The roofs and floors of timber houses are often forced to behave as horizontal diaphragms because the shear force from the upper storey are transferred to the walls of the first storey, particularly when the second storey is set back from first storey. A large number of timber houses collapsed in the Hyogoken-Nanbu earthquake in Japan in 1995, because of the lack of shear capacity of their roofs. The seismic design procedure for the torsional effects on a structural wall systems connected by a ridged horizontal diaphragm has been proposed by T. Paulay. A new design procedure which extends this method the shear transfer to a ductile wall system via flexible diaphragms is presented in this paper. The method compiles the shear stress in each diaphragm and the lateral forces carried in each wall are computed by superimposing the local diaphragm shear angles on the overall twist of the floor computed assuming a rigid diaphragm. This simple design method is verified by using inelastic dynamic analyses.
