Multi-storey buildings with asymmetric setbacks possess both vertical and in-plan irregularities. The former is due to the abrupt changes in the plan dimension at the level of the setback while the latter results from the asymmetry between the upper tower portion and the lower base portion. These irregularities introduce mass, stiffness and resistance eccentricities in the structure. As a result, the structural behaviour under lateral seismic excitation in the direction perpendicular to the setback is coupled in translation-torsion, which could increase the demand on the structural response. This coupling behaviour has not been studied in detail, and its effect is largely ignored by many conventional seismic design methods. In this research, the effects of both the degree and level of setback on the seismic response of multi-storey building frames were evaluated. The degree of setback refers to the ratio of the plan area of the lower base portion to the plan area of the upper tower. An analytical study was carried out to identify the governing parameters of the structural behaviour and their effects on the overall response of setback buildings. A total of nine different configurations were examined. The study showed that both the degree and level of setback have a considerable influence on the overall seismic behaviour of asymmetric setback buildings loaded in the direction perpendicular to the setback.
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