Influence of Out-of-Plane Behavior of Floor Diaphragm on Lateral Load Distribution

The validity of the rigid diaphragm assumption in building structures has mainly been discussed in relation to lateral load distribution, considering the relative stiffness of a floor diaphragm (hereafter, diaphragm) to the lateral force-resisting system (LFRS). This analytical study shows that load distribution can be amplified for the central LFRS elements in diaphragms generally considered rigid. This amplification is due to nonlinear variation in the end-fixity of the central and outer LFRS elements, and is referred to as the end-fixity-inequality effect in this study. Coefficients that reflect the end-fixity-inequality effect were introduced into the modified equivalent beam-on-spring model (MEBM) proposed in a previous study, and the model accurately predicts the location and magnitude of the load amplification. Amplifications of approximately 17% for diaphragms with three rows of LFRS elements and approximately 12% for diaphragms with four rows of LFRS elements, relative to the rigid diaphragm case, were confirmed in a regular flat-plate single-story structure. The corresponding amplification factor remained nearly constant as the aspect ratio varied, provided that the number of LFRS elements remained the same. Furthermore, for a fixed number of LFRS elements, the location of the local maximum value remained nearly constant with respect to the beam-to-column stiffness ratio as the aspect ratio varied. Contrary to the common belief that the load distribution in a concrete diaphragm can be calculated by assuming a rigid diaphragm, structural design that accounts for the end-fixity-inequality effect is required to achieve conservative result.