Single-storey non-residential buildings form a large proportion of the building inventory of Canada. Such buildings may be used for industrial, commercial, recreational or institutional purposes and often have a large footprint. Their common structural framing is comprised of steel deck roof diaphragm supported by the vertical system that may consist of steel braced frames, masonry or concrete shear walls. Although, wood panels, lumber and other engineered wood products can be used as materials for the diaphragms (roofs) of such buildings, wood has not been the material of choice so far in Canada. One of the reasons is the lack of knowledge of the performance of these, in general very flexible diaphragms, that are sometimes significantly more flexible than the vertical lateral force resisting elements. As a result, the seismic response of such buildings is strongly influenced by the flexibility of the diaphragms and the seismic provisions of codes, which are generally suitable for the design of buildings with rigid diaphragms, are not applicable to the design of buildings with flexible diaphragms. Flexibility of the diaphragm affects the period of the building and hence its response to seismic forces. Flexibility also affects the distribution of shears and moments in the diaphragm.
The present study examines the effect of diaphragm flexibility on the building period and reviews the methods currently recommended for classifying the diaphragm as flexible, rigid, or semi rigid. A simplified method of obtaining a better estimate of the period of a building with flexible diaphragm is also suggested. In addition the effect of diaphragm flexibility on the distribution of shear forces and bending moments is examined through time history analysis of a building with flexible diaphragm subjected to an earthquake ground motion.