AISI Codes, Standards and Design Guides on Cold-Formed Steel Framing

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AISI Codes, Standards and Design Guides on Cold-Formed Steel Framing J. W. Larson, P.E., F. ASCE 1 1

Director of Construction Standards Development, American Iron and Steel Institute, 1140 Connecticut Ave, NW, Suite 705, NW, Washington, DC 20036; PH (601) 6916334; email: [email protected]

Abstract To enable the increased use of cold-formed steel framing, the American Iron and Steel Institute (AISI) in its role as an ANSI-accredited standards development organization has been actively turning state of-the-art research and industry practices into a suite of design and installation standards. In 2004, AISI released updates to its General Provisions, Header Design, Prescriptive Method and Truss Design Standards. In 2005, the AISI completed a Code of Standard Practice. A Steel Stud Brick Veneer Design Guide and a Cold-Formed Steel Framing Design Guide have also been developed to assist practicing structural engineers and architects to design coldformed steel framing systems. This presentation will provide an introduction to these significant industry documents. Introduction The efforts of AISI in standards development began with the sponsorship of research at Cornell University under the direction of Professor George Winter and the first publication of the AISI Specification in 1946. This initial work was started because “the acceptance and the development of cold-formed steel construction in the United States faced difficulties due to the lack of an appropriate design specification. Various building codes made no provisions for cold-formed steel construction at that time” (Yu et al., 1996). AISI has continued its efforts in this area, under its Committee on Specifications, with a very significant activity being the continuous improvement of the Specification through ongoing investments in research and development. The latest Specification is the 2001 Edition with 2004 Supplement (AISI, 2004a). In 1997, the AISI Construction Marketing Committee authorized the formation of the Committee on Framing Standards (COFS). This was done due to the

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“increased interest in cold-formed steel for residential and light commercial framing” and the sense that “there were a number of design issues that were not adequately addressed for this emerging market. (Bielat and Larson, 2002). The COFS established as its mission: “To eliminate regulatory barriers and increase the reliability and cost competitiveness of cold-formed steel framing in residential and light commercial building construction through improved design and installation standards.” The committee also established as its primary objective: “To develop and maintain consensus standards for cold-formed steel framing, manufactured from carbon or low alloy flat rolled steel, that describe reliable and economical design and installation practices for compliance with building code requirements.” The COFS organized itself under the same ANSI-approved operating procedures that govern the Committee on Specifications. These procedures provide for balance between producer, user and general interest categories; voting, including the resolution of negatives; public review, interpretations and appeals. Numerous task groups have been added under various subcommittees; however, the main committee always maintains control of all decisions through the balloting process. In its first few years, the COFS accomplished many things. The committee established subcommittees and task groups, recruited active members and conducted many meetings. By 2001, the COFS had completed four standards for cold-formed steel framing; namely, General Provisions, Truss Design, Header Design, and a Prescriptive Method for One and Two Family Dwellings. In 2003, a Commentary to the Prescriptive Method, including design examples, was completed. But by no means had the COFS completed its mission. It continued to improve the existing standards. In 2002 it began working on standards for Wall Stud Design and Lateral Design, and began leading an effort to develop an industry Code of Standard Practice. General Provisions Standard The General Provisions standard addresses those things that are common to prescriptive and engineered design. It provides a link between all of the industry stakeholders and code enforcement agencies, ensuring everyone is “on the same page” with respect to the basic requirements of cold-formed steel framing. It provides general requirements for material, corrosion protection, products, member design, member condition, installation, and connections. There were two significant changes included in the 2004 edition of the General Provisions standard (AISI, 2004b); cutting and cut edge protection, and alignment framing tolerances. In the section on materials, the standard now states, “Additional corrosion protection is not required on edges of metallic-coated steel framing members, shop or




field cut, punched or drilled.” And, in the section on cutting and patching the standard requires that “All cutting of framing members be done by sawing, abrasive cutting, shearing, plasma cutting or other approved methods.” These two provisions really go hand-in-hand, and recognize zinc's ability to galvanically protect steel at cut edges when proper cutting techniques are employed (AISI, 1996). The second change is in the section on alignment framing tolerances. Based on testing at the University of Waterloo (Fox, 2003), an additional limitation was added to address those cases where the bearing stiffener is located on the backside of the floor joist. The previous limitation alone, that “each joist, rafter truss and structural wall stud shall be aligned vertically so that the centerline (mid width) is within 3/4 inch (19 mm) of the centerline (mid width) of the load bearing member beneath”, could result in a significant misalignment in the load path, as shown in Figure 1.

