Modal strength reduction factors for seismic design of plane steel braced frames

A performance based seismic design method for eccentrically braced frames (EBFs) and buckling restrained braced frames (BRBFs) is developed herein. The method is a force-based seismic design one where the design base shear is obtained by using different modal strength reduction factors for each one of the first four modes of the frame, instead of using a constant behavior factor for all modes as in all current design codes. These modal strength reduction factors incorporate the dynamic characteristics of the structure, different soil types and different performance targets. Thus, the proposed method can automatically satisfy deformation demands at all performance levels without requiring deformation checks, as it is the case with code-based design methods. The basic concept of the method is associated with the determination of an equivalent elastic structure that retains the mass and initial stiffness of the original nonlinear one, and is characterized by high amounts of viscous damping in order to equilibrate the nonlinear energy of dissipation by the viscous one. The above mentioned factors are obtained through the use of extensive parametric studies on 98 steel plane frames subjected to 100 far-field ground motions. Empirical expressions for those modal strength reduction factors as functions of 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 steel EBFs and BRBFs, are provided. The proposed method is illustrated with representative numerical examples that demonstrate its advantage over code-based seismic design methods.