ESTIMATING MODAL PARAMETERS FOR A SIMPLE SOFT-SOIL SITE HAVING A LINEAR DISTRIBUTION OF SHEAR-WAVE VELOCITY WITH DEPTH

Simple approximate expressions are derived for estimating equivalent m odal parameters of a single soft-soil layer that has a straight line d istribution of shear wave velocities and is on a flexible half space w hich is more rigid than the overlying soil layer. It is shown that rad iation damping from the flexibility of the half space can be converted approximately into an equivalent material damping and that a site of a single soft-soil layer on a flexible half space can be replaced by a n equivalent single layer, with the same shear wave velocity distribut ion, on a rigid bedrock. The implementation for modal analyses and non -linear analyses of the equivalent site under earthquake excitation is discussed.