Is the Rigid-Diaphragm Assumption Conservative? Seismic Response of One-Story Buildings with Flexible Corrugated Steel Deck Diaphragms

Roof diaphragm flexibility can significantly influence the seismic response of one-story buildings by altering their dynamic characteristics, elongating the fundamental period, and changing the distribution of seismic demands within the diaphragm and seismic force-resisting system (SFRS). This study investigates whether the commonly used rigid-diaphragm assumption provides a conservative representation of the seismic response for one-story buildings with flexible corrugated steel roof diaphragms. A set of 36 prototype buildings with fully designed diaphragms is analyzed in both the short and long directions, resulting in 72 building-direction cases. Each case is subjected to 40 ground motions scaled to the Los Angeles design uniform hazard spectrum. The buildings are modeled using a simplified analytical approach that accounts for the in-plane response of the roof diaphragm and are assumed to use buckling-restrained braced frames (BRBFs) as the SFRS. For each ground motion, the elastic base shear is first determined and the corresponding SFRS yield strength is established using a response modification factor of R = 8, consistent with BRBF design. Nonlinear time-history analyses are then performed for each building for two assumptions: (1) a rigid-diaphragm model, and (2) a model that accounts for the in-plane flexibility of the diaphragm. The results show that diaphragm flexibility increases the fundamental period and reduces the elastic base shear demand. Thus, the rigid-diaphragm assumption results in a larger global seismic demand and may initially appear to be conservative. However, the larger base shear does not necessarily result in conservative estimates of internal and nonlinear response demands. Diaphragm flexibility increases in-plane diaphragm bending moments at midspan, diaphragm shear demands near the quarter-span regions, and ductility demands on the SFRS. The results demonstrate that designing based on the larger base shear obtained from a rigid-diaphragm assumption is not always conservative, as diaphragm flexibility redistributes seismic demands and increases local force demands on the diaphragm and deformation demands on the SFRS. The findings highlight the importance of explicitly considering diaphragm flexibility in the seismic analysis and design of one-story buildings with flexible steel roof diaphragms.