The following is an excerpt from an article published in the April 2007 issue of Construction Specifier magazine. We've republished it here with Simpson Strong-Tie's permission.
Since the early 1990s, cold-formed steel shear wall design has evolved in step with the increased understanding of their performance. Significant advances and additions in the provisions for these systems have occurred since their first inclusion in the 1997 Universal Building Code (UBC). These include the addition of values for steel and gypsum sheathing, allowance for thicker framing members, shear walls with openings, and deflection equations.
The 2006 International Building Code (IBC) Section 2210.5 requires that the design of light-framed cold-formed steel shear walls be in accordance with the 2004 edition of the American Iron and Steel Institute’s (AISI’s) Standard for Cold-Formed Steel Framing–Lateral Design [LATERAL-04]. The Lateral Design standard provides further information and clarifies design and detailing requirements of lateral-force resisting, cold-formed steel systems than what was previously available. This article discusses and illustrates the information and requirements of this relatively new standard.
Light-framed shear walls
A typical light-framed shear wall transfers lateral loads, in the plane of the wall, through the mechanically attached sheathing, and into the framing members. The in-plane shear loads are transferred from the wall to the floor framing or foundation along the length of the bottom horizontal member (bottom track). The induced overturning forces are transferred through the vertical boundary members (end studs) and over-turning restraint system (hold-downs) at the ends of the wall.
Typical lateral loads on shear walls result from either wind or seismic demand. Design wind loads are the actual expected forces, whereas design seismic loads are reduced based on the type of lateral system used, how many lateral elements are employed in the structure, and the level of seismic detailing performed. Designing for a reduced seismic load can significantly lower the cost of construction, but the tradeoff is damage in the structure during a major earthquake.
Typically, light-framed shear wall assembly strengths are determined through monotonic tests per ASTM International E 564, Standard Practice for Static Load Test for Shear Resistance of Framed Walls for Buildings, for wind load resistance. Another monotonic test standard that is used to determine sheathing strength and not system performance or strength, is ASTM International E 72, Standard Test Methods of Conducting Strength Tests of Panels for Building Construction.
Cyclic tests are performed to determine strengths for light-framed shear walls used to resist seismic forces. Cylic test protocols that have typically been used for light-framed shear wall assemblies are the Sequential Phase Displacement (SPD) protocol and the CUREE protocol for seismic resistance. The SPD protocol was originally developed for masonry walls and then required to be used for light-framed wood shear walls as well. However, research from around the world (e.g., Forintek Canada, Inc., Dan Dolan of Washington State University, Ario Ceccotti in Italy) has shown that the energy demand of this protocol is up to seven times of the actual demand to a low-rise building under during a design level earthquake. The Curee protocol is discussed in detail in Curee publication No. W-13 entitled “Cyclic Response of Woodframe Shearwalls: Loading Protocol and Rate of Loading Effects” with a printing date of May 2002. This cyclic test protocol considers a history of minor to moderate seismic events prior to the design level earthquake and is considered by many to be a more realistic test protocol.
Member strengths and system failure modes are important considerations for seismic design. Generally, a system that will fail suddenly is classified as ‘brittle,’ while a ‘ductile’ one is detailed to sustain more deformation without loss of load-carrying capability. This is typically done by designing the connections and members that are not supposed to yield—or are incapable of yielding (e.g. compression columns)—with a strength in excess of what is needed to fully develop the strength of the designated yielding elements. Use of the special load combinations that employ the ‘over-strength’ factor to determine the design level demand of the non-yielding components is one way to ensure yielding of the designated ductile elements. Codes encourage the use of ductile systems by assigning them a higher R-value, which results in lower required design loads.
Shear wall types
The AISI standard recognizes two basic types of cold-formed steel-framed shear walls. Type I is defined as a fully sheathed shear wall resisting in-plane forces, with hold-downs at each end of each wall segment, and where “detailing for force transfer around the openings is provided” if the wall has openings. A Type II shear wall contains multiple wall segments resisting in-plane forces, with wood or steel sheathing that contains openings between wall segments, and with hold-downs only at the ends of the wall. There is no requirement to detail for shear transfer around openings in a Type II wall.
Basically, Type II shear walls use Type I published strength values modified by a coefficient based on wall and opening height (shear resistance adjustment factors). Type II shear walls also have special considerations, including design for a uniform uplift force along the wall bottom plates, in addition to the typical Type I design for uniform shear at the bottom plates.
Shear wall tables
The Lateral Design standard has three shear wall tables tabulating nominal strengths based on sheathing material, fastener spacing, framing thickness, and seismic or wind loading. The first table focuses on wood- or steel-sheathed assemblies resisting wind loads, while the second deals with gypsum board-sheathed assemblies resisting wind or seismic loads. The final table is for wood- or steel-sheathed assemblies that resist seismic loads.
The values in the tables represent the nominal (or in this case, ultimate) wall capacities. They have to be adjusted to obtain the appropriate design resistance—this is done by multiplying by a resistance factor (φ, phi) to obtain a load- and resistance-factored design (LRFD)-based resistance, or dividing by a safety factor (Ω) to obtain an allowable strength design (ASD) level resistance. Ω is 2.0 for wind and 2.5 for seismic, whereas φ is 0.65 for wind and 0.60 for seismic.
LRFD is defined by the Lateral Design standard as a “Method of proportioning structural components such that the design strength equals or exceeds the required strength of the component under the action of the LRFD load combinations.” ASD is defined as a “Method of proportioning structural components such that the allowable strength equals or exceeds the required strength of the component under the action of the ASD load combinations.” LRFD load combinations include load factors where ASD typically does not, except that a reduction factor is applied to the dead load for the combination including dead plus seismic load and also a reduction factor is applied to the seismic load that is calculated per the IBC at an LRFD level and, thus, when ASD is used it must be converted down to the ASD level.
General requirements and design procedure
Some of AISI’s Lateral Design standard shear wall basic requirements include use of framing members with a minimum thickness of 33 mil, no shear panels less than 305 mm (12 in.) in width, and 610-mm (24-in.) maximum framing spacing. For seismic applications, framing member thickness cannot be beyond the limits set in the table. Summing the strength of shear walls with different sheathing material on the same wall face is not permitted. The wood- sheathed shear wall strength can be increased by 30 percent if gypsum board is used on the opposite side as permitted by the standard’s first table.
Per the AISI Lateral Design standard, the wood and steel-sheathed shear panels may be installed either perpendicular or parallel to the framing members and all panel edges are to be blocked. However, gypsum board sheathing must be installed perpendicular to the framing members. Additionally, where the shear wall tables permit, when the height (h) to width (w) aspect ratio of a shear wall segment may exceed 2:1, but not greater than 4:1, if the tabulated nominal shear wall strengths are reduced by 2w/h. Using an aspect ratio greater than 2:1 is not permitted for gypsum sheathed shear wall assemblies.
Read more
To read more about cold-formed steel framed shear walls, download the full article or view the article on Simpson Strong-Tie’s website.
About the author
Jeff Ellis, P.E., S.E., is a senior engineering project manager for Simpson Strong-Tie, and is a member of the Structural Engineers Association of California (SEAOC), the American Society of Civil Engineers (ASCE), the Building Seismic Safety Council Technical Subcommittee 6 (BSSC TS 6), and the American Iron and Steel Institute (AISI) Committee on Framing Standards (COFS).
