The International Code Council’s International Building Code (IBC) contains three types of shear wall design methods. The first, the segmented shear wall method, uses full-height shear wall segments that comply with aspect ratio requirements and are usually restrained against overturning by hold-down devices at the ends of each segment.

The second method—force transfer-around openings—considers the entire shear wall with openings and the wall piers adjacent to openings are segments. The method requires the forces around the perimeter of the openings to be analyzed, designed, and detailed. With this method, the hold-down devices generally occur at the ends of the shear wall, not at each wall pier, and special reinforcement around the opening is often required. See IBC Figure 2305.3.5.

The third and newest method is the perforated shear wall method—which is an empirical approach that does not require special detailing for force transfer adjacent to the openings. The perforated shear wall method, however, specifically requires hold-down devices at each end of the perforated shear wall.

Two most common methods
Section 2302 of the IBC defines a shear wall as a vertical-resisting element that is designed to resist lateral, seismic, and wind forces parallel to the plane of the wall.
Traditional wood-frame shear wall design (segmented shear wall design) uses full-height segments that comply with the aspect ratio requirements, and the wall areas between segments that do not comply with aspect ratio are ignored and treated as openings. The sheathing above and below the openings, if present, is also ignored. Collectors between segments distribute diaphragm shear to the full-height segments. In general, hold-down devices for overturning restraint are located at each end of the full-height segments as shown in Figure 1.

Fig. 1


IBC Section 2305.3.8.2 permits unreinforced openings in shear walls with no force-transfer design provided the restrictions outlined in the section are met. This method evolved into what is now referred to as the perforated shear wall method in the 2003 IBC—which was later expanded further in the 2006 IBC. A perforated shear wall is defined in Section 2302 as a wood-structural, panel-sheathed wall with openings that have not been specifically designed and detailed for force transfer around the openings.

The perforated shear wall design method of IBC Section 2305.3.8.2 is a recently developed empirical method that recognizes the strength and stiffness contributed by sheathing above and below the wall openings. Historically, designers have used principles of mechanics to design wall areas above and below openings similar to coupling beams. This approach usually results in very specific and complicated detailing requirements that are often difficult to inspect and construct properly in the field. Because the perforated shear wall approach does not require special analysis and detailing for force-transfer continuity around openings, it is easier to design and construct and inspect in the field. Although the perforated shear wall is required to have a hold-down device at the ends of the perforated shear wall, there is no need for overturning restraint at intermediate wall segments. A perforated shear wall is illustrated in Figure 2.

Fig. 2


History of perforated shear wall provisions
In 1981, Professor Hideo Sugiyama at the University of Tokyo, Japan, proposed an empirical equation to estimate the shear capacity and stiffness of shear walls with openings without intermediate overturning restraints. Sugiyama’s equation serves as the basis for the perforated shear wall design method in the IBC. Sugiyama’s equation and the resulting design procedure are discussed in several articles published by the American Forest & Paper Association (AF&PA) that are available on the American Wood Council website: www.awc.org. Full-scale tests (Dolan et al, 1996) provided further verification of the empirical equation that was used as the basis for the procedure. The Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings—1995 SBC High Wind edition, first recognized the perforated shear wall system without intermediate overturning restraint and unreinforced openings as an acceptable design method. The method was subsequently incorporated into the 2000 IBC. The wording used in these documents created some confusion regarding proper application of the procedure. The language was improved in the Federal Emergency Management Agency’s 2000 National Earthquake Hazard Reduction Program’s (NEHRP) Provisions in an attempt to clarify the intent of the code. Several refinements and clarifications were incorporated into the 2003 and 2006 IBC. The current procedure in the 2006 IBC has specific limitations prescribed in Section 2305.3.8.2.1. Additional research is expected to modify the limitations and further expand the application of the method.

