Shear Wall Design

 

Perforated Shear Wall Design
 
          The perforated shear wall (PSW) design methodology is similar to the segmented shear wall (SSW) method but only requires two hold-downs for each wall (one at each end), thus eliminating the intermediate overturning restraints located adjacent to window and door openings in the SSW method.  The PSW method recognizes that the entire wall acts to resist overturning, rather than a series of individual full-height segments.  The result is a wall with slightly lower capacity (a function of the sizes of openings) and reduced cost in material and labor due to elimination of intermediate hold-downs.  Intuitively, a PSW with a small opening would have nearly the same capacity as the same wall without openings.  As the size of the opening(s) increases, the capacity is correspondingly reduced.  In many cases wood-framed walls have excess capacity when designed according to the SSW methodology, while the PSW method results in a more economical design.
 


Figure 3 - Wood-frame shear wall with openings
 


Figure 4 - Perforated shear wall model
 

          Determining the capacity of a PSW, or shear wall with unrestrained openings, is similar to the SSW method.   However, in the PSW method all sheathed portions of the shear wall (shaded in green) are assumed to provide resistance to lateral loads. PSW capacity is based on the capacity of the wall from the SSW method, reduced by an opening adjustment factor that is related to the percentage of full-height sheathing in the wall. 

           The first step in PSW design is to determine the unit shear capacity (v) of the SSW using IBC Table 2306.4.1 or UBC Table 23-II-I-1, as was demonstrated previously for segmented shearwalls.  Next, the shear capacity adjustment factor (Co) must be calculated using the tabulated information below.  Note that the value of Co is always less than or equal to unity since a PSW has fewer hold-downs than a SSW.
 

Percent full-height sheathing

 


                        

where:

L =  total length of the wall
bi length of full-height sheathing segment
h =  height of shear wall
  

Maximum Unrestrained Opening Height
(Door or Window)

h/3 h/2 2h/3 5h/6 h
8 ft wall 2'-8" 4'-0" 5'-4" 6'-8" 8'-0"
10 ft wall 3'-4" 5'-0" 6'-8" 8'-4" 10'-0"
Percent full-height sheathing Shear Capacity Adjustment Factor
(Co)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.67
0.69
0.71
0.74
0.77
0.80
0.83
0.87
0.91
0.95
1.00
0.50
0.53
0.56
0.59
0.63
0.67
0.71
0.77
0.83
0.91
1.00
0.40
0.43
0.45
0.49
0.53
0.57
0.63
0.69
0.77
0.87
1.00
0.33
0.36
0.38
0.42
0.45
0.50
0.56
0.63
0.71
0.83
1.00
Table adapted from Line & Douglas and IBC Table 2305.3.7.2.

 
          The unrestrained opening height is the vertical dimension of any opening in the wall.  The maximum unrestrained opening height is the vertical dimension of the tallest opening in the wall, expressed as a fraction of the total shear wall height (h). Openings may be located vertically between two sheathing elements (windows), between a sheathed element and the floor (doors), or between the top of the wall and the floor (large doors, or windows and doors without structural sheathing above/below openings).   Any areas of the wall that are not sheathed with structural panels such as plywood or OSB are considered unrestrained openings.  The PSW method makes the conservative assumption that all unrestrained openings in the wall have the same height as the maximum vertical opening dimension in the wall.

           Once the shear capacity adjustment factor (Co) is determined, it is used to calculate the allowable shear capacity of the perforated shear wall (VPSW) as follows: 

VPSW = Co v Sbi

 
 


Topics of this module include:

IntroductionLoad Path, Segmented Design Method, UBC Design Table, Wall Shear, Dimension Ratios,
  Chord Design,  Anchorage, DeflectionMethod Comparison, Shearwall Failures

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