Mainshock and aftershock effect on the performance of code designed steel plate shear walls

Earthquake events are typically composed of foreshocks, a mainshock and aftershocks.\ud Mainshock events release the largest amount of energy and often cause most of the structural damage. Once the structure is damaged by the mainshock event its capability to resist another ground motions is largely reduced. In this thesis the performance of the code designed steel plate shear walls (SPSWs) is evaluated by subjecting a SPSW system to series of nonlinear response analyses. The infill plates and the boundary elements are sized using the capacity design principle and an analytical model is used to check the inter-story drift code requirements. It is found that designing the SPSW system using the current code provisions satisfies the code maximum inter-story drift requirements, but it does not give an insight in the nonlinear behavior of infill plates and boundary elements. In order to better understand the nonlinear behavior of the SPSW system nonlinear time history analyses using the tension strip model for the infill plates are performed. The nonlinear model is validated with the experimental results from Driver et al. (1997; 1998) of a four story quasi-static cyclic testing of a single bay SPSW specimen. Upon validation, an analytical strip model of an 8-story SPSW is developed and analyzed using OpenSees (McKenna, et al., 2006). The 8-story SPSW is subjected to forty median and forty 84 percentile mainshock-aftershock pairs of ground motions using earthquakes developed for the SAC project (Somerville, et al., 1997). Each pair consists of a 2% probability of exceedance in 50 years mainshock ground motion and a 10% probability of exceedance in 50 years ground motion as aftershock. In addition to the nonlinear time history analysis, a nonlinear static pushover analysis is also performed to quantify the damage state thresholds. The nonlinear analysis results show that demands in vertical boundary elements in design level shaking are considerably less than those from the capacity design. It is also found that steel plate shear walls designed meeting current code requirements satisfy maximum interstory drift requirements considering design level earthquakes and have maximum interstory drifts of less than 4% for maximum considered earthquakes. When subjected to a sequence of mainshock-aftershock ground motions it was found that once the SPSW is damaged by the mainshock earthquake the aftershocks do not significantly increase the peak nor residual drift demands