach glued interface, hr βη = nominal char rate = 1.5 in./hr From NDS Section 16.2.1: ⎛h ⎞ tgi = ⎜ lam ⎟ ⎝ βη ⎠ n lam =

1.23

t tgi

Alternatively, effective char depths for common fire-resistance ratings can be taken directly from NDS Table 16.2.1B. Design Example 6.3—Fire Resistance Rating of 5-Ply CLT Panel For a 5-ply CLT element with a total thickness of 6.66 in. with 1-hour fire exposure, calculate the effective panel thickness. Given: h = 6.66 in. hlam = 1.33 in. nlam = 1 lamination t = 1 hr βη = 1.5 in./hr

Fig. 6-5.  NDS Table 16.2.2 Adjustment Factors for Fire Design (courtesy of the American Wood Council, Leesburg, Va.) (AWC, 2018). 64 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Solution: 1.23 ⎛ 1.33 in. ⎞ ⎝1.5 in./hr⎠ = 0.862 hr

t gi =

achar = 1.2 ⎡n lam hlam + β η ( t − nlam tgi ) ⎣

0.813 ⎤

{

(from NDS Eq. 16.2-4)

⎦

= 1.2 (1) (1.33 in.) + (1.5 in./hr )[1 hr − (1) ( 0.862 hr )] = 1.96 in.

0.813

} 

h fire = h − achar = 6.66 in. − 1.96 in. = 4.70 in. Thus, the CLT will have 4.70 in. of wood remaining after 1 hour of fire exposure, equivalent to a 1-hour fire resistance test to ASTM E119. The panel design exercise from Section 6.1.2 can be repeated utilizing the calculated reduced panel depth and the fire design adjustment factors from NDS Table 16.2.2, shown in Figure 6-5. Designers will frequently run a simple but conservative check neglecting the partial ply remaining—especially in situations where the partial ply is running in the weak direction. To account for the partial ply, designers are typically forced to calculate section properties by hand because these properties are not frequently provided in tables from manufacturers or the industry at large. 6.2

LATERAL SYSTEM DESIGN

6.2.1 Steel Lateral Force-Resisting Systems A key advantage of the steel framing in a hybrid system is the flexibility of the steel LFRS to minimize the impacts of the lateral system in the building architecture. A steel LFRS also provides a proven path to code compliance. Braced frames and moment frames are the most commonly used LFRS; however, any steel LFRS allowed by the AHJ can be used in a hybrid project. In high-seismic areas, more ductile LFRS are used in accordance with the AISC Seismic Provisions for Structural Steel Buildings (AISC, 2016b). CLT panels can help reduce the mass of the building, which in turn reduces the seismic forces. Consideration for bolted connections should be given special care where plates and bolts can impact the bearing of the CLT panel. Where possible, conditions for bolts on the top flange should be minimized. If they cannot be avoided, the final connection design needs to be coordinated with mass-timber panels to rout out the underside of the panel and ensure firm bearing. See Section 6.6 for additional detailing considerations. 6.2.2 Diaphragms Diaphragm design in steel and mass-timber construction must be considered appropriately to ensure global stability under lateral loads. The engineering mechanics of diaphragm behavior in hybrid buildings is similar to metal decking or metal deck with composite concrete diaphragms. However, the designer needs to consider the behavior of the mass-timber panels under in-plane loading and ensure the load path between the timber and the steel is addressed appropriately. 6.2.2.1 CLT Diaphragm Design Approaches One of the inherent benefits of CLT panel construction is that the mass-timber panels have significant in-plane strength to resist lateral loads. However, until recently, there has not been a clear path to code compliance using CLT panels as diaphragms in the IBC or NDS. A new codified approach for using CLT panels as diaphragms is now outlined in the ANSI/AWC Special Design Provision for Wind and Seismic (SDPWS). Historically, in high-wind and high-seismic regions, the approach to diaphragm design utilized supplemental diaphragm systems that were already codified for a simpler permit approval process, such as plywood sheathing or unbonded concrete toppings. For special loading conditions, horizontal steel in-plane bracing can be utilized.

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 65

CLT Diaphragm Design per 2021 SDPWS The main benefit to using CLT panels as the diaphragm is that it can eliminate any additional structural sheathing or topping. SDPWS Section 4.5 allows a fully codified approach to using CLT panels as the diaphragm system. There is a design guide being published by WoodWorks for working though these provisions; therefore, the methodology will not be covered in full detail here. As mentioned earlier, CLT panels by themselves have significant in-plane shear capacity and generally do not govern the shear capacity of a CLT diaphragm. Rather, it is the connection between CLT panels that generally controls the shear capacity of CLT diaphragms. CLT panels are typically connected together with thin plywood splines that are then nailed or screwed to the CLT panels. Half-lap connections are another type of CLT-to-CLT panel connection. The spacing and size of the plywood and connectors then determines the shear capacity of the diaphragm. This is very similar to plywood sheathing diaphragms. Figure 6-6 shows a CLT-only diaphragm with panel splines at a roof condition. Similar to plywood sheathing, SDPWS Table 4.2.2 limits the aspect ratio of CLT (double-layer diagonally sheathed lumber) diaphragms to 4:1. The aspect ratio will have a significant impact on whether the diaphragm behaves as flexible, rigid, or semi-rigid. In determining the flexibility of the CLT diaphragm, the SDPWS does not provide equations for determining the flexibility, but rather notes that the engineer should use the principles of engineering mechanics. There are several papers available that outline approaches for determining the diaphragm flexibility including the following: • 2015 white paper on CLT Diaphragms (Spickler, 2015) • WoodWorks solutions paper on CLT modeling (WoodWorks, 2017) • The forthcoming WoodWorks CLT diaphragm design guide Because determining the diaphragm stiffness for a semi-rigid analysis can be an intensive exercise, a common approach by the industry is to envelope the behavior of an idealized flexible or rigid diaphragm design. Once the diaphragm forces have been determined, the spline connections can then be appropriately designed. The SDPWS outlines the requirements for dowel-type fasteners in determining the connection capacity. In the NDS, dowel-type fasteners are governed by four types of failure behaviors between the fastener and the connection materials, classified as Modes I through

Fig. 6-6.  CLT diaphragm with plywood spline connections and steel strapping for chord/collector across CLT panel joints (photo courtesy of Arup). 66 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

IV. Modes I and II are considered to be less ductile modes because they are controlled by crushing of the connection material. Modes III and IV are considered to be more ductile modes because they are governed by yielding in the fastener. The key goal for the connection design in the diaphragm is to ensure ductile connection behavior by having the fasteners controlled by Mode III or IV failures. Connections to steel, wood elements, or chord splines are also amplified by a factor up to 2.0. Where jurisdictions allow, a common approach is to delegate the connection design to the CLT manufacturer by providing the design shear forces at the spline connections. Typical economic shear panel splices have a shear capacity of 500 lb/ft to 1,500 lb/ft (LRFD). For higher forces, steel splines may need to be used, which can increase cost. Figure 6-7 shows an example plan where shear design forces are called out for the CLT manufacturer to design. Where forces are relatively low, it is common to design the CLT panel to resist the chord and collector axial forces and simplify the steel design. For this approach, the engineer first needs to determine the chord and collector axial forces utilizing an effective panel width based on the capacities of the CLT panel. The out-of-plane bending of the panel also needs to be considered with the in-plane axial loads. Once the chord and collectors within the panels have been identified, the connections between the panels are sized for this load transfer in tension (the compression forces are generally transferred via panel bearing). SDWPS 2021 notes that connections used to transfer diaphragm shear forces cannot be used to resist diaphragm tension forces. Therefore, either the spline connections need to be locally sized for the tension forces with appropriate amplification, or steel strapping is placed on top of the CLT panels to transfer the forces across the panel joint. Chord and collector steel strapping can use steel plates with self-tapping screws where forces are high, or light-gauge strapping where forces are lower. Similar to shear design forces, these connections can also be delegated to the CLT manufacturer if they are clearly documented in the contract documents and if allowed by the AHJ. The transfer of the collector forces in the CLT diaphragm to the steel LFRS is a critical connection that has to be carefully considered. Where collector forces are low enough, the collector force transfer can occur locally at the braced frame or moment frame only via self-tapping screws at a tighter spacing pattern locally. Figure 6-8 shows a steel braced frame with close spacing of screws to transfer the shear force locally between the CLT diagram and the steel braced frame. The CLT diaphragm is then designed to distribute the collector forces. Where chord and collector forces are high, the steel framing can be used as collectors to transfer and distribute the lateral forces over a longer distance into the diaphragm. The steel collector framing needs to consider the additional axial loads in the member design, including any second-order effects as a beam-column. The connections also need to be designed for the additional axial forces. Large collector forces can be accommodated efficiently in steel design; this is one of the strengths of a hybrid steel and CLT structural system. If steel collector elements are desired to be hidden, steel drag struts can also be provided atop the CLT decking as shown in Figure 6-9. This approach is easiest to implement at roof levels where the insulation can typically hide the collector element. For jurisdictions that have not adopted the 2021 version of the SDPWS, one of the alternative diaphragm approaches may be required unless approval for its use is given by the AHJ. Additionally, there is currently a code limitation on cantilevered timber diaphragms of 35 ft that is currently being challenged by designers to be modified in future codes. If a concrete topping slab is

