Impact of diaphragm flexibility on dynamic properties and seismic design parameters of irregular buildings in plan

Abstract

One of the established conditions of structural regularity in modern Mexican seismic codes is that a rigid and strong diaphragm has to be provided at each story. However, this condition is one of the least rigorously reviewed in the design of buildings in Mexico. During the past 15–20 years, a modern version of flatslab floor systems are being dominantly used in new building construction in Mexico City. However, their ability to transmit and carry adequately important seismic loads has not been demonstrated. In this paper, the authors present a study where two buildings irregular in plan using a cast-in-place flatslab system lightened with large Styrofoam (EPS) blocks were designed according to Mexican codes as done by most engineers: assuming a rigid and strong diaphragm. Then, the diaphragm flexibility was evaluated using detailed finite element models, as well as equivalent solid slabs with orthotropic modeling and struts models to account for the diaphragm flexibility. It was found that when modeling the diaphragm flexibility, lateral displacements increased from 2.1 to 3.4 times those of the rigid diaphragm assumption. When modeling the diaphragm flexibility, the natural periods of vibration increased, due to the decrement of the effective lateral stiffness for the buildings. It was found that the effective lateral stiffness for the buildings decreased by 54.3% for a subject 8-story confined masonry building and by 66.7% for a subject 15-story RC moment frame building. Switching of dominant mode shapes between the rigid and the flexible diaphragm assumption was also observed.