Practical Analysis for Horizontal Diaphragm Design of Wood-Frame Single-Family Dwellings

Abstract

Seismic design of wood-frame single-family dwellings’ (WFSFD) lateral force–resisting systems requires determination of the stiffness of horizontal diaphragms and shearwalls. During design, sizes and locations of shearwall openings are often changed, altering shearwall stiffness and loads and requiring a significant redesign effort. Rigid and tributary area method analyses are examined for different geometries of L-shaped WFSFD and include stiffness reductions for roof geometry and pitch. These methods are applied to historic earthquake damage reports and compared using rigid, semirigid, or flexible horizontal diaphragm analyses useful in design practice. Most WFSFD should be designed using an envelope method because they contain a mix of horizontal diaphragm types owing to the effects of roof pitch and geometry on diaphragm stiffness. Cases occur where determination of semirigid or flexible diaphragm behavior is difficult because the analysis results are contradictory or unclear. This suggests that semirigid modeling or an envelope method is prudent. The use of simple rigid plate, flexible plate or semirigid plate methods can be practical for analyzing WFSFD with a reasonable level of detail and effort.