Fig. 6-7.  Delegated shear connection diagram for Houston Endowment Headquarters, including strapping above CLT for chords and collectors (image credit: Arup). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 67

used as the diaphragm, then this limitation does not apply. If mass timber is used (e.g., sheathed NLT/DLT or unsheathed CLT), then discussions with the building department are recommended, or horizontal steel in-plane bracing could be used. Unbonded Concrete Topping Slabs One approach to mass-timber diaphragm design is to use an unbonded concrete topping and ignore the CLT panels. This approach can be efficient where an unbonded concrete topping is already required as part of the acoustic design. Typically, this concrete topping is poured atop an acoustically resilient mat. This acoustics mat results in the concrete being unbonded from the CLT panel. Concrete diaphragms also have significant capacity, which is a benefit where large lateral loads are present, and are inherently fire resistant.

Fig. 6-8.  Collector nail attachment of CLT to top flange of steel braced frame at Mansfield Airport (photo courtesy of Arup and Michael Shearer). 68 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

The minimum concrete topping thickness is recommended to be no thinner than 3 in. per ACI 302.1R-04 (ACI, 2004). However, special care should be taken with thin topping slabs to ensure there is sufficient clear cover for reinforcement, especially if significant chord reinforcement is required. Care should be taken in the concrete mix for thin unbonded concrete pours to minimize the effects of curling. Any floor boxes in the topping slab should also be considered by the engineer when determining the topping thickness, as floor boxes or AV boxes can have significant depth. The thickness of the topping slab can also affect flexible versus rigid diaphragm behavior. All these design aspects should be considered when determining the appropriate concrete topping thickness for a project.

Fig. 6-9.  CLT collector (drag strut) element above CLT deck collecting forces to steel braced frame (photo courtesy of Arup). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 69

Once the concrete topping thickness has been selected, the design and behavior of the diaphragm should be designed per ACI 318 (ACI, 2019) code requirements. There are many design guides on the design and behavior of concrete diaphragms so it will not be covered in this Design Guide. However, the load path of the concrete diaphragm to the steel LFRS needs to be carefully detailed, as well as the transferring of the mass-timber inertial mass into the concrete diaphragm. The most direct load path of the diaphragm to the steel braced frames or moment frames is through bearing of the concrete topping slab against the steel columns. Steel brackets or deformed bar anchors at the columns and concrete slab interface could be used for force transfer into the steel. In high-seismic regions, appropriate overstrength values should be used for these interfaces. Approaches could also use steel headed stud anchors or other mechanical connections between the steel beams and concrete topping; however, there needs to be consideration for impact to the acoustical performance for this approach. These steel headed stud anchors would also need to be detailed such that they engage the topping and disrupt the CLT panel continuity. In wind-controlled regions, if the façade is attached directly to the mass timber, careful detailing is also required to transfer the lateral loads back into the concrete diaphragm. Plywood Sheathing The 2021 SDPWS provides a clear path for code compliance for CLT diaphragms in high-seismic zones. For jurisdictions that have adopted this version of the SDPWS, the inherent strength of the CLT diaphragm will be more cost-effective than adding an additional redundant plywood sheathing layer. However, historically in high-seismic and wind regions, there has not been a clear path to code compliance; one solution has been to add plywood sheathing directly on top of CLT panels. This approach is conservative because it ignores any CLT diaphragm capacity and is not recommended for new designs where the CLT diaphragm can be used on its own with a clear path to code compliance through the 2021 SDPWS. Because not all jurisdictions may have adopted this version of the SDPWS, this section is included for historical purposes and to provide an alternative to unbonded concrete diaphragms where the clear path for code compliance with CLT diaphragms may still be difficult. Plywood sheathing diaphragms are designed in accordance with the SDPWS and are governed by the fastener connection capacity. As the plywood sheathing is fastened directly to the CLT panels, the diaphragm is treated as blocked, similar to applications where plywood is attached to timber decking. There are many design guides for plywood diaphragms; they will not be discussed in full detail here. However, one of the main limitations of the SDPWS are the aspect ratios of the diaphragm, which is limited to 4:1 for double-layer diagonally sheathed lumber in SDPWS Table 4.2.2. The engineer needs to determine the classification of the diaphragm as flexible, rigid, or semi-rigid by calculating the deformation of the diaphragm relative to the deformation of the lateral system per ASCE/SEI 7. The diaphragm deflection is calculated using SDPWS Section 4.2.3, which accounts for the contribution of both the sheathing deformation as well as the slip from the connections. Once the diaphragm is appropriately classified, the appropriate provisions of IBC and ASCE/SEI 7 for a flexible, rigid, or semi-rigid diaphragm are then applied. Chords and collector forces in plywood diaphragms can be transmitted either through the steel elements or locally with additional strapping. Where collector or chord forces are high, using the steel beams as the lateral load path can be an efficient approach to take advantage of the inherent strength of steel members and connections. In this approach, the steel beams need to be designed for the additional axial loads as beam-columns, including consideration for any second-order effects. Likewise, the steel connections need to accommodate the additional axial loads. Where chord and collector forces are low, using the steel elements for load transfer can result in steel connections being increased for the axial loads compared to designing for gravity loads only. One approach to minimize this increase of steel elements and connections is to design portions of the CLT panel for the chord or collector axial forces. Additional strapping across panel joints could be required to transfer concentrated tension forces across panel joints. Horizontal Steel In-Plane Bracing Steel in-plane bracing below the CLT can also be used as a diaphragm. This approach is generally used sparingly to minimize steel tonnage; however, it can be advantageous at local conditions where in-plane forces are significant or where greater design flexibility is needed.

