Special Design Provisions for Wind and Seismic (SDPWS) specifies diaphragm unit shear values for lateral loads due to wind or seismic. What diaphragm unit shear values should be used to resist lateral loads from soil?


Guidance for this issue can be found in the commentary to the Permanent Wood Foundation Design Specification. The PWF commentary states in C5.4.5:

    Since nominal unit shear capacities for shear walls and diaphragms published in the Special Design Provisions for Wind and Seismic (SDPWS) Specification are based on short-term load duration, it is necessary to multiply nominal unit shear capacities for seismic by 0.281. The 0.281 multiplier results from the combination of the 2.0 allowable stress design (ASD) reduction factor and a 0.9/1.6 factor for adjusting from the ten-minute load duration basis for wind and seismic design in the SDPWS to a permanent load duration basis for PWF applications.