Seismic code provisions are now adopting performance-based methodologies, where structures are designed to satisfy multiple performance objectives. Most codes rely on approximate methods to predict the desired seismic demand parameters. Most of these methods are based on simple SDOF models, and do not take into account neither MDOF nor degradation effects, which are major factors influencing structural behavior under earthquake excitations. More importantly, most of these models can not predict collapse explicitly under severe seismic loads. This study presents a newly developed model that incorporates degradation effects into seismic analysis of MDOF structures. A new energy-based approach is used to define several types of degradation effects. Collapse under severe seismic excitations, which is typically due to the formation of structures mechanisms, was modeled in this work through the degrading hysteretic structural behavior along with P-Delta effects due to gravity loads. The model was used to conduct extensive statistical dynamic analysis of different structural systems subjected to a large set of recent earthquake records. To perform this task, finite element models of a series of generic MDOF structures were developed. An ensemble of recent earthquake records was used in the work, and a variety of degrading MDOF structures that cover a wide range of periods, yield values, and levels of degradation were considered. For each MDOF structure, collapse was investigated and inelastic displacement ratios curves were developed in case collapse doesn’t occur. In addition, seismic fragility curves for a collapse criterion were also developed. The findings provide necessary information for the design evaluation phase of a performance-based earthquake design process.
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