70 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

In-plane steel bracing typically ignores any benefit in the CLT diaphragm to provide a direct and more easily detailed load path for lateral forces. Horizontal steel bracing is typically only used for portions of the diaphragm with an aspect ratio higher than 4:1, at lateral load transfers, or at other nontypical load transfer configurations that have a more clearly defined load path through steel framing. An example of this use is shown in Figure 6-10 at the Houston Endowment Headquarters at a horizonal in-plane discontinuity in the vertical LFRS, transferring forces from a moment frame in the north of the building to a moment frame in the south of the building. The diagram shows the in-plane load path where shear force is transferred to a drag strut in the steel framing via horizontal in-plane WT framing. Due to required openings in the diaphragm and the high magnitude of lateral force, an efficient in-plane transfer would not have been possible without the in-plane steel framing. The flexibility of using steel for these types of conditions is another benefit of hybrid steel and mass-timber panel projects. NLT, DLT, and GLT Diaphragm Discussion While this Design Guide is focused on CLT and steel hybrid structures, many of the principles are the same when using NLT/ DLT/GLT and steel structures. A key difference for NLT/DLT/GLT panels is that the in-plane shear strength and stiffness of the NLT/DLT/GLT panels is generally ignored. An unbonded concrete topping or plywood sheathing is often used as the diaphragm. A more detailed discussion of NLT diaphragms can be found in the Nail Laminated Timber U.S. Design and Construction Guide (BSLC, 2017). Design Example 6.4—CLT Diaphragm Design This design example follows the provisions of SDPWS Section 4.5. The design example uses a rectangular floor plate with lateral force-resisting frames at the perimeter of the building as shown in Figure 6-11. Check the adequacy of the 6d in., 5-ply CLT diaphragm (SPF), and determine the required connections to transfer the forces through the diaphragm into the lateral force-resisting frames at the perimeter of the building. Given: CLT panel properties: 6d in., 5-ply, Grade E1 CLT panel width = 8 ft Diaphragm width, L = 240 ft (dimension perpendicular to applied load) Diaphragm depth, W = 100 ft (dimension parallel to applied load) E = 1,700,000 psi W27×84 collectors Panel-to-panel connection type: 1-in.-thick × 6-in.-wide plywood spline with 16d common nails Loading: wEQ = 500 lb/ft (earthquake loads govern for this design example) Solution: Diaphragm Aspect Ratio SDPWS Table 4.2.2 limits the diaphragm aspect ratio for CLT diaphragms (blocked structural panel) to L / W ≤ 4. L 240 ft = W 100 ft = 2.40 < 4

o.k.

Calculate the required shear forces in the diaphragm at the support: VEQ =

( 500 lb/ft )( 240 ft )

2 = 60,000 lbs

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 71

Fig. 6-10.  In-plane lateral force transfer via steel in-plane brace and steel elements at Houston Endowment Headquarters (image credit: Arup).

Fig. 6-11. Example diaphragm.

72 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Calculate the required shear force per foot width of the diaphragm: 60,000 lbs 100 ft = 600 lb/ft

vEQ =

Determine the required shear force per foot width, factoring the seismic loads as specified in ASCE/SEI 7, Sections 2.3.6 (LRFD) and 2.4.5 (ASD): LRFD

ASD

vu,EQ = (1.0 ) ( 600 lb/ft )

va,EQ = ( 0.7 ) ( 600 lb/ft )

= 600 lb/ft

= 420 lb/ft

According to SDPWS Section 4.5, the nominal unit shear capacity, vn, of CLT diaphragms is based on the nominal shear capacity for dowel-type fastener connections used to transfer diaphragm shear forces. The following additional requirements are valid for CLT diaphragm design from SDPWS Section 4.5.4: 1. The nominal shear capacity for dowel-type fastener connections used to transfer diaphragm shear forces between CLT panels and between CLT panels and diaphragm boundary elements (chords and collectors) shall be taken as 4.5Z *, where Z * is Z multiplied by all applicable NDS adjustment factors except CD, KF, ϕ, and λ; Z shall be controlled by Mode IIIs or Mode IV fastener yielding in accordance with NDS Section 12.3.1. 2. Connections used to transfer diaphragm shear forces shall not be used to resist diaphragm tension forces. 3. Wood elements, steel parts, and wood or steel chord splice connections shall be designed for 2.0 times the diaphragm forces associated with the shear forces induced from the design loads. Exceptions outlined in SDPWS Section 4.5.4 apply. From SDPWS Section 4.5.4, the nominal capacity of a CLT diaphragm shear connection fastener can be summarized as: Zn = 4.5Z * The adjusted design capacity, Z*, is determined using NDS Table 11.3.1 with the following exceptions: • CD = 1.0, KF = 1.0, ϕ = 1.0, λ = 1.0 (from SDPWS Section 4.5.4) • Group action factor, Cg = 1.0 from NDS Section 11.3.6.1 for dowel-type fasteners with D < 4 in. • Diaphragm factor, Cdi = 1.1 for CLT diaphragm shear connections (from NDS Section 12.5.3) This example assumes interior conditions; therefore, all other factors are equal to 1.0. From NDS Table L4 for 16d common nails: D = diameter = 0.162 in. H = head diameter = 0.344 in. L = length = 32 in. From NDS Table 12.3.3A for SPF CLT panel with E = 1,700,000 psi: G = 0.42 From NDS Table 12.3.3B for plywood, “other grades”: G = 0.42 From NDS Table 12R: Z = 109 lb Calculating Z* using Cdi = 1.1: Z* = 1.1(109 lb) = 120 lb

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 73

From NDS Section 12.1.6.4, the minimum main member penetration is 6D: 6D = 6 ( 0.162 in.) = 0.972 in. < 32 in. − 1 in. = 22 in.

o.k.

NDS Table 12R assumes 10D penetration: 10D = 10 ( 0.162 in.) = 1.62 in. < 22 in.

o.k.

As stated previously, from SDPWS Section 4.5.4: Z n = 4.5Z* = 4.5 (120 lb ) = 540 lb The unit shear capacity is calculated by dividing the nominal shear capacity by the fastener spacing: vn =

Zn S

SDPWS Section 4.1.4 states that for the seismic design of diaphragms, the ASD allowable shear capacity is determined by dividing the nominal shear capacity by the ASD reduction factor (RF) of 2.8, and the LRFD factored shear resistance is determined by multiplying the nominal shear capacity by a resistance factor, ϕD, of 0.50. For wind design, the factors are 2.0 and 0.80, respectively. Calculate the required nail spacing: LRFD vu,EQ ≤ ϕ D S

⎛ Zn ⎞ ⎝S⎠

⎛ Z ⎞ ≤ ϕD ⎜ n ⎟ ⎝ vu,EQ ⎠ ≤ 0.50

ASD Zn S ( RF ) Zn ≤ va,EQ ( RF )

va,EQ ≤ S

⎛ 540 lb ⎞ (12 in./ft ) ⎝ 600 lb/ft ⎠

≤ 5.40 in.

⎡ ⎤ 540 lb ≤⎢ ⎥ (12 in./ft ) ⎣ ( 420 lb/ft ) ( 2.8 ) ⎦ ≤ 5.51 in.

Therefore, use 4 in. nail spacing. From NDS Commentary Table C12.1.6.6, the minimum nail spacing is 15D: 15D = 2.43 in. < 4 in.

o.k.

Similar procedures are followed for the connection of the panel to the perimeter W27×84 that acts as a collector to the lateral force-resisting frame. For this connection, 6-in.-long #12 wood screws are used. From NDS Table L3 for #12 wood screws: D = 0.216 in. Dr = root diameter = 0.171 in. L = 6 in. As previously determined, for a SPF CLT panel: G = 0.42

74 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

From NDS Table 12M: Z = 172 lb Note: W27×84 flange thickness is greater than the maximum value in Table 12M. This example has conservatively taken the value for the maximum flange thickness available. Z n = 4.5Z * = 4.5 (172 lb) = 774 lb LRFD vu,EQ ≤ ϕ D S

ASD

⎛ Zn ⎞ ⎝ S ⎠

Zn S ( RF ) Zn ≤ va,EQ ( RF )

va,EQ ≤

⎛ Z ⎞ ≤ ϕD ⎜ n ⎟ ⎝ vu,EQ ⎠

S

774 lb ⎤ ⎡ ≤⎢ ⎥ (12 in./ft ) 420 lb/ft 2.8 ( ) ( ) ⎣ ⎦ ≤ 7.90 in.

⎛ 774 lb ⎞ (12 in./ft ) ⎝ 600 lb/ft ⎠ ≤ 7.74 in.

