Torsional response of seismically isolated structures revisited

The torsional response of seismically isolated structures is revisited. Experimental results on the torsional response of seismic isolated structures in the past have been conducted with unrealistically torsionally stiff seismic isolation systems. It is shown in this paper that the measured torsional amplification ratios correspond to accidental eccentricities of about half of the code-described value of 5-percent of largest plan dimension. Current practice to calculate the amplification factor for the displacement demand due to torsion is discussed and a rational and simple improvement for predicting these amplification factors is presented. The revised equations are useful in predicting the torsional response of torsionally stiff seismic isolation systems and in assessing the utility of shake table experimental results on the basis of scaling principles.