Progressive Collapse of Damaged Structures Under Seismic Excitation

Abstract eng:
During earthquake, it is quite often seen that two similar structures (building frames) constructed at the same time and at the same location behave differently; one may completely collapse and the other may remain standing. One of the possible causes is that a progressive collapse might have initiated by the earthquake for one, while for the other the progressive collapse is arrested at a certain damaged state. The initiation of progressive collapse in a structure is largely due to concentration of localized damages. In the present study, progressive collapse of a damaged building frame is investigated under seismic excitation. A ten story building frame is artificially damaged to create different damage scenarios having the same total damage expressed in terms of the total reduction of plastic moment capacities of beams and columns. Plastic analysis is carried out for each damage scenario in order to obtain the plastic moment capacities of damaged members by minimizing the collapse load factor subject to constraint that complete collapse mechanism is formed. This is achieved by using genetic algorithm. Push over analysis is carried for all cases, and the behavior of buildings at different stages of inelasticity including the final collapse are studied. The seismic demand curves represented by response spectra is superimposed on push over curves to find different performance points. The peak ground accelerations (PGA) of demand curves are adjusted to obtain the performance points to coincide with the collapse state of the capacity curves. The damage scenario, for which the capacity curve provides the performance point with lowest value of PGA, is most vulnerable to earthquake. The results of the study show that i) there exists certain localized damage states for a building frame which may trigger progressive collapse leading to complete failure under an earthquake; ii) it is possible to identify these localized damage states using plastic analysis, genetic algorithm and push over analysis; iii) for an earthquake with a certain PGA level, similarly constructed buildings may have different progressive damages depending upon their initial damage states; iv) if the damage state of the building can be evaluated beforehand, vulnerability of building to collapse, under future earthquakes can be predicted using the methodology presented here.

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