Effect of structural characteristics on damping modification factors for floor response spectra in RC buildings

Abstract

Floor response spectra (FRS) are used to compute the design floor accelerations for seismic design of acceleration-sensitive non-structural components (NSCs) in a building. In the present study, the elastic FRS at different floor levels are investigated for a set of linear and nonlinear models of both low- and mid-rise reinforced concrete (RC) frame structures. The damping modification factors (DMFs) for the elastic FRS are derived for seven different damping ratios of the NSC’s (i.e. 0.1%, 0.2%, 0.5%, 1%, 2%, 5%, and 10%), and six different levels of inelastic response in the building structure. It is observed that DMFs for the elastic FRS are influenced by both NSC and building structure characteristics. The factors affecting the DMFs for the elastic FRS include the damping ratio of the NSC, the tuning ratio (i.e. the ratio between the period of vibration of the NSC, Ts, to the fundamental period of vibration of the building structure, T1), the modal periods (T1 and T2), and the level of the building structure’s inelasticity. Conversely, the vertical location of the NSC in the building structure does not affect DMFs significantly. Further, in the case of a tuned response of the NSC, the DMFs for elastic FRS are significantly higher for NSC damping ratios less than 5% and lower for NSC damping ratios greater than 5%, than those conventionally used for the elastic ground response spectra (GRS). This is true for the elastic response as well as for low levels of inelasticity in the building structure. On the other hand, at high levels of the building structure’s inelasticity, the DMFs for elastic FRS approach to the conventional DMFs derived for the elastic GRS. Considering these observations, a design-oriented multilinear model is proposed to compute DMFs for the elastic FRS, accounting for the specific characteristics of the NSC (i.e. the period and the damping ratio of the NSC) and the building structure (i.e. the modal periods and the level of the inelasticity). The proposed model for computing the DMFs for elastic FRS is validated by comparing the model predictions with those obtained by time-history analyses, using a different RC frame building and an entirely different suite of ground-motion records than those that were used to develop the proposed model. The proposed model to compute DMFs for the elastic FRS can be used with both the current code-based and the performance-based seismic design of NSCs located in the inelastic building structures.