SEISMIC VULNERABILITY OF BUILDINGS 4 LOCATED ON HILL SLOPES

Abstract

The frontier slates of India which are situated on highly seismically active region of the l-Iimalayas are undergoing rapid development. Further, the building structures in this region are irregular due to resting on hill slopes. The seismic vulnerability of such buildings is higher as compared to buildings resting on flat ground. Absence of design rules for such buildings is creating the need for the present study, where performance of those buildings is to be understood. Therefore, this study is oriented in the direction of more reliable estimation of seismic performance of buildings located on hill slopes. A field survey was conducted, in which it is found that the buildings on hill slopes cannot be classified, for the purpose of seismic vulnerability, just on the basis of the framing and flooring systems. For detailed vulnerability classification of the observed buildings, structural configuration is also to be taken into consideration. Based on the field survey, some generalised model building typologies for hill buildings have been identified. The most common configuration on hill slopes is considered for the detailed analysis. The chosen configuration is designed for two different levels of forces (i.e. for gravity load only and as special moment resisting frame, SMRF). Further, analysis was carried out using Incremental Dynamic Analysis (IDA) procedure. As the hill buildings are torsionally coupled, the IDA has been performed using bi-directional ground motion. Orthogonal components of recorded ground motions are chosen for a wide range of source and site parameters. These components are incrementally scaled for the spectral acceleration of the major component at the fundamental period of the building. The dynamic pushover curves are obtained in terms of spectral acceleration and the peak interstorey drift ratio. It is also observed that in case of torsionally coupled buildings subjected to bi-directional excitation, the peak interstorey drift may or may not occur in the direction of application of the major component. Using the IDA curves, fragility analysis of these buildings is carried out, considering the variability in demand and capacity and modelling uncertainty. In hill buildings the collapse occurs mostly due to shear failure of short columns. In some cases formation of a mechanism due to plastic yielding of a large number of frame members can also lead to failure. The fragility curves for incipient collapse are plotted using the fragility analysis procedure of Wen et al. [21]. It is found that the hill buildings are susceptible to collapse at much lower spectral accelerations and interstorey drifts, as compared to buildings located on flat ground. Further, it is observed that the median interstorey drift for incipient collapse is independent of the design force level. The hill buildings are able to sustain very low levels of seismic intensity, resulting in large down scaling factors for the ground motions observed in case of medium and large magnitude earthquakes. This is in corroboration with the observed damage in hilly areas even for low intensity events.