Figure 1: Potential Misalignment in Load Path The new limitation prescribes a maximum distance of 1/8 inch (3 mm) from the web of the horizontal framing member to the edge of the vertical framing member, as well, when a bearing stiffener is located on the backside of the horizontal framing member. Truss Design Standard The Truss Design standard applies to cold-formed steel trusses used for load carrying purposes in buildings. Without such a document, the industry would be at a significant disadvantage with respect to competitive materials. The standard is not just for design. It also applies to manufacturing, quality criteria, installation and testing as they relate to the design of cold formed steel trusses. The requirements of the truss




standard apply to both generic C-section trusses, as well as the various proprietary truss systems and were developed, in part, based on extensive research at the University of Missouri-Rolla. For the 2004 edition, the Truss Standard (AISI, 2004c) was revised to recognize the Load and Resistance Factor Design (LRFD) method. This was not included in the previous edition because the industry is still heavily rooted in Allowable Strength Design (ASD). However, with the elimination of the 1/3 stress increase from ASD, the industry feels that there may now be compelling reasons to use LRFD. Header Design Standard The Header Design standard is aimed at giving design professionals the tools they need to design headers over door and window openings in buildings. The design methodologies are based on testing at the NAHB Research Center, the University of Missouri-Rolla and industry, and were developed under the guidance of Dr. Roger LaBoube of the University of Missouri-Rolla. The Header Design standard provides general, design and installation requirements. The only substantive change to the Header Standard for 2004 (AISI, 2004d) was the addition of single L-headers, shown in Figure 2. Based on testing at the NAHB Research Center, single L-headers will be allowed for openings up to 4 feet wide. The design methodology is very similar to that for double L-headers (LaBoube, 2003), except specific limitations are defined based on what was tested.

Figure 2: Single L-Header




Prescriptive Method The Prescriptive Method for One- and Two-Family Dwellings is essentially an updated version of previous building code submittals. The document has gone through the rigorous consensus process, earning it ANSI recognition, giving it instant credibility and making it easily accepted by the various building codes. The standard incorporates the latest cost saving developments of the Steel Framing Alliance, such as the L-header, coupled with the latest engineering and load combination developments, such as the LRFD provisions of the AISI Specification. Significant changes approved for 2004 (AISI, 2004e) include a typical wall-tofloor connection detail, a change to the provisions to allow direct connection of wall track to floor sheathing, based on testing at the NAHB Research Center (NAHB, 2003), details to illustrate the various ceiling joist top flange bracing options, including new provisions for using cold-rolled channel or hat channel, and a detail for connecting an uplift strap to a back-to-back header. In 2004, the Commentary to the Prescriptive Method (AISI, 2004f) was also updated. Wall Stud Design Standard The Wall Stud Design standard (AISI, 2004g) addresses general requirements, loading, design and installation of cold-formed steel wall studs. It addresses certain items not presently covered by the AISI Specification, including load combinations specific to wall studs, a new, more rational approach for sheathing braced design, and methodologies for evaluating stud-to-track connections and deflection track connections. It should be noted that the sheathing braced design provisions in section D4.1 of the previous edition of the Specification have been eliminated. Included in the Wall Stud Design standard is a requirement that when sheathing braced design is used, the wall stud shall be evaluated without the sheathing bracing for the dead loads and loads that may occur during construction or in the event that the sheathing has been removed or has accidentally become ineffective. The load combination is taken from ASCE 7-02 (ASCE, 2002) for special event loading conditions. Sheathing braced design in the Wall Stud Design standard is based on rational analysis assuming that the sheathing braces the stud at the location of each sheathingto-stud fastener location. Axial load in the stud is limited, therefore, by the capacity of the sheathing or sheathing-to-wall stud connection. Provisions are provided for the stud-to-track connection, and recognize that when the track thickness is equal to or greater than the stud thickness, an increase in web crippling strength can be realized. This increased strength is attributed to the favorable synergistic effect of the stud-to-track assembly. The provisions are based on research conducted at the University of Waterloo (Fox and Schuster, 2000) and the



University of Missouri-Rolla (Bolte, 2003). Provisions are also provided for a Csection wall stud installed in a single deflection track application.