≤ 0.50

Therefore, use 7 in. spacing. Chords are used to resist the bending moment in the diaphragm. In hybrid structures, the steel framing may be used as the chord where beams are placed in appropriate locations and where connections from the CLT panel to the beam are sufficient to transfer chord forces. Alternatively, a strip of the CLT panel may be used as the chord. Combined bending and axial loading must be considered using the appropriate load combinations combining dead, live, and lateral loading. The chords, collectors, and panels are designed using an overstrength approach as outlined in SDPWS Section 4.5.3: γ v ≤ v′ where ν = wind or seismic force demand ν′ = adjusted capacity calculated per the NDS From SDPWS Section 4.5.4: γ = 2.0 for wood and steel components (typ.) = 1.5 for wood members resisting wind loads = 1.5 for chord splice connections controlled by Mode IIIs or IV (seismic) = 1.0 for chord splice connections controlled by Mode IIIs or IV (wind) The maximum chord force is determined by calculating the diaphragm moment at midspan and dividing the moment by the effective depth. The effective depth is 100 ft. The diaphragm moment at midspan (120 ft), M, is calculated as: wEQ l 2 8 ( 500 lb/ft )( 240 ft )2 = 8 (1,000 lb/kip )

M=

= 3,600 kip-ft

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 75

The chord force is then calculated as: M d 3,600 kip-ft = 100 ft = 36.0 kips

v=

Using the amplification factors given previously, the chord is designed to resist: γ v = 2.0 ( 36.0 kips ) = 72.0 kips 6.2.2.2 Acoustic Consideration for Diaphragm Design Note that there may be acoustic implications regarding the structural diaphragm design—namely, if a concrete topping layer is structurally connected to the mass-timber floor-ceiling panel below, thereby “short circuiting” any resilience that may have been intended to provide impact sound separation between floors. Close coordination with the acoustic designer is recommended to confirm the structural diaphragm design does not negate the acoustic design elements required to meet the project requirements. Even if structural diaphragm connections (i.e., acoustically rigid connections) exist only at the beams or building core, there exists a “short circuit” at those locations that may create an acoustic flanking path. There is little to no test data (lab or field) for the “partially connected” diaphragm design to inform the resulting acoustic performance from the variety of design options. Further study and/or physical testing may be warranted to better understand the project-specific conditions. Reference Chapter 4 for a more in-depth discussion of acoustic considerations. 6.3

COMPOSITE SYSTEMS

6.3.1 Composite vs. Noncomposite Behavior Composite action between materials is commonly used to increase the overall efficiency, strength, and stiffness of structural systems. This is commonly seen in steel construction with steel elements behaving compositely with a concrete deck or in reinforced concrete construction where concrete behaves compositely with reinforcing steel as shown in Figure 6-12. Research, product development, and design have sought to improve mass-timber structural systems through composite behavior. These efforts have focused on various components in the gravity system, with extensive concentration on achieving composite action between masstimber slabs and the concrete topping usually added for acoustic separation. These systems aim to increase the spanning capacity, for both strength and serviceability, by utilizing material already present in the structural build-up. Additional industry focus has been placed on achieving composite action with the steel beams in hybrid systems. These systems aim to regain some of the efficiency lost from comparable conventional composite floors by capitalizing on the timber floor, concrete topping, or combination of the two, as a large compression flange for the steel beam. Achieving this action can be challenging, as many approaches are currently not yet codified in the United States, and fire testing must be done to ensure the elements transferring shear between the steel beams and the slabs can maintain strength in a fire condition. Considerations must also be given to any composite connector (e.g., steel headed stud anchors) that would conventionally be field welded that now pose a fire risk due to the presence of timber. Ideally, items requiring welding are attached in a shop condition. Further, the acoustic concrete topping slab is typically detailed to be structurally independent from the timber slab below, separated by an acoustic membrane, and introducing mechanical connections through that separation may negatively impact acoustic performance of the system. Attention should be given to these connections, and possible acoustic testing should be done once constructed to ensure targeted performance is met. As another reference, the Eurocode provides guidance on timber and concrete composite design. AISC offers resource design examples for composite beam design between steel beams and concrete slabs (AISC, 2019). These are valid references if pursuing a design solution that looks to engage the concrete topping slab. With adjustments for material and connector type (see Section 6.5), similar calculations can be produced to design for composite action between steel beams and mass-timber slabs. The details of these designs should be evaluated by the full design team, with specific input from acoustic and fire consultants.

76 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Figure 6-13 shows examples of noncomposite, steel-timber composite, and steel-concrete composite options. AISC also sponsored the 2017 report, AISC Steel & Timber Research for High-Rise Residential Buildings, produced by Skidmore, Owings, and Merrill (SOM, 2017). This report investigates an innovative steel and timber hybrid flat slab solution for residential construction that utilizes composite behavior with custom asymmetric steel shapes, as shown in Figure 6-14.

Fig. 6-12.  Timber concrete composite (TCC) with NLT composite slab (StructureCraft, 2021).



Noncomposite Steel (a) frame with wood floor Noncomposite



(b) Steel-timber (c) Steel-concrete composite Steel frame with composite wood floor Steel frame with wood floor Steel-timber composite Steel-concrete composite Fig. 6-13.  Steel frame with wood floor composite approaches. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 77

Fig. 6-14.  AISC-SOM composite timber deck and steel beam detail (SOM, 2017).

78 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Design Example 6.5—Composite Hybrid Steel Beam with CLT Panel Given: Using the floor geometry from Design Example 6.1, depicted in Figure 6-15, check an alternate primary beam size, ASTM A992/ A992M W24×62, assuming composite action between the beam and the concrete topping. The CLT panel is placed to create a gap between the panels at the beam location, allowing for a concrete beam region above the steel beam. Figure 6-13(c) depicts this steel-concrete composite approach. Geometry and loading conditions are taken from Design Example 6.1. Consider the concrete beam region over the primary beam and 3 in. NWC (150 pcf) topping slab (ƒc′ = 4 ksi) with composite action. Consider 20 psf construction live load with unshored construction. Serviceability limits: l 360 ( 30 ft )(12 in./ft ) = 360 = 1.00 in.

Δ LL =

l 240 ( 30 ft )(12 in./ft ) = 240 = 1.50 in.

Δ TL =

Fig. 6-15.  Example composite steel-framed structure with mass-timber floors. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 79

Solution: From AISC Manual Tables 1-1 and 2-4, the material and geometric properties for the trial W24×62 beam are: A = 18.2 in.2 Fy = 50 ksi Ix = 1,550 in.4 Zx = 153 in.3 bf = 7.04 in. d = 23.7 in. tw = 0.430 in. tf = 0.590 in. h/ tw = 50.1 Determine the concrete beam region: Depth of beam region = CLT depth, 6d in. Width of beam region = gap between CLT panels Considering 2 in. of CLT bearing on both sides of the beam, the beam region width is 3.04 in., and the concrete beam area, Aconc beam, is: Aconc beam = ( 3.04 in.) ( 6d in.) = 20.9 in.2 Consider the effective area topping slab acting in composite action using AISC Specification Section I3.1: beff = min ( distance to adjacent beam /2, span/8, distance to edge of slab ) ⎡( 20 ft ) (12 in./ft ) ( 30 ft ) (12 in./ft ) ⎤ = min ⎢ , , not applicable⎥ 2 8 ⎣ ⎦ = 45.0 in. Aconc topping = ( 45.0 in.) ( 3 in.) = 135 in.2 Aconc = Aconc topping + Aconc beam = 135 in.2 + 20.9 in.2 = 156 in.2 Loads: From Example 6.1, the unfactored live load component of the point load at midspan, PL, is: PL = wL

⎛ 40 ft + 20 ft ⎞ ⎝ ⎠ 2

⎛ 1,200 lb/ft ⎞ ( 30 ft ) ⎝ 1,000 lb/kip ⎠ = 36.0 kips =

The unfactored construction live load (20 lb/ ft2) component of the point load at midspan, PL,constr, is: ⎛ 20 lb/ft 2 ⎞ PL,constr = ⎜ ⎟ ( 20 ft + 10 ft ) (15 ft ) ⎝ 1,000 lb/kip ⎠ = 9.00 kips 80 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

From Example 6.1, the unfactored superimposed dead load (SDL) component of the point load at midspan, PD, is: PD = wD

⎛ 40 ft + 20 ft ⎞ ⎝ ⎠ 2

⎛ 1,250 lb/ft ⎞ ( 30 ft ) ⎝ 1,000 lb/kip ⎠ = 37.5 kips =

The self-weight of the steel and concrete beam region imposes a uniform dead load, wD. The calculated value of PD already accounts for the self-weight of the 3 in. concrete slab and the CLT over the entire tributary area. The variable wD,conc beam is used to account for the increased self-weight of the concrete beam region because concrete now takes the place of CLT in the 3.04 in. panel gap. (The weight of the 6d in. CLT panel is 35 lb/ ft3.) wD,conc beam =

