Influence of symmetric setbacks on the performance of high-rise buildings under blast and earthquake loading

Abstract

. With the increase in terrorism worldwide, blast loading has received considerable attention. Blasts due to bomb explosions have become a new threat to buildings designed for normal static loads in Sri Lanka. Earthquakes are generally considered to be among the worst natural disasters on Earth. 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. As such it is prudent to undertake detailed studies to better understand the behaviour of typical high-rise buildings in Sri Lanka under these two extreme events. Computer simulation has become an efficient tool in the analysis of structures under extreme loads. This study explores three-dimensional nonlinear dynamic analyses of a typical high-rise building in Sri Lanka under blast or earthquake loadings, with and without setbacks. 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. These 20 storey reinforced concrete buildings have been designed for normal (dead, live and wind) loads. The influence of the setbacks on the lateral load response due to blasts and earthquakes in terms of peak deflections, accelerations and bending moments at critical locations (including hinge formation) is investigated. Structural response predictions were performed with the finite element analysis program SAP2000, using non-linear direct integration time history analyses. Results obtained for buildings with different setbacks are compared and conclusions drawn. From the comparisons it is revealed that buildings having symmetrical setbacks that protect the tower part above the setback level from blast loading show considerably better response in terms of peak displacement and inter-storey drift, when compared to buildings without setbacks, while the detrimental effects of symmetric setbacks for seismic response are not that significant. 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. From the analyses it is revealed that these twenty storey tall buildings with shear walls and frames that are designed for just normal loads perform reasonably well, without catastrophic collapse, when subjected to a blast that is equivalent to 500 kg TNT at a standoff distance of 10m, and also when subjected to a seismic excitation having a PGA of 0.1 g. The study helps to understand the relative performance of buildings with different setbacks, designed for normal static loads subjected to blast and earthquake loadin