Lateral Design Standard The Lateral Design (AISI, 2004h) standard addresses general and design requirements for walls and diaphragms that provide lateral support to a building structure. This standard addresses design requirements for shear walls (Type 1 or segmented and Type 2 or perforated), diagonal strap bracing (that is part of a structural wall), wall anchorage and diaphragms. Previously, these requirements were scattered among various building code provisions, design guides, technical notes and research reports. The intent of this document is to pull them together into one document that is recognized by the codes. A companion Commentary has also been developed to help provide further technical substantiation of the provisions. The requirements for Type I shear walls in the Lateral Design standard were based on studies by Serrette (1996, 1997, 2002). This series of investigations included reverse cyclic and monotonic loading and led to the development of the design values and details for plywood, oriented strand board, and gypsum wallboard lightweight shear wall assemblies. The requirements for Type II shear walls, also known as perforated shear walls, in the Lateral Design standard were based on recognized provisions for wood systems. Research by Dolan (1999, 2000a, 2000b) demonstrated that the design procedure is as valid for steel framed systems as for all wood systems. Also included in the Lateral Design standard are new provisions for estimating the deflection of Type I shear walls. This method considers the bending, overturning, shear and inelastic effects and is based on a recent study at the University of Santa Clara (Serrette and Chau, 2003). Design values for diaphragms with wood sheathing were developed by Serrette (LGSEA, 1998), as was the methodology for determining the design deflection of diaphragms, which was based on a comparison of the equations used for wood frame shear walls and diaphragms, coupled with similarities in the performance of cold-formed steel and wood frame shear walls. Code of Standard Practice In 2005, the COFS developed of an industry Code of Standard Practice for Cold-Formed Steel Structural Framing (AISI, 2005). It covers general requirements, classification of materials, contract documents, installation drawings, materials, installation, quality control, and contractual relations.




This document, developed by the COFS, was reviewed by several peer committees within the industry. It defines and set forth accepted norms of good practice for fabrication and installation of cold-formed steel structural framing. It is not intended to conflict with or supercede any legal building regulations, but serves to supplement and amplify such laws and is intended to be used unless there are differing instructions in the contract documents. Other industries have such documents. This one was patterned after these other documents, but was tailored to the needs of cold-formed steel structural framing industry. Steel Stud Brick Veneer Design Guide In 2003, under the auspices of its Committee on Specifications, AISI released the Steel Stud Brick Veneer Design Guide (AISI, 2003). This document provides background on the key issues and industry references, provides definitions and explanations of terms, describes the function and behavior of the various components, and provides an understanding of overall system behavior and design considerations. Several design approaches are described and a clear set of recommendations is provided for the designer and installer. The recommendations in the guide are based on significant industry references, which are cited, with particular emphasis on a comprehensive long-term investigation funded by the Canada Mortgage and Housing Corporation. The recommendations include suggestions about the bracing of the stud system, the type of brick ties and what design load must be used for them, the amount of movement that is safely permitted for crack control, and insulating techniques in different climates to help prevent condensation within the wall and encourage drying of wall cavities that may experience some moisture. The document includes a very extensive bibliography. For the designer or builder preparing to install a brick veneer system over steel studs, this resource provides excellent insight into how the system should be designed, detailed and installed. Proper anticipation, mitigation and management of heat, air and moisture within the wall system can go a long way to preserving the integrity of the overall building. Cold-Formed Steel Framing Design Guide In 2002, also under the auspices of its Committee on Specifications, AISI released the Cold-Formed Steel Framing Design Guide (AISI, 2002). This document provides a basic introduction to design methods, loads and load combinations, design strength determination, member design as a function of bracing and design strength of connections. But the bulk of this document is devoted to the solution of four detailed design examples. Each example starts with the applied loads and illustrates how to analyze load paths, determine member and connection forces, select members, establish proper bracing conditions, design bracing, and design connections.




