Design Factors

The following factors are applied to shear capacities listed in SDPWS Tables 4.3A, 4.3B, 4.3C and 4.3D:

Wind Increase Factor

This factor arises from differences in the ratio between allowable strength to ultimate strength for wind vs. seismic design, which is the safety factor in allowable stress design. Seismic design uses a higher safety factor because wind loads are unidirectional - the wind blows in one direction and the structure recovers, whereas seismic loads have a reverse load component in the ground motion oscillations. The published shear tables that were originally implemented in Shearwalls were based based upon the seismic safety factor, and wind increase factor was applied according to decisions made by the code authorities.

The wind shear strength increase factor is incorporated in SDPWS Tables 4.3A, 4.3B and 4.3D such that the wind capacities are 1.4 times the seismic values, rounded to the nearest 5 plf. Shearwalls uses the seismic values and multiplies by 1.4, without rounding, so the capacities are slightly different than those listed in the table.

ASD Factor

According to 4.3.3, all unit listed in Tables 4.3A-D are to be divided by the ASD reduction factor of. 2.0 All capacities listed in the Shear Resistance and Provisions by Sheathing Material Type topics have had the reduction factor applied, so are half of those listed in SDPWS.

Specific Gravity Adjustment Factor

Design values for wood structural panels are based on sheathing nailed into Southern Pine or Douglas Fire framing. For other materials, a specific gravity factor of 1 - ( 0.5 - G) is applied, where G is the wall stud specific gravity from NDS Table 12.3.3.A. It is expressed in this form rather than 0.5 + G to indicate that it cannot be greater than 1.

Aspect Ratio Factor

Aspect ratio factors are applied to wood structural panel and fiberboard shear wall segments and force transfer piers that are within specified ranges of aspect ratios h/b_s, where b_s = segment length and h = wall height. The factors depend on material type and to whether deflection-based or capacity-based load distribution is used.

For perforated walls, these factors are not applied directly, instead the lengths of the wall segments are factored by 2b,/ h in the calculation of the Shear Capacity Adjustment Factor C_o . This has the effect of decreasing C_o . which is applied to the shear capacity.

Refer to Aspect Ratio Limits and Factors for the factors and ranges of h/b_s to which they apply.

Unblocked Factor C_ub

SDPWS Table 4.3.3.2 lists factors C_ub applied to unblocked shear walls, which depend on the nail spacing in intermediate framing members ("field spacing") and on the stud spacing, and also specifies that the capacity for 24" stud spacing and 6" edge nail spacing be used regardless of the actual spacings when walls are unblocked. Refer to Unblocked Factor for more details.

Perforated Shear Wall Shear Capacity Adjustment C_ub

Refer to Shear Capacity Adjustment Factor for the

Summary of Factors

The table below gives the value of these factors, the reference in SDPWS and and whether they pertain to the major classes of shear wall materials. The column Capacity-based Force Distribution indicates whether capacity-based force distribution of design shear force to shear resisting elements within a line by relative shear capacity is allowed. Otherwise distribution must be done such as to equalize deflection of all elements.

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