Evaluation of Damping Modification Factors in Codes with a Consideration of Effect of Earthquake Magnitude

Abstract:

Damping modification factors (DMF) are used in current codes to adjust elastic response spectral values corresponding to 5% of viscous damping to other damping levels. However, the impact of moment magnitude (Mw) is not considered in codes. The DMF in the NEHRP 2003 and Eurocode 8 are derived from displacement response spectra while those in the China 2010 are obtained from absolute acceleration response spectra. Based on the DMF for different moment magnitude ranges computed from seismic records located at the same site class, except for site Class E, statistical analysis of results shows that the median DMF for Mw>6.5 derived from displacement response spectra are close to those values in NEHRP 2003 and Eurocode 8 while those derived from absolute acceleration response spectra are similar to those values in China 2010. However, the DMF for Mw=5.5~6.5 are obviously different from those values in codes, further, the DMF in codes are probably unsafe. Besides, for site Class E, the DMF in codes are in most cases slightly conservative. Finally, nonlinear regression analyses are used to develop expressions for DMF corresponding to various site classes and moment magnitude ranges.