Period Dependence of Response Spectrum Damping Modification Factors due to Source- and Site-Specific Effects

Response spectrum damping modification factors are key components of displacement-based seismic design methods. This paper examines the period dependence of damping modification factors as a result of near-source forward directivity, basin-induced surface waves, and surficial soil response by using recorded ground motions from the Canterbury, New Zealand, earthquakes as examples. It is illustrated that spectral peaks in the 5% damped response spectra have systematically different damping modification factors than those suggested by conventional empirical formulas; this is also supported by arguments based on forced vibration theory. Because source- and site-specific effects are increasingly being considered in the development of region- or site-specific design response spectra, this work illustrates the critical need to adequately consider adjustments to damping modification factors to ensure that displacement-based seismic design procedures remain consistent.