( 3.04 in.)( 6d in.) (150 lb/ft 3 − 35 lb/ft 3 ) 2 (12 in. /ft )

= 16.7 lb/ft wD = wD,steel + wD,conc beam 62 lb/ft + 16.7 lb/ft 1,000 lb/kip = 0.0787 kip/ft =

For a precomposite section (during construction), ASCE/SEI 7-22, Load Combination 2a, from Section 2.3.1 (LRFD) and Section 2.4.1 (ASD), controls: LRFD

ASD

wu = 1.2D

wa = D

= 1.2 ( 0.0787 kip/ft )

= 0.0787 kip/ft

= 0.0944 kip/ft

Pa = PD + PL = 37.5 kips + 9.00 kips

Pu = 1.2PD + 1.6PL

= 46.5 kips

= 1.2 ( 37.5 kips ) + 1.6 ( 9.00 kips ) = 59.4 kips Determine the precomposite shear and moment demands: LRFD

ASD

2

Pu l wu l + 4 8 (59.4 kips )( 30 ft )(12 in./ft ) = 4 ( 0.0944 kip/ft ) ( 30 ft )2 (12 in./ft ) + 8 = 5,470 kip-in.

Mu =

Pu + wul 2 59.4 kips + (0.0944 kip/ft )( 30 ft ) = 2 = 31.1 kips

Vu =

2

Pa l wa l + 4 8 ( 46.5 kips)( 30 ft )(12 in./ft ) = 4 ( 0.0787 kip/ft )( 30 ft )2 (12 in./ft ) + 8 = 4,290 kip-in.

Ma =

Pa + wal 2 46.5 kips + (0.0787 kip/ft )( 30 ft ) = 2 = 24.4 kips

Va =

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 81

Check precomposite shear strength: The nominal shear strength, Vn, is determined using AISC Specification Equation G2-1: Vn = 0.6Fy AwCv1

(Spec. Eq. G2-1)

Aw = dt w = ( 23.7 in.) ( 0.430 in.) = 10.2 in.2 Check the h/tw ratio: h E ≤ 2.24 tw Fy h = 50.1 tw 2.24

E 29,000 ksi = 2.24 Fy 50 ksi = 54.0

Because

h E : < 2.24 tw Fy

ϕv = 1.00 Ωv = 1.50 Cv = 1.0 

(Spec. Eq. G2-2)

Vn = 0.6 ( 50 ksi )(10.2 in. ) (1.0 ) 2

= 306 kips LRFD

ASD

ϕ vVn = 1.00 ( 306 kips ) = 306 kips > 31.1 kips

Vn 306 kips = Ωv 1.50 = 204 kips > 24.4 kips

o.k.

o.k.

Check precomposite flexure: The nominal flexural strength, Mn, is the lower of the values obtained according to the limit states of yielding and lateral-torsional buckling. Because the beam flange in compression is braced by the CLT deck, lateral-torsional buckling does not apply. The nominal flexural strength of the beam for the limit state of yielding is: Mn = Mp = Fy Z x

(Spec. Eq. F2-1)

= ( 50 ksi ) (153 in. ) 3

= 7,650 kip-in.

 LRFD

ASD

ϕb Mn = 0.90 ( 7,650 kip-in.) = 6,890 kip-in. > 5,470 kip-in.

o.k.

Mn 7,650 kip-in. = 1.67 Ωb = 4,580 kip-in. > 4,290 kip-in.

82 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

o.k.

Check precomposite deflection: Δ PC =

PDl 3 5wDl 4 + 48EI 384EI ⎛ 1 ft ⎞

5 ( 0.0787 kip/ft ) ( 360 in.)4 (37.5 kips )( 360 in.)3 ⎝ 12 in.⎠ = + 48 ( 29,000 ksi )(1,550 in.4 ) 384 ( 29,000 ksi ) (1,550 in.4 ) = 0.843 in. < 1.50 in.

o.k.

For the composite section, ASCE/SEI 7-22, Load Combination 2a, from Sections 2.3.1 (LRFD) and 2.4.1 (ASD), controls: LRFD

ASD

wu = 1.2D

wa = D

= 1.2 ( 0.0787 kip/ft )

= 0.0787 kip/ft

= 0.0944 kip/ft

Pa = PD + PL = 37.5 kips + 36.0 kips

Pu = 1.2PD + 1.6PL

= 73.5 kips

= 1.2 (37.5 kips ) + 1.6 (36.0 kips) = 103 kips Determine shear and moment demands: LRFD

ASD

Pu l wu l 2 + 4 8 (103 kips )( 30 ft )(12 in./ft ) = 4 ( 0.0944 kip/ft ) ( 30 ft )2 (12 in./ft ) + 8 = 9,400 kip-in.

Mu =

Pu + wul 2 103 kips + (0.0944 kip/ft )( 30 ft ) = 2 = 52.9 kips

Vu =

Pa l wa l 2 + 4 8 (73.5 kips) ( 30 ft )(12 in./ft ) = 4 (0.0787 kip/ft )( 30 ft )2 (12 in./ft ) + 8 = 6,720 kip-in.

Ma =

Pa + wa l 2 73.5 kips + (0.0787 kip/ft )( 30 ft ) = 2 = 37.9 kips

Va =

Check composite shear strength: LRFD

ASD

ϕ vVn = 1.00 ( 306 kips) = 306 kips > 52.9 kips

o.k.

Vn 306 kips = Ωv 1.50 = 204 kips > 37.9 kips

o.k.

The shear strength of the bare steel without any composite action satisfies the demands. Check composite flexural strength: Assuming the concrete is at a uniform stress of 0.85 fc′ from AISC Specification Section I3.2d (AISC, 2016c) and that the steel is at a uniform stress of Fy, find the location of the plastic neutral axis (PNA) as shown in Figure 6-16. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 83

Locate the PNA between the steel and the concrete: The maximum tension force in the steel is: Py = As Fy = (18.2 in.2 ) ( 50 ksi ) = 910 kips The maximum compression force in the concrete is: C = 0.85 fc′Aconc = 0.85 ( 4 ksi ) (156 in.2 ) = 530 kips And therefore, the PNA is in the steel.

Fig. 6-16.  Locating the plastic neutral axis. 84 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Determine the location of the PNA in the steel: Pflange = A flange Fy = ( 7.04 in.) ( 0.590 in.) ( 50 ksi ) = 208 kips Pflange + C = 208 kips + 530 kips = 738 kips Py − Pflange = 910 kips − 208 kips = 702 kips Because Pflange + C > Py − Pflange, the PNA is in the top flange. Balanced force = Py + C = 910 kips + 530 kips = 1,440 kips 1,440 kips 2 = 720 kips =C =

T

Cs = Compressive force in steel = 720 kips − 530 kips = 190 kips CCT = Compressive force in concrete topping = 0.85 ( 4 ksi ) (135 in.2 ) = 459 kips CCB = Compressive force in concrete beam = 720 kips − 459 kips − 190 kips = 71.0 kips T flg = Tensile force in top beam flange = 208 kips − 190 kips = 18 kips Tweb = Tensile force in beam web = 720 kips − 208 kips − 18 kips = 494 kips Determine the depth of the PNA from the top flange, a: 190 kips ( 50 ksi )( 7.04 in.) = 0.540 in.

a=

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 85

Sum forces about the PNA as shown in Figure 6-17 to determine the composite flexural strength, M: 3 in.⎞ 6d in. ⎞ ⎛ ⎛ ⎛ 0.540 in.⎞ M = ( 459 kips ) 0.540 in. + 6d in. + + ( 71.0 kips ) 0.540 in. + + (190 kips ) ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ 2 2 2 22.5 in.⎞ ⎛ 0.590 in. − 0.540 in.⎞ ⎛ + (18 kips ) + ( 494 kips ) 0.590 in. − 0.540 in. + ⎝ ⎠ ⎝ 2 ⎠ 2 0.590 in.⎞ ⎛ + ( 208 kips ) 0.590 in. − 0.540 in. + 22.5 in. + ⎝ ⎠ 2 = 14,800 kip-in. LRFD

ASD

ϕb Mn = 0.90 (14,800 kip-in.) = 13,300 kip-in. > 9,400 kip-in.

o.k.