Current provisions in the 2006 IBC
2305.3.8.2.1 Limitations—Specific limitations were added to the 2003 IBC, as follows:
1) Full-height segments are required at each end of a perforated shear wall. Openings are permitted to occur beyond the ends of the perforated shear wall, provided the width of the openings are not included in the width used to design the perforated shear wall.
2) The allowable shear used in the design from Table 2306.4.1 cannot exceed 490 pounds per linear foot (plf).
3) Where there are out-of-plane offsets in a perforated shear wall, the portions of the wall on each side of the offset must be considered separate perforated shear walls.
4) Collectors for shear transfer must be provided throughout the full length of the perforated shear wall.
5) A perforated shear wall must have uniform top of wall and bottom of wall elevations, and if not, the shear wall must be designed by other methods.
6) The perforated shear wall height, h, cannot exceed 20 feet.

2305.3.8.2.2 Perforated shear wall resistance—There were some changes in the 2003 IBC that clarified the procedure for determining perforated shear wall resistance. The percentage of full-height sheathing is determined by calculating the sum of the widths of segments divided by the total width of the perforated shear wall, including openings. The maximum opening height is the maximum opening clear height that occurs in the perforated shear wall. If areas above and below an opening are unsheathed, then the height of the opening is the height of the wall. The unadjusted shear resistance is the allowable shear given in Table 2306.4.1 if the height-to-width ratio of the segments do not exceed 2:1 for seismic design, and 3-1/2:1 for wind design. Where the height-to-width ratio of any perforated shear wall segments is greater than 2:1, but does not exceed 3-1/2:1, the unadjusted shear resistance is multiplied by 2w/h for seismic design, where w is the wall’s width. The adjusted shear resistance is calculated by multiplying the unadjusted shear resistance given in Table 2306.4.1 by the shear resistance adjustment factors given in Table 2305.3.8.2. The section permits interpolation of adjustment factors in Table 2305.3.8.2 for intermediate percentages of full-height sheathing. The total perforated shear wall resistance is equal to the adjusted shear resistance times the sum of the widths of the perforated shear wall segments.

2305.3.8.2.3 Anchorage and load path — The design requirements for perforated shear wall anchorage and load path detailing are given in Sections 2305.3.8.2.4 through 2305.3.8.2.8 or must be based on recognized principles of mechanics. Wall framing, sheathing, sheathing attachment, and fastener schedules must conform to the requirements of Section 2305.2.4 and Table 2306.4.1 except as specifically modified by these sections.

2305.3.8.2.4 Uplift anchorage at perforated shear wall ends — Although hold-down devices to resist overturning forces are not required at intermediate wall segments, anchorage devices to resist uplift forces due to overturning must be provided at each end of the perforated shear wall. The uplift anchorage must comply with the general overturning-restraint requirements of Section 2305.3.7, except that this section prescribes a minimum tension chord uplift force determined by Equation 23-3.

2305.3.8.2.5 Anchorage for in-plane shear—The unit shear force, v, transferred into the top and out of the base of the perforated shear wall at full-height segments and into collectors connecting shear wall segments is determined by Equation 23-4.

2305.3.8.2.6 Uplift anchorage between perforated shear wall ends—In addition to the uplift anchorage for the ends of the perforated shear wall required by Section 2305.3.8.2.4, the bottom plate of full-height segments are required to be anchored for a uniform uplift force, t, equal to the unit shear force, v, determined by Equation 23-4.

2305.3.8.2.7 Compression chords—The section requires each end of each wall segment to be designed to resist a compression chord force, C, equal to the tension chord uplift force, T, determined by Equation 23-4.

2305.3.8.2.8 Load path—A continuous load path to the foundation shall be provided for uplift forces T and t, for shear forces, V and v, and for compression chord force, C. This section explicitly requires elements resisting shear wall forces contributed by multiple stories to be designed to resist the sum of forces contributed by each story.

2305.3.8.2.9 Deflection of shear walls with openings — The section requires the deflection of shear walls with openings to be taken as the maximum individual deflection of the shear wall segments determined in accordance with Equation 23-2, and divided by the applicable shear resistance adjustment factor from Table 2305.3.8.2. Note that Equation 23-2 applies to fully blocked shear walls that are uniformly fastened throughout.

Conclusion
Perforated shear wall methods are relatively new to design engineers, but with proper analysis and design, they could be an attractive option to the traditional method.