Evaluation of damping modification factors for seismic response spectra

Seismic response spectra with structural damping ratio other than nominal 5% (of critical damping) are essential for the design and evaluation of structures in performance-based seismic engineering. Such response spectra are also essential for the design and evaluation of structures with seismic isolation and energy dissipation systems. A number of formulations for damping modification factors (DMF) have been proposed in the literature for scaling the 5% damped response spectra. Dependence of the DMF on several ground motion parameters has also been identified. Few seismic design codes have already incorporated simplified DMF based on these studies. This paper critically reviews the available formulations for DMF for seismic response spectra. Analytical investigations on the ground motion response spectra at soil sites, based on a wide range of simulated ground motion records, have been carried out. It has been observed that the DMF for ground motion response spectra at soil sites is significantly dependent on site period, which has not been identified in previous studies. The influences of earthquake shaking level, earthquake source-site distance (nearfield and far-field events), soil plasticity index, and the rigidity of bedrock have also been investigated.