Developing fragility curves for Conventional and Base-isolated buildings

Nallagatla, Bhagya Sree (2020) Developing fragility curves for Conventional and Base-isolated buildings. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

Base isolation of the building is a widely accepted and practiced technique for improving the seismic performance of buildings. Using isolators at the base of buildings reduces base shear and interstorey drift demand of buildings making the building less vulnerable to earthquakes. Fragility curves of the building are plots of probability of exceedance of damage state of a structure for Intensity Measure (IM) of earthquake. These curves form an essential component in seismic reliability and risk assessment of structures. Currently, researchers are moving towards developing the fragility curves of base-isolated buildings and using them for making reliable decisions of retrofitting structures. In this thesis, the differences in the fragility of fixed building models and base-isolated buildings are studied along with the failure of isolators. To this aim, an isolation system with Lead rubber and Flat slider bearings is designed for Indian structural design code-compliant three and seven storey buildings. A separate design methodology is made for the isolation system. This design methodology requires fixing some design parameters of isolator, remaining parameters are varied until isolator design satisfies some predefined constraints. With this developed design methodology, isolator design options are populated, and some selective design options are taken for study. It is observed that the number of design options of isolation system and Factor of safety of buckling (For lead rubber base isolator) decreases with a decrease in stiffness of the isolator stiffness. For studying the effect of degradation of the isolator, the variation of buckling load capacity and stiffness of isolator, Lead rubber bearings are modeled with state of the art LeadRubberX element in OpenSees. Incremental Dynamic Analysis (IDA) is done with two ground motion suites (The first suite contains only pulsetype ground motions, and the second suite includes equal number of non-pulse-type pulse-type ground motions). Fragility curves are developed for two extreme damage measures. The effect of pulse-type ground motions and degradation of isolators on the fragility of buildings is studied in this work. Apart from developing fragility curves of buildings, different modes of failures of isolators are plotted, and an attempt made to find a trend in isolator failure. Buckling of isolators is found to be predominant when variation in stiffness, buckling load, and strength degradation of the isolator are considered in isolator modeling. Shear failure of isolators is prominent when degradation of isolators is not considered. Pulse ground motion set causes high responses in base-isolated buildings, increasing the fragility of buildings. This effect is more pronounced when the degradation effect of the isolator is accounted. Isolators are failing at very low drifts of the superstructure, implying the need for designing reliable isolation systems.

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