Code Update: Provisions for Cold-Formed Steel Nonstructural Members Clarified in AISI S220
For most cold-formed steel framing projects, industry or manufacturers’ design tables are sufficient for specifying and constructing nonstructural partitions. But for projects where the requirements are outside the usual limits, design professionals and building officials now have a resource to address those challenges.
Prior to 2011, designers consulted AISI S200, North American Standard for Cold-Formed Steel Framing—General Provisions, for both structural and nonstructural members. In an effort to more clearly delineate and eliminate confusion between requirements for cold-formed steel structural members and nonstructural members, the American Iron and Steel Institute (AISI) Committee on Framing Standards developed a new standard: AISI S220, North American Standard for Cold-Formed Steel Framing—Nonstructural Members. This standard intends to provide a level playing field for cold-formed steel manufacturers to develop new, efficient products for nonstructural applications.
AISI S220 applies to commonly recognized nonstructural members such as wall studs used in interior partition walls and other members that meet the nonstructural definition and limitations. Having this information contained in one standard makes it easier for design professionals and building officials to specify and approve cold-formed steel nonstructural framing for building projects. The standard recognizes that the consequence of failure for a nonstructural member is less severe than for a structural member, and therefore permits a lower reliability for nonstructural members. AISI S220 includes several important updates and clarifications for designers working with cold-formed steel.
Redefining Nonstructural Members
AISI S220 also includes an updated definition for a nonstructural member. Although previous standards focused on wall assemblies, AISI S220 expands and generalizes the definition to include other nonstructural members such as those used in interior fascia and ceiling construction:
Nonstructural member: A member in a steel-framed system that is not a part of the gravity load resisting system, lateral force resisting system or building envelope.
Historically, nonstructural wall assemblies have been load tested in accordance with ICC-ES AC86, which considered the design capacity of the composite wall assembly (including gypsum wallboard), and were based on a safety factor of 1.5. However, designers often were also required to use the non-composite assembly design approach and neglect the composite-action contribution of attached sheathings based on the design provisions of AISI S100, North American Specification for the Design of Cold-Formed Steel Structural Members. This required a safety factor of 1.67 for a bending member or 1.8 for an axial-loaded member to be imposed. AISI S220 addressed this inconsistency between composite and non-composite nonstructural members.
Equivalent Thickness Sections
To achieve more-economical design solutions, manufacturers developed proprietary cross sections below the minimum thickness of 0.0179 inch, but achieved the strengths required by ASTM C645 by using higher-yield-strength materials. The proprietary cross sections have been labeled equivalent (EQ) sections, so AISI S220 does not include a minimum thickness requirement.
Generally, nonstructural members are relatively thin, and they often do not meet the dimensional limitations prescribed within Section B1 of AISI S100 for structural members. To facilitate design of the nonstructural member sections meeting and not meeting the AISI S100 Section B1 requirements, AISI S220 includes provisions for non-composite and composite assembly applications.
Chapter D of AISI S220 provides additional design guidance regarding screw installation, stripped screws, spacing and edge distance of screws and attachment of gypsum board for nonstructural members.
AISI standards, including AISI S220, are available to download free of charge at www.aisistandards.org as a service to the design and construction community. AISI also offers several resources, design manuals and guides to support the design and practical application of cold-formed steel, including a suite of test standards covering the testing of individual connections and members as well as complete systems. These additional resources are available at www.buildusingsteel.org.