Figure 3: Wind Bearing In-Fill Wall Design Example #1, for a wind bearing infill wall (Figure 3), is based on the sheathed design approach which assumes that the sheathing is structurally adequate to resist the torsional component of loads not applied through the shear center and to resist the effects of lateral instability. Members are de-signed using simple beam theory. All connections are fastened with self-drilling screws. Design Example #2, also for a wind bearing infill wall, is based on welded connections and an unsheathed design approach where the secondary effects of torsion and lateral instability are included. Design Example #3, reviews three alternative methods for framing strip windows with cold-formed steel framing. Detailed design calculations are presented for the third alternative, studs outside the face of the structure. The calculations assume welded connections and an all steel system where the restraint of the sheathings is ignored. Design Example #4 covers the design of a cold-formed steel framing floor system bearing on a steel stud wall with a window opening (Figure 4).




Detailed calculations are included for all elements including the stud bridging and its anchorage.

Figure 4: Cold-Formed Steel Floor System Bearing on a Steel Stud Wall Conclusions The American Iron and Steel Institute has effectively leveraged its experience and expertise in standards development to support the growing needs of the coldformed steel framing industry. Charged with a mission, to eliminate regulatory barriers and increase the reliability and cost competitiveness of cold-formed steel framing through improved design and installation standards, the Committee on Framing Standards has built on the internationally recognized AISI Specification and developed and published six ANSI-accredited consensus standards. The model building codes have adopted these Standards for Cold-Formed Steel Framing, which include General Provisions, Truss Design, Header Design, Wall Stud Design, and Lateral Design, as well as a comprehensive Prescriptive Method for One and Two Family Dwellings. AISI also helps facilitate the development of industry recommendations and guidance, including an industry Code of Standard Practice and design guides on Steel Stud Brick Veneer and Cold-Formed Steel Framing. These documents have widespread application and are available from AISI (www.steel.org) and Steel Framing Alliance (www.steelframingalliance.com).




Acknowledgements The members of the AISI committees, subcommittees and task groups responsible for bringing these standards to fruition are to be commended for their time and effort. It is through the participation of representatives from steel producers, fabricators, users, educators, researchers, and building code officials in this consensus process that such progress is made. The partner organizations, Steel Framing Alliance, Light Gauge Steel Engineers Association, Steel Stud Manufacturers Association, Canadian Sheet Steel Building Institute and Center for Cold Formed Steel Structures, are to be thanked for their active participation. Particular gratitude is owed to the American Iron & Steel Institute and the Steel Framing Alliance, and their members, for their long-term vision for this market and financial support of this technical effort. References American Iron and Steel Institute (1996), Durability of Cold-Formed Steel Framing Members, Washington, D.C., 1996. American Iron and Steel Institute (2002), Cold-Formed Steel Framing Design Guide, Washington, D.C., 2002. American Iron and Steel Institute (2003), Steel Stud Brick Veneer Design Guide, Washington, D.C., 2003. American Iron and Steel Institute (2004a), North American Specification for the Design of Cold-Formed Steel Structural Members, 2001 Edition with 2004 Supplement, Washington, D.C., 2004. American Iron and Steel Institute (2004b), Standard for Cold-Formed Steel Framing – General Provisions, 2004 Edition, Washington, D.C., 2004. American Iron and Steel Institute (2004c), Standard for Cold-Formed Steel Framing – Truss Design, 2004 Edition, Washington, D.C., 2004. American Iron and Steel Institute (2004d), Standard for Cold-Formed Steel Framing – Header Design, 2004 Edition, Washington, D.C., 2004. American Iron and Steel Institute (2004e), Standard for Cold-Formed Steel Framing – Prescriptive Method for One and Two Family Dwellings, 2004 Edition, Washington, D.C., 2004. American Iron and Steel Institute (2004f), Commentary on the Standard for ColdFormed Steel Framing – Prescriptive Method for One and Two Family Dwellings, 2004 Edition, Washington, D.C., 2004. American Iron and Steel Institute (2004g), Standard for Cold-Formed Steel Framing – Wall Stud Design, 2004 Edition, Washington, D.C., 2004. American Iron and Steel Institute (2004h), Standard for Cold-Formed Steel Framing – Lateral Design, 2004 Edition, Washington, D.C., 2004. American Iron and Steel Institute (2005), Code of Standard Practice for Cold-Formed Steel Structural Framing, 2005 Edition, Washington, D.C., 2005. American Society of Civil Engineers (2002), Minimum Design Loads for Buildings and Other Structures, ASCE Standard 7-02, Reston, VA, 2002.