Mn 14,800 kip-in. = Ωb 1.67 = 8,860 kip-in. > 6,720 kip-in.

o.k.

Check deflection: The location of the neutral axis, y, can be determined by applying the principle of moments with the axis of moments at the top of the slab. Concrete components need to be transformed to the equivalent steel values to account for the differing modulus of elasticity. From AISC Specification Section I2.16: Ec = wc1.5 fc′ = (150 lb/ft 3 ) 4 ksi = 3,670 ksi 29,000 ksi 3,670 ksi = 7.90

n =

Using the values calculated in Table 6-1, the value of y can be calculated as shown. Figure 6-18 shows y for the composite section and each component. y=

∑ Ay ∑A

440 in.3 38.0 in.2 = 11.6 in. from top of combined section in the steel =

45.0 in.

Plastic neutral axis 0.540 in.

0.270 in.

18.0 kips (T)

Fig. 6-17.  Composite flexural strength. 86 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Table 6-1.  Calculation of Ay Component Concrete flange (topping)

A, in.2 135

Concrete web (beam)

20.9

W24×62

18.2

Total

Transformed A, in.2 17.1 2.65 18.2

y , in. 1.50 6.44 21.8

38.0

Ay, in.3 25.7 17.1 397 440

Table 6-2.  Determination of Iequivalent Component Concrete flange Concrete web W24×62

A, in.2 17.1 2.65 18.2

d, in. 10.1 5.16 10.1

I , in.4

12.8 10.4 1,550

I + Ad2, in.4

1,760 81.0 3,410 5,250

The equivalent moment of inertia, Iequivalent, is determined as shown in Table 6-2. Figure 6-19 shows d for each component of the composite section. I equivalent = 5,250 in.4 Check deflection: Δ LL = =

PL l 3 48EI ( 37.5 kips )( 360 in.)3

48 ( 29,000 ksi ) ( 5,250 in.4 )

= 0.239 < 1.00 in.

o.k.

Fig. 6-18.  Section and component values of y . AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 87

Δ TL = Δ PC + Δ LL = 0.843 in. + 0.239 in. = 1.08 < 1.50 in.

o.k.

The W24×62 member is adequate for deflection, flexure, and shear. The designer also needs to confirm that the appropriate shear transfer can be achieved using w-in.-diameter steel headed stud anchors atop the steel beam. 6.4 VIBRATION 6.4.1 Principles of Vibration Design Human-induced vibration examines the ability for occupants of a space to excite the structural floor plate. This excitation most often comes from people walking within the space but may include running, walking up and down stairs, and aerobic or rhythmic activities. This often can result in human perception of the vibrations that result from these excitations and can lead to discomfort if not appropriately considered in design. This is a well-known phenomenon and has been well studied for a variety of structural systems and materials. Reference AISC Design Guide 11, Vibrations of Steel-Framed Structural Systems Due to Human Activity (Murray et al., 2016), or CCIP-016, A Design Guide for Footfall Induced Vibration of Structures (CCIP, 2007), for past documentation from the United States and United Kingdom pertaining to steel and concrete structures. Several factors contribute to a structural system’s adequacy for vibration performance, but it is most important to understand the planned use of the space and expected levels of vibration from its occupants. Assessing how the space is intended to be used will help to not only evaluate the expected levels of excitation seen by the structural framing, but also to help determine what levels of vibration will be expected and tolerated by the building’s occupants. The vibration behavior of a floor plate is often a function of the system’s weight or mass, damping, and its stiffness. These three factors drive the overall dynamic performance of a system, and through modifications to these factors, the vibration performance of a structural system may be improved. Due to the lightweight nature of mass-timber construction, it is particularly susceptible to human-induced vibration. Footfall vibration typically occurs between excitation frequencies of 1 to 2.5 Hz, and thus, it is recommended that floorplates have modal frequencies over 8 to 9 Hz to ensure the floorplate is not susceptible to the fundamental or harmonic frequencies of excitation. Appropriate analysis must be conducted to understand the modal properties of a structural system. Hybrid systems utilizing steel framing are generally stiffer than all-timber buildings and therefore are more appropriate for vibration sensitive spaces such as labs. 6.4.2 Methods of Analysis Analysis of vibration of floor plates is a complex exercise that must adequately consider strength, stiffness, and fixity of elements under the expected design excitations being considered. Simplified design methods exist to consider vibration design at a high level—for example, mass-timber slab span tables produced by manufacturers or span limit tables provided by the CLT Handbook. These recommendations can be useful in early phases of design; however, they may not be fully applicable at later stages of design or for unique structural systems.

Fig. 6-19.  Component values of d. 88 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

To help provide a consistent vibration analysis approach in the U.S. market, WoodWorks produced the first edition of the U.S. Mass Timber Floor Vibration Design Guide in 2021 (WoodWorks, 2021b). The document aims to consolidate various analysis recommendations provided in the industry, including AISC Design Guide 11 and CCIP-016. This document is a recommended resource for approaching vibration analysis of timber floors, but also provides valuable background information on humaninduced vibration in general. Figure 6-20 shows an example of footfall analysis following the CCIP-016 approach for a steelframe structure with a 3-ply CLT floor. 6.5

MASS TIMBER-TO-STEEL CONNECTION TYPES

6.5.1 Fastener Types Typical mass-timber connections use dowel-type fasteners, including bolts, screws, nails, and pins. Screws are the most commonly used fastener type and are typically proprietary, self-tapping screws 4  in. diameter and greater. Lag screw design is covered by NDS. Proprietary screws are fabricated using high-strength steel, resulting in significant single fastener capacity for mass-timber connections; installation requirements are provided by manufacturers. There are two types of screws used in mass-timber connections—partially threaded and fully threaded (see Figure 6-21). Partially threaded screws are used for fastening two members together when the primary force transfer mechanism is shear. Fully threaded screws are used when thread withdrawal capacity is required on both sides of a connection because the primary force transfer mechanism is tension. Fully threaded screws can also be used for reinforcing of a single member. Screws are available with a variety of head types. The most common screw heads are countersunk, cylindrical, and flange. Countersunk heads are used when a flush finish is desired. Cylindrical heads are used when the screw head is embedded beneath the timber surface. Flange heads are used when pull-through resistance is important. Countersunk heads are the most common head type when fastening steel plates to timber. Plates should either be fabricated with countersunk holes or flared washers should be used. Inclined screws should be installed using an appropriate angled washer, in accordance with the manufacturer’s recommendations. Design of fasteners in hybrid steel-timber construction between steel beams and mass-timber floor panels is often governed by diaphragm forces at floor levels and diaphragm forces combined with uplift at roof levels. In steel-timber hybrid construction, mass-timber floor panels can also be used to brace the steel beams against lateral-torsional buckling. The screws provide the shear connection between the steel beam and mass-timber panel required to properly brace the beam.

Fig. 6-20.  Footfall analysis of steel-framed structure with 3-ply CLT floor. AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 89

Standard screw layout for steel beam-to-timber panel connections will vary by project but is typically no more than 24 in. on center with screws fastened through the beam flange on each side of the web, installed vertically from the underside of the steel beam flange. Standard detailing practice is to stagger the screws on each side of the web to prevent splitting of the wood. NDS and AISC provisions provide appropriate technical guidance on end and edge distances for fasteners in timber and steel, respectively, as well as guidance on appropriate fastener spacing. An example of steel beam-to-timber panel connections is shown in Figure 6-22.

Fig. 6-21.  Partially threaded and fully threaded fasteners (photo courtesy of MTC Solutions).

Fig. 6-22.  CLT-to-steel beam typical connection (photo courtesy of Odeh Engineers, Inc.). 90 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Table 6-3.  Standard Steel Hole Size Based on Fastener Diameter Major Diameter, D 4 in. c in. a in. 2 in.

