SEISMIC FRAGILITY ANALYSIS OF DEGRADING STRUCTURALSYSTEMS

摘要

Seismic fragility of a system describes the probability of the system to reach or exceed different degrees ofdamage, including possible collapse. These curves serve to predict the damage level and possible collapseof a structure due to a specific earthquake ground motion. Earlier work focused on developing seismicfragility curves of systems for several values of a calibrated damage index. A damage index is a factor thatrepresents the degree of damage of the structure, and typically ranges from 0 to 1, with the value of 1representing complete collapse. Collapse was therefore expressed implicitly as the state of the structurewhen its damage index approaches a value of 1. This research work focuses on developing seismicfragility curves for a collapse criterion, in an explicit form. To perform this task, new degradingconstitutive material models that represent collapse explicitly are developed. The newly developed modelstake into account strength softening, defined as the material degradation in strength after reaching its fullcapacity under static loading, and cyclic degradation in strength and stiffness under repeated reversedloading. Collapse is defined when the system completely loses its full capacity. Several degrading modelswere developed, a bilinear model, a modified Clough model, and a pinching model. An energy-basedcriterion is used in all models to estimate strength and stiffness degradation. The previously describeddegrading models were used to conduct the fragility study. An ensemble of recent earthquake records wasused in the work, and a variety of degrading systems that cover a wide range of periods, yield values, andlevel of degradation were considered. Several fragility curves are developed for a collapse criterion, andconclusions are drawn for each case. The newly developed fragility curves represent a major advancementover damage index-based fragility curves.