Bielat, K.R., Larson, J.W. (2002), AISI Committee on Framing Standards – Enabling the Widespread and Economic Use of Steel Framing, Proceedings of the 16th International Specialty Conference on Cold-Formed Steel Structures, St. Louis, MO, 2002. Bolte, W. G. (2003), “Behavior of Cold-Formed Steel Stud-to-Track Connections”, Thesis in partial fulfillment of the degree Masters Science, Department of Civil Engineering, University of Missouri-Rolla, Rolla, MO, 2003. Dolan, J.D. (1999), “Monotonic and Cyclic Tests of Long Steel-Frame Shear Walls with Openings,” Report No. TE-1999-001, Virginia Polytechnic Institute and State University, Blacksburg, VA. Dolan, J.D., and Easterling, W.S. (2000a), “Monotonic and Cyclic Tests of LightFrame Shear Walls with Various Aspect Ratios and Tie-Down Restraints,” Report No. TE-2000-001, Virginia Polytechnic Institute and State University, Blacksburg, VA. Dolan, J.D., and Easterling, W.S. (2000b), “Effect of Anchorage and Sheathing Configuration on the Cyclic Response of Long Steel-Frame Shear Walls,” Report No. TE-2000-002, Virginia Polytechnic Institute and State University, Blacksburg, VA. Fox, S.R. (2003), “The Strength of Stiffened CFS Floor Joist Assemblies with Offset Loading,” American Iron and Steel Institute, Washington, DC. Fox, S. R., and Schuster, R. (2000), “Lateral Strength of Wind Load Bearing Wall Stud-to-Track Connections,” Proceedings of the 15th International Specialty Conference on Cold-Formed Steel Structures, University of Missouri-Rolla, Rolla, MO, 2000. LaBoube, R.A. (2003), “Summary Report on Strength of Single L-Headers,” American Iron and Steel Institute, Washington, DC, 2003. Light Gauge Steel Engineers Association (1998), Lateral Load Resisting Elements: Diaphragm Design Values, Tech Note 558b-1, Washington, DC, 1998. NAHB Research Center (2003), “Hybrid Wood and Steel Sole Plate Connection Walls to Floors Testing Report”, U.S. Department of Housing and Urban Development, Washington, DC, 2003. Serrette, R.L. (1996), “Shear Wall Values for Light Weight Steel Framing,” Final Report, Santa Clara University, Santa Clara, CA. Serrette, R.L. (1997), “Additional Shear Wall Values for Light Weight Steel Framing,” Final Report, Santa Clara University, Santa Clara, CA. Serrette, R.L. (2002), “Performance of Cold-Formed Steel-Framed Shear Walls: Alternative Configurations,” Final Report: LGSRG-06-02, Santa Clara University, Santa Clara, CA. Serrette, R.L., and Chau, K. (2003), “Estimating the Response of Cold-Formed SteelFrame Shear Walls,” Santa Clara University, Santa Clara, CA. Yu, W.W., Wolford, D.S., Johnson, A.L. (1996), Golden Anniversary of the AISI Specification, Proceedings of the 13th International Specialty Conference on Cold-Formed Steel Structures, St. Louis, MO, 1996.



AISI Codes, Standards and Design Guides on Cold-Formed Steel Framing

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