A seismic design method for reinforced concrete moment resisting frames using modal strength reduction factors

A performance-based seismic design method for plane reinforced concrete (R/C) moment-resisting frames (MRF) is proposed. The method is a force-based seismic design one, utilizing not a single strength reduction factor as all modern codes do, but different such factors for each of the first significant modes of the frame. These modal strength reduction factors incorporate dynamic characteristics of the structure, different performance targets, and different soil types. Thus, the proposed method can automatically satisfy deformation demands at all performance levels without requiring deformation checks at the end of the design process, as it is the case with code-based design methods. Empirical expressions for those modal strength reduction factors as functions of the period, deformation/damage and soil types, which can be used directly in conjunction with the conventional elastic pseudo-acceleration design spectra with 5% damping for seismic design of R/C MRFs, are provided. These expressions have been obtained through extensive parametric studies involving non-linear dynamic analyses of 38 frames under 100 seismic motions. The method is illustrated by numerical examples which demonstrate its advantages over code-based seismic design methods.