Response of tall buildings with symmetric setbacks under Earthquake loading

Abstract

Earthquakes are one of nature’s greatest hazards to life. Reinforced concrete structures built in zones of low seismicity such as Sri Lanka have not had seismic effect taken into consideration in the design. The seismic performance evaluation and upgrading for non-seismic designed building structures is the most urgent issue for seismic hazard mitigation. The behaviour of a building during earthquakes depends critically on the shape, size and geometry of the building. A setback is a common geometric irregularity consisting of abrupt reduction of floor size in multi-storey buildings above certain elevations. Setbacks usually arise from urban design demands for illumination and aesthetic requirements. Computer program simulations are very valuable in testing a wide range of building types especially under disaster loads which are difficult and economically not viable to analyse with experimental methods. This paper explores three-dimensional nonlinear dynamic responses of a typical tall building under earthquake loading, with and without setbacks. These 20 storey reinforced concrete buildings were first designed for normal (dead, live and wind) loads. The influence of the setbacks on the lateral load response due to earthquake loading in terms of peak deflections, accelerations, inter-storey drift and bending moments at critical locations (including hinge formation) was then investigated. Structural response predictions were performed with a commercially available three-dimensional finite element analysis program using non-linear direct integration time history analyses. Results obtained for buildings with different setbacks are compared and conclusions made. It is revealed that the detrimental effects for seismic response due to symmetric setbacks are not that significant, for PGA values of 0.1g to 0.15g. Abrupt changes in moments and shears are experienced near the levels of the setbacks. Further it is revealed that there needs to be a balance between the stiffness and mass of the building to get the optimum response under seismic loading. Finally, these analyses give an indication of the integral seismic resistance of typical reinforced concrete wall-frame structures although specifically not designed for seismic loading. This is of significance to Sri Lanka, a zone with low seismicity and having many urban high-rise buildings that are not originally designed for seismic loading.