Design Factors

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

ASD Factor

According to 4.3.3 and note 1 to tables Tables 4.3A-D, all unit shear values listed in those tables are to be divided by the ASD reduction factor of 2.0 for wind design and 2.8 for seismic design. The tables listing shear capacities in Resistance and Provisions by Sheathing Material Type give the ASD-factored capacities for wind and for seismic as they appear in the Shearwall output, and also list the unfactored capacity as shown in the SDPWS tables.

This difference between the seismic and wind factors 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.

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 the simpler 0.5 + G to indicate that it cannot be greater than 1.

Aspect Ratio Factor

Aspect ratio factors from SDPWS 4.3.3.2 and from 4.3.5.5.1 Exceptions 1 and 2. are applied to wood structural panel and fiberboard shear wall segments and FTAO wall piers that are within specified ranges of aspect ratios h/b, where b = segment length and h = wall height. The factors depend on material type and on whether deflection-based or capacity-based load distribution is used. Refer to Aspect Ratio Limits and Factors for the factors and ranges of h/b to which they apply.

For perforated walls, these factors are not applied directly or referred to as Aspect Ratio factors. Instead, the lengths of the wall segments b_i are factored and the ratio of the sum of the adjusted and unadjusted segment lengths is effectively the aspect ratio factor for the wall. Refer to the perforated wall Shear Capacity Adjustment for more details.

For FTAO walls, factors are applied to piers beside or below openings, which are less likely to have a high h/b ratio than full-height wall segments.

Unblocked Factor C_ub

SDPWS Table 4.3.5.3 lists factors C_ub that are applied to unblocked shear wood structural panel shear walls. The value of this factor depends on the nail spacing in intermediate framing members ("field spacing") and on the stud spacing, It 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. The unblocked factor is applied only to the capacity of the sheathing on the side of the wall that is made of wood structural panels and not nailed to blocking. Refer to Unblocked Factor for more details.

Note that FTAO walls require blocking to distribute shear force so that it would be rare to designated an FTAO wall as unblocked.

Perforated Shear Wall Shear Capacity Adjustment C_ub

Designating a shear wall as a perforated shear wall allows you to design the wall without hold-down connections a the sides of the openings, however this comes at a penalty, Refer to Shear Capacity Adjustment Factor for an explanation of the Perforated Shear Wall Shear Capacity Adjustment factor, C_o .

Note that if there are small and/or few openings on the wall, the C_o factor is often 1.0, so it is advantageous to designate such walls as perforated walls.

Hold-downs with 10d Nails

According to note 10 of Table 4.3A, those wood structural panels using 10d common nails, on shear walls for which a hold-down is affixed to the inside of the end post, a factor of 0.92 is applied. Shearwalls currently assumes that there is such a hold-down attached to all walls, and applies the factor to all Structural I and Structural sheathing panels using 10d nails.

This provision was recently added to program with the implementation of SDPWS 2021. Future versions may include a more sophisticated implementation that allows you to specify the hold-down is on the outside of the post, to avoid the factor, and which identifies locations that do not require a hold-down because dead loads completely counteract overturning. The implementation will also take into account current plans to implement tie-down systems./

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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