Pushover Analysis Of R/C Setback
Bulding Frames

Abstract

The behaviour of a multi-storey framed building during strong earthquake motions depends on the distribution of mass, stiffness, and strength in both the horizontal and vertical planes of the building. In multi-storeyed framed buildings, damage from earthquake ground motion generally initiates at locations of structural weaknesses present in the lateral load resisting frames. Further, these weaknesses tend to accentuate and concentrate the structural damage
through plastification that eventually leads to complete collapse. In some cases, these weaknesses may be created by discontinuities in stiffness, strength or mass between adjacent storeys. Such discontinuities between storeys are often associated with sudden variations in the frame geometry along the height. There are many examples of failure of buildings in past earthquakes due to such vertical discontinuities. Irregular configurations either in plan or elevation were often recognised as one of the main causes of failure during past earthquakes. A common type of vertical geometrical irregularity in building structures arises from abrupt reduction of the lateral dimension of the building at specific levels of the elevation. This building category is known as the setback building. Many investigations have been performed to understand the behaviour of irregular structures as well as setback structures and to ascertain method of improving their performance. Pushover analysis is a nonlinear static analysis used mainly for seismic evaluation of framed
building. Conventional pushover analysis outlined in FEMA 356:2000 and ATC 40:1996 is limited for the buildings with regular geometry. It may not be possible to evaluate the seismic performance of setback building accurately using conventional nonlinear static (pushover)analysis outlined in FEMA 356:2000 and ATC 40:1996, because of its limitations for the irregular structures with significant higher modes effects. There is no research effort found in
the literature to use this analysis procedure for setback building. It is instructive to study the performance of conventional pushover analysis methodology as well as other alternative pushover methodologies for setback buildings and to suggest improvements suitable for setback buildings. In the present study an improved procedure for estimating target displacement of setback buildings is proposed. This proposal is a simple modification of the displacement coefficient method as outlined in FEMA 356: 2000. A parametric study is also carried out to understand the applicability of existing lateral load patterns on the pushover analysis of setback building. It is found that mass proportional uniform load pattern is most suitable amongst others for pushover analysis of setback buildings. The results of the study show that pushover analysis carried out by mass proportional uniform load pattern and proposed modification in target displacement estimation procedure consistently predicting the results close to that of nonlinear dynamic analyses.