More and more applications exist of base-isolated buildings. So far much attention has been paid to the improvement of design approaches. In this paper, the equation of motion is derived for a torsionally coupled base-isolated structure under the action of seismic ground motion. Based on numerical analyses of the structure, torsional responses and influencing parameters have been investigated and some rules obtained for sites with different soils, in which the superstructure is idealized as a series-rigid floor model, while the isolation system adopts the Bi-linear resilience curve. Applying the method of least squares to the numerical results, three simplified formulae have been produced to approximately calculate the torsional seismic accelerations of base-isolated structures. The results calculated by the practical method proposed in this paper approximate to those obtained by time history analyses. It is shown that the method is of good accuracy in comparative terms and suffices for the needs of engineering designs for base-isolated eccentric structures with rubber bearings under earthquake action.
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