Modeling viscous damping in nonlinear response history analysis of steel moment-frame buildings: Design-plus ground motions

Investigated in this paper is the question: Can seismic demands on steel moment-frame buildings due to maximum considered earthquake (MCER) design-level ground motions (GMs) be estimated satisfactorily using linear viscous damping models or is a nonlinear model, such as capped damping, necessary? This investigation employs an enhanced model with several complex features of a 20-story steel moment-frame building. Considered are two linear viscous damping models: Rayleigh damping and constant modal damping; and one nonlinear model where damping forces are not allowed to exceed a predefined bound. Presented are seismic demands on the building due to two sets of GMs: MCER design-level GMs (2% probability of exceedance [PE] in 50 years) and rarer excitations (1% PE in 50 years). Based on these results, we conclude that linear damping models are adequate for estimating seismic demands on steel moment-frame buildings-designed to satisfy current story drift and plastic rotation limits due to MCER design-level GMs. Between the two linear damping models, constant modal damping is preferred; it is available in commercial computer codes for earthquake structural analysis.