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 re sponse for structures with flexible diaphragms and counterpart structu res with rigid diaphragms. Case studies of three existing buildings wi th flexible diaphragms and analogous systems with rigid diaphragms are presented to illustrate these differences. Each building was subjecte d to the 1989 Loma Prieta Earthquake. The structures were: (1) A two-s tory firehouse in Gilroy with unreinforced masonry walls; (2) a two-st ory timber office building in Pale Alto with grouted and reinforced cl ay-unit masonry shear walls; and (3) an eight-story hotel in Oakland w ith 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 in creases. Further, approximate expressions prescribed in current seismi c codes can underestimate the period of vibration of systems with flex ible diaphragms.