DAMPING MODIFICATION FACTORS INCLUDING SOIL-PILE KINEMATIC INTERACTION

The seismic hazard at a site is usually quantified by means of 5%-damped pseudo-acceleration response spectrum. Thus, for structural damping ratios other than 5%, it is necessary to apply period-dependent Damping Modification Factors, which adjust spectral ordinates to the desirable structural damping ratio. On the other hand, piles, depending on their stiffness, may modify the seismic excitation at the base of a structure with respect to the free-field motion, by filtering the seismic demand imposed by the soil. This phenomenon, which is known as kinematic soil-pile interaction, is also period-dependent and it stems from the inability of piles to follow short wavelengths of the free-field motion. The problem is explored by means of a rigorous Finite Element model of a single, fixed-head pile embedded in a generalized inhomogeneous soil. Average spectral acceleration ratios between the pile-head and the ground surface at free-field conditions are derived for various soil-pile configurations and structural damping ratios. The effect of critical model parameters is discussed, and a new spectral reduction factor is defined which relates the acceleration response spectrum at the pile-head for any structural damping ratio to a 5%-damped acceleration spectrum at the free-field.