Evaluation of soil-structure interaction effects on the damping ratios of buildings subjected to earthquakes

This paper evaluates the effects of soil-structure interaction on damping ratios of buildings subjected to earthquake ground motions. Explicit expressions of transfer functions of absolute horizontal accelerations of multistory buildings on an elastic half-space subjected to horizontal ground motions are developed. An optimization procedure is then used to obtain the effective modal properties of a replacement or equivalent fixed-based multi-story building that minimize the difference in the ordinates of the absolute acceleration transfer functions in the building on flexible-base relative to those in the replacement fixed-base building. A parametric study was conducted to study the variation of effective modal damping ratios of the fundamental mode and of higher modes of vibration with changes on the wave parameter. Results indicate soil-structure interaction effects may either increase or reduce the effective modal damping ratio of the fundamental period. Typically, it reduces the effective damping ratios of slender long period structures such as tall buildings and increases the damping ratio of short and medium period structures. It is shown that the reduction of effective modal damping ratio of the fundamental mode with increasing height computed in this study follows a similar trend to that observed in instrumented buildings, indicating that the reduction in damping ratio observed in buildings as their height increases is primarily due to soil-structure interaction effects. Furthermore, it is shown that soil-structure interaction effects lead to an approximately linear trend in effective modal damping ratios with increasing modal frequency.