For estimating the response spectrum of offshore engineering structures with different damping ratios

the damping modification factor (DMF) model of offshore engineering response spectrum was studied. Based on 5 680 horizontal offshore ground motion records from S-net network

we analyzed the effects of source parameters such as moment magnitude

fault depth

and earthquake type

as well as source distance and sediment thickness on DMF

and proposed a DMF model for horizontal acceleration spectrum from offshore ground motions considering damping ratio and spectral period

which was compared with onshore models. Results show that moment magnitude and source distance had significant impact on DMF. DMF with low damping ratios increased with the increase in moment magnitude and source distance

while DMF with high damping ratios decreased with the increase in moment magnitude and source distance

and DMF was not sensitive to the influence of fault depth and sediment thickness. Compared with the study of onshore area

the DMF values of shallow crustal and upper-matle earthquakes in this study were slightly larger under low damping ratios and medium-long periods. When spectral periods were greater than 0.1 s or less than 0.1 s

the influence of damping ratio on DMF could be simulated by cubic or quadratic logarithmic polynomials

and the influence of spectral period could be expressed by quartic or cubic polynomials. There were significant differences between offshore DMF model and onshore models. The DMF model for response spectrum from offshore ground motions proposed in this paper provides reference for the determination of seismic design spectrum of offshore engineering under various damping ratios.