Peak seismic demands on soft and weak stories models designed for required code nominal strength

Abstract

The performance of buildings with soft or weak stories in past and recent strong earthquakes has been unacceptable. Nevertheless, this structural vertical irregularity is still frequently used in new or modern buildings in seismic regions because it is an architectural solution in urban areas for parking and retail purposes. Modern seismic codes have some special provisions for the design of such structures, but some of them have not been formally evaluated. Therefore, in this paper a parametric study where two-degree of freedom (2DOF) simplified models are used to represent structures likely to develop soft or weak stories is presented. Different stiffness and strength balances were considered to define an ample range of structures with the potential to develop soft stories. 338 models were studied for all the considered stiffness and strength combinations which represent typical building structures with soft or weak stories currently built in the lakebed zone of Mexico City. Step by step nonlinear dynamic analyses were conducted using 10 artificial acceleration records generated to be compatible with the design spectrum for zone III-a of 2004 Mexico's Federal District Code, corresponding to soft soil conditions in Mexico City. Peak ductility demands, story drifts, drift concentration factors and residual drifts were obtained. The results obtained from this parametric study mostly confirm that a soft story mechanism may be triggered or prevented with a right combination of strength and stiffness balances for the structural system, which are close to what it is currently proposed in Mexican seismic building codes.