Modeling of soil heterogeneity and its effects on seismic response of multi-support structures

This paper addresses the analytical evaluation of soil lateral heterogeneity effects, especially the random fluctuations of the soil layer's predominant frequency, on the spatial coherency of ground motion and the seismic response of multi-support structures. A coherency probabilistic model is proposed. In this model, the spatial variation of motion is attributed to wave passage effects, effects of loss of coherence in the bedrock motion and particularly site response effects (based on the assumption of vertically propagating shear-waves through a horizontal layer with random characteristics). The results indicate that soil lateral heterogeneity effects tend to cause diminution of the values of the total coherency function. This diminution is not limited to the vicinity of the mean resonant frequency of the layer, but reaches considerably high frequencies even for relatively low values of coefficient of variation (CV of 5 to 15%). Therefore, the trend of the total coherency function (exponential decay) can be influenced significantly by site effects. Finally, the proposed coherency model is applied for two different support seismic excitations. Study results indicate that the greater the soil heterogeneity, the larger are the dynamic displacements and shear forces in the columns of the oscillator (i.e., support structure). Furthermore, these two components of the response are influenced differently by soil heterogeneity effects.