Seismic fragility analysis of irregular buildings under bi directional earthquake

Abstract

Seismic demand of structures may be potentially exaggerated due to bi-directional shaking which is strongly influenced by the local site condition. Further, challenging situation may rise if structure rests on sloping ground. It is to be mentioned that, seismic structural fragility constitutes an important step for performance based seismic design. This paper deals with reliability-based fragility analysis of various types of vertical and plan irregular buildings situated on different types of soil. In addition to that, isolated foundations situated on various uniform and layered soils and sloping ground by using finite element software Abaqus. Uniform soil types include hard, medium and soft soils whereas layered soil type is based on shear wave velocity variation through depth. In this study soil-structure interaction have been considered with near field and far field bidirectional earthquakes (Denali Alaska earthquake and Loma Prieta Santa Cruz Mountains earthquake). It is to be noted that, by studying the characteristics of different properties of the accelerograms across orientations, combination of structural responses has been carried out by 30% and 40% rules as per AASHTO 1998 and ATC 1996; NCHRP 2001. For studying the effect of pile foundation, analysis has been carried out using SeismoStruct. Results reveal that spectral acceleration and characteristic length of Loma Prieta and Denali earthquake follow desired characteristics over different orientations. Also, building is subjected to an earthquake motion situated on hard soil, it has the least probability of exceeding a limit state. However, it has the largest probability of exceeding a limit state when located on soft soil. In case of most G+9 building configurations, the probability of exceedance of a specific limit state for that soil-structure system is less when it is subjected to a near-field earthquake. Exceptionally, for vertically irregular building, the probability of exceedance value of the soil-structure system will be greater under the influence of near-field earthquake. Further, highest probability of exceedance value of the soil-structure system has been observed in case of building on sloping ground as compared to building on plane ground in case of far field earthquake. On the contrary, for near field earthquake, lowest probability of exceedance value of the soil-structure system has been noted in case of building situated on sloping ground as compared to building situated on plane ground. Furthermore, in case of G+9 building having pile foundation, has the largest probability of exceeding a limit state when located on medium soil for both far and near field earthquake.