Steel Hole T in. 23 64

in.

v in. 33 64

in.

6.5.2 Shop Fabrication All holes required for steel-to-timber connections should be shop fabricated to avoid onsite labor and to leverage the prefabrication benefits of both steel and mass timber. Mass-timber installers prefer to use smaller diameter screws for constructability purposes; however, in hybrid steel-timber construction, the ability to fabricate small diameter holes through steel members should be considered. For pre-drilled holes through steel members, the standard hole diameters shown in Table 6-3 are recommended based on fastener major diameter. Installation instructions for proprietary fasteners are provided by the manufacturer. Fabrication of small diameter holes through thicker steel members may be difficult and should be considered in consultation with the steel fabricator. 6.6

DETAILING CONSIDERATIONS

6.6.1 Steel Moment Connections Any elements that prevent the mass-timber panels from bearing firmly on the supporting steel framing must be accounted for in the panel design and fabrication, including any top-flange connection plates or bolts at moment frame connections. To minimize panel conflicts and fit-up, the engineer should consider avoiding any protruding plates and bolts on the top flange at steel moment connections. Where forces are low to moderate, bolted end-plate moment connections should be considered. This solution avoids top plates that interfere with the mass-timber panels, avoids field welding, and is quick to install. Where forces are large, the detail requires careful consideration weighing the ease of installation against the cost and risk of field welding. Field-welded moment frame connections, without top-flange plates, require the least coordination with CLT panels. However, significant field welding can impact structural costs and pose a fire risk during construction. Appropriate fire safety measures need to be in place if field welding is required. Alternatively, bolted moment connections with plates or bolts on the top flanges can be used, but these elements need to be coordinated carefully and accounted for in the CLT panel design, fabrication, and installation. 6.6.2 Steel Beam Camber Large camber in a steel beam can lead to fit-up issues with CLT panels that come in 8 to 10 ft widths. One option to avoid fit-up issues is to eliminate camber in the steel design. Due to the lower mass of timber floor plates, vibration design can sometimes control the steel beam sizing; eliminating camber can often benefit the vibration design. There may not be a significant tonnage impact if the beam depth can be increased to accommodate the vibration and eliminate or reduce camber. Camber can be considered for longer spans where the camber would be more proportional to typical radial cambering of a glulam beam. The engineer should contact the CLT panel fabricator for maximum allowable camber requirements based on the suggested panel widths and layout. For preliminary design it is recommended that a maximum w in. camber is assumed for steel framing, unless measures are taken to mitigate effects of panel fit-up. 6.6.3 Timber Panel Penetrations Penetrations for mechanical, electrical, plumbing, and fire protection need to be accounted for in the panel design. Whenever possible, penetrations should be coordinated during the design phase so they can be fabricated with the CLT panels, and the appropriate reinforcing provided.

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 91

Small penetrations spaced far apart can often be accommodated without reinforcement in 5-ply CLT or higher by using the weakaxis capacity of the CLT panel to redistribute forces around the opening. For larger openings, steel reinforcement may be required. Steel opening reinforcement can be provided as channels or angles below the CLT as part of the steel design. Where there is room above the CLT, angles or channels could also be located above the CLT to redistribute forces around the opening and provide a cleaner ceiling. Examples of opening size limitations and steel reinforcement are shown in Figure 6-23. The ability for a 3-ply CLT panel to accommodate even small penetrations tends to be much more limited, as the weak-axis direction of the 3-ply panel is only a single lamination. For 3-ply construction, openings should be grouped together as much as possible to limit reinforcement above or below the CLT panel. If the contractor requires greater flexibility to field install a significant number of penetrations, the engineer should consider whether a 5-ply CLT panel with fewer openings with reinforcement is more appropriate than a 3-ply panel with more steel reinforcing around the openings.

(a) Penetration > 4 in. diameter and ≤ 8 in. diameter

(b) Penetration 4 in. diameter or smaller Fig. 6-23.  CLT angle reinforcing detail at penetrations. 92 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

6.6.4 Integrity and Redundancy For steel-timber hybrid buildings where the primary frame is constructed using steel, engineers should consider load path redundancy in detailing to meet ASCE/SEI 7 requirements relating to general structural integrity as with any other steel project. Some buildings may require more stringent requirements relating to disproportionate, or progressive, collapse. Disproportionate collapse occurs when the failure of one structural element leads to the progressive failure of the other structural elements, disproportionate in magnitude to the original failure. Federal standards and guidelines contain prescriptive requirements to prevent disproportionate collapse where applicable. Where mass-timber floor panels are relied upon to contribute to global stability or redundancy, the compatibility with the timber panel framing must be considered carefully to ensure ductility and that redistribution can occur in the steel framing. 6.6.5 Façade Supports A benefit to hybrid steel buildings is that any additional façade framing can be easily accommodated in the steel framing. Additionally, connecting directly to the steel framing avoids creep and moisture movement issues. Steel framing is often provided at the perimeter of the building due to the stricter deflection requirements at the façade. The perimeter steel framing can often be utilized to greatly simplify the façade connection interface of the building. The simplest way to support the façade is generally to utilize the perimeter steel framing in lieu of the timber panels. Detailing façade systems attachments to mass-timber panels can be difficult, particularly in the weak direction of mass-timber panels or where eccentricities and moments are present. AISC Design Guide 22, Façade Attachments to Steel-Framed Buildings (Parker, 2008), is a good reference to address façade and steel framing details. The examples and principles in this Design Guide still largely apply to hybrid steel and mass-timber floor panel projects. Where lateral façade forces in perimeter steel beams are resisted by the CLT diaphragm panel, edge distance perpendicular to the panel edge should be accounted for in the fastener design at the steel beam and CLT panel condition. Additional perimeter steel framing members can also be added where required for façade support. Steel framing provides great flexibility and simplicity in accommodating virtually any façade condition. Figures 6-24 and 6-25 show examples of an eccentric façade condition that was easily accommodated with supplemental steel façade framing. Steel-framed buildings can also easily accommodate thermal breaks to maintain a good thermal building envelope, which is often required by newer energy codes. Thermal breaks need to be detailed to allow for CLT panel installation. An example of a thermal break is shown in Figure 6-26. 6.6.6 Timber Shrinkage and Swelling Timber is a natural material that shrinks and swells as its moisture content changes below the fiber saturation point. The fiber saturation point varies by species and individual piece of wood but is approximately 30% for most structural timber. Moisture content (MC) is defined as follows:



MC =

⎛ Moist weight − Oven dry weight ⎞ (100%) ⎝ ⎠ Oven dry weight 

(6-1)

Timber can be provided “green” with moisture content greater than 19% at manufacture or “dry” with moisture content less than or equal to 19% at manufacture. Once delivered to site, the ambient temperature and humidity can increase or decrease the equilibrium moisture content of the timber resulting in swelling and shrinking, respectively. CLT panels are manufactured at a maximum moisture content of 12%; the moisture content will eventually fall to a range between 6% and 8% once enclosed in a conditioned space and the panel reaches an equilibrium with the local climate. CLT panels should never be directly exposed to the elements and prolonged exposure during construction should be avoided.

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 93

Fig. 6-24.  Example of eccentric façade with perimeter HSS tube for façade bearing at Houston Endowment Headquarters.

94 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

The magnitude of swelling and shrinking can be calculated by estimating the moisture content at installation and the maximum and minimum expected moisture content based on the site-specific exposure. A simplified approach can be used where the percentage of shrinkage and MC are as follows: Shrinkage = 0% at MC = 30% Shrinkage = 6% at MC = 0% Using linear interpolation, the expected shrinkage or swelling can be calculated using the estimated installation and maximum moisture content. Connections should be detailed to accommodate this magnitude of movement. Note that timber has three primary directions—tangential, radial, and longitudinal. Longitudinal shrinkage is very small and considered negligible for the purpose of design. The simplified approach described previously is appropriate for radial and tangential shrinkage. CLT composed of alternating lamination directions is significantly more dimensionally stable than dimensional lumber or one-way systems such as NLT or DLT. CLT is most prone to panel thickness changes from shrinkage, and not commonly subject to dimensional changes in panel width or length. This property of CLT allows panels to be more closely fit together in the field, without the panel gaps needed for less dimensionally stable products.

