Seismic behavior of structures with flexible diaphragms

The influence of floor flexibility on the seismic response of building structures is discussed through comparison of the computed seismic response for structures with flexible diaphragms and counterpart structures with rigid diaphragms. Case studies of three existing buildings with flexible diaphragms and analogous systems with rigid diaphragms are presented to illustrate these differences. Each building was subjected to the 1989 Loma Prieta Earthquake. The structures were: (1) A two-story firehouse in Gilroy with unreinforced masonry walls; (2) a two-story timber office building in Palo Alto with grouted and reinforced clay-unit masonry shear walls; and (3) an eight-story hotel in Oakland with unreinforced clay-unit masonry and reinforced-concrete shear walls. The analytical studies show that, in some cases, diaphragm and shear-wall accelerations can increase with the flexibility of the diaphragm. Torsional forces can reduce considerably as diaphragm flexibility increases. Further, approximate expressions prescribed in current seismic codes can underestimate the period of vibration of systems with flexible diaphragms.