Effect of torsional ground motion on seismic response of symmetric multistorey buildings incorporating soil-structure interaction

Effect of torsional ground motion is considered in the seismic codes indirectly by considering accidental eccentricity which also includes uncertainties in storey masses and stiffnesses of structural elements of the buildings. However, there is no explicit mention of the effect of torsional ground motion on the response for different types of buildings including soil-structure interaction (SSI), as it is reported in the literature that torsionally flexible low-rise buildings are susceptible to torsional ground motion. A systematic study has been carried out on the effect of torsional ground motion including SSI on the response of torsionally flexible through torsionally stiff multistorey buildings. The effect of torsional ground motion resulting from the propagation of motion through different soil mediums: firm soil, medium-firm soil and soft soil is reported in the study. For these soil mediums, two cases of ground motion: (i) translation ground motion only and (ii) combined translation and torsional ground motion are considered. A set of multistorey buildings with fundamental period of vibration T = 0.3 s and T = 0.8 s having frequency ratios omega = 0.65, 1.0 and 1.5 (omega = ratio of uncoupled fundamental torsional frequency, omega(theta) to uncoupled fundamental translational frequency, omega(t)) is considered. These chosen buildings cover practically the entire spectrum of torsionally flexible and torsionally stiff low-rise buildings. Frequency domain analysis has been carried out to incorporate SSI effects. The results obtained from the frequency domain have been transformed into the time domain using Fast Fourier Transform (FFT) and the storey shears obtained from ten earthquake time histories (translation and torsional) have been averaged. It is found that the torsional ground motion effect is significant in the case of torsionally flexible buildings with low periods of vibration and the effect of SSI is insignificant in all types of buildings. Further, the combined effect of torsional ground motion and soil-structure interaction on the distribution of storey shears along the height varies significantly for high period buildings (T =0.8s).