Fig. 6-25.  Steel façade support condition (photo courtesy of David Barber, Arup).

AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 95

Fig. 6-26.  Example of thermal breaks included in steel framing where steel elements are required to extend past building envelope (Houston Endowment Headquarters).

96 / HYBRID STEEL FRAMES WITH WOOD FLOORS / AISC DESIGN GUIDE 37

Chapter 7 Constructability 7.1 PROCUREMENT

7.2 ERECTION

The decision to utilize a hybrid steel and mass-timber structure should be made at the early stages of design. Pricing information to inform this decision benefits from cost estimates from multiple potential subcontractors and/or suppliers that would carry the mass-timber scope. Several mass-timber panel suppliers are PRG 320 certified in North America. There are also European suppliers who can supply mass-timber products to the United States at competitive pricing. For design-bid-build projects, it is recommended that a design-assist partner is engaged during the design process, typically during the design development phase. Because mass-timber panel sizing is not standardized and varies by supplier, design-assist frameworks allow the supplier early design engagement to optimize the design based on their panel properties. The design-assist package can be competitively bid based on schematic design or design development level documentation. For steel-timber hybrid construction, there are several commonly used procurement options for the mass-timber scope, as detailed in Table 7-1.

Steel-timber hybrid construction is erected floor-by-floor with steel framing placed prior to timber panels. Masstimber panels are craned in using lifting attachments typically provided by the mass-timber supplier and attached to the top (unexposed) face of the panel. Mass-timber panels and their connections can be designed to serve as the temporary stability system during erection. CLT panels can be cut to allow for placement around columns to allow columns to be erected over multiple floors as is standard in steel construction. This method was utilized in the construction of RISD North Hall (Figure 7-1). Mass-timber panels are provided with piece markings similar to steel so that erection can follow a prescribed order with specific pieces lifted into place based on their piece marking and final layout. The steel and mass-timber panels need to be installed in tandem and therefore it is key that there is appropriate planning for material delivery to avoid delays. It is recommended that the steel and mass-timber panels are installed by the same erector to ensure coordination of these trades.

Fig. 7-1.  RISD hybrid structure under construction (photo courtesy of Odeh Engineers, Inc.). AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 97

Table 7-1.  Mass-Timber Procurement Pathways Procurement Path

Supply Scope

Erection Scope

Design-Assist Scope

General contractor led

General contractor procures directly from mass-timber supplier

Steel contractor

Mass-timber supplier

Steel contractor led

Steel contractor procures directly from mass-timber supplier

Steel contractor

Mass-timber supplier

Specialist timber contractor led

Specialist timber contractor supplies or procures mass timber

Specialist timber contractor Specialist timber contractor

7.3 TOLERANCES

7.4

Construction tolerances can be separated into two categories—fabrication and erection. Timber and steel tolerances are governed by different documents for fabrication: • Glulam: ANSI A190.1, Standard for Wood Products— Structural Glued Laminated Timber (APA, 2017)

Protection of timber against moisture is of the utmost importance, particularly during transportation and construction. Trapped water can result in fungal growth, decay, and unexpected dimensional movement. General good practice of protecting mass timber during transportation and storage may include, at minimum, the following: • Penetrating sealant applied to end-cuts and both surfaces of mass-timber panels

• CLT: ANSI/APA PRG 320, Standard for PerformanceRated Cross-Laminated Timber • Steel: AISC Code of Standard Practice for Steel Buildings and Bridges (AISC, 2016a) No standard code of practice exists for erection tolerances of mass-timber construction. Realistic erection tolerances and tolerance compatibility should be discussed with the contracting team, but the following erection tolerances are suggested as a reasonable starting point: • For rectangular areas, the corner-to-corner diagonal measurements should not deviate from each other by more than 2 in. or 0.25% of the length of the shortest side of the rectangle, whichever is greater. • Overall surface levelness (floors and flat roofs): 4 in. in 10 ft maximum. • Elevation: ±a in. from theoretical. • Joints: x in. maximum gap between panels unless noted otherwise. Another reference document for tolerances is the UKbased Timber Research and Development Association (TRADA) National Structural Timber Specification (NSTS) (TRADA, 2017). Note that some hybrid projects may require interfaces with cast-in-place concrete or masonry construction. It should be noted that the tolerances for these materials are larger than steel and timber. Connections to concrete and masonry should be carefully considered to accommodate the tolerances allowable for these materials.

TIMBER PROTECTION

• Wrapping of individual members, removed only after wrapping no longer serves a useful purpose • Storage of all wrapped members off the ground with proper ventilation and drainage provided Prior to commencing construction, it is recommended that the contractor develop and provide a moisture management plan that outlines procedures to protect against moisture. This moisture management plan should include procedures for recording and documenting moisture content at various stages throughout construction to ensure project requirements for the final moisture content of the members is achieved. Good practice of protecting mass timber during construction includes the following: • Providing water-resistant taping at all seams and lifting pockets • Providing temporary flashing at perimeter and interior edges • Removing standing water A more in-depth discussion on moisture control during construction is also provided in the WoodWorks Mass Timber Construction Manual (WoodWorks, 2021a). Additional lines of defense against moisture include justin-time delivery to eliminate on-site storage of mass timber and proper detailing of permanent protection of mass timber at roof levels, including redundant levels of waterproofing and ventilation. Chemically treated timber, species with

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naturally high levels of tannins, and high-moisture environments all present potential corrosion issues for metal fasteners and connectors. The risk of corrosion should be evaluated, and the use of stainless steel or hot-dip galvanized coatings should be considered. 7.5

FIRE RISK DURING CONSTRUCTION

Fires during construction are a significant issue with lightframe timber buildings because the fire protection of gypsum board and sprinkler protection are not installed until the building is nearly completed, placing the incomplete building at risk. A major advantage of mass-timber construction is that structural members, such as CLT, do not rely on additional protection measures, such as gypsum board, to resist fire. The required structural fire ratings are provided by each member as soon as it is installed. Preventing construction fires from starting is always important, and the methods used include increasing construction

site security to prevent deliberate fires, eliminating cooking onsite, storage away from the building, and hot-work supervision (where required). The use of NFPA 241, Standard for Safeguarding Construction, Alteration, and Demolition Operations (NFPA, 2021b), provides a means to reduce the fire risk and consequences of construction fires. For steeltimber hybrid systems it is recommended to avoid onsite welding, flame-cutting, and grinding in order to eliminate one of the major possible ignition sources. If onsite welding is required, the welding should be sequenced to occur prior to the installation of the mass timber, otherwise appropriate safety precautions should be followed once the mass timber is in place. For Types IV-A, -B, and -C construction, there are also specific fire protection requirements during construction that must be met. This includes protecting the exposed mass timber when the construction is above six floors.

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Spickler, K. (2015), Cross Laminated Timber Horizontal Diaphragm Design Example, Structurlam, Penticton, British Columbia. StructureCraft (2021), Timber-Concrete Composite, Structure­Craft Builders, Inc., British Columbia, Canada, https://structurecraft.com/materials/mass-timber/ timber-concrete-composite, accessed 12/6/2021. Tally (2020), Tally® computer software, version 2020.06.09.01, KT Innovations, www.choosetally.com/download/. TDI (2019), CLT Buildings: A WBLCA Case Study Series, Tallwood Design Institute, Corvallis, Ore. Think Wood (2021), Mass Timber Design Manual, Woodworks, Think Wood, Washington, D.C. TRADA (2017), National Structural Timber Specification (NSTS), V2.0, The Timber Research and Development Association, High Wycombe, UK. UL (2020), Fire Tests of Building Construction and Materials, ANSI/UL 263, Underwriters Laboratories, Northbrook, Ill.

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Smarter. Stronger. Steel. American Institute of Steel Construction 312.670.2400 | www.aisc.org D837-22