9. Design Using Modal Behavior Factors

A performance based seismic design method for plane steel moment resisting and braced framed structures is described. It is a force-based seismic design method employing different modal (or strength reduction) factors for the first four significant modes of the frame, instead of the same constant behavior factor for all modes as in all current design codes. These modal behavior factors are functions of the modal periods of the structure, different soil types and different performance targets. Thus, the method automatically satisfies deformation demands at all performance levels without requiring deformation checks, as in all current design codes. The method is theoretically based on the construction of the equivalent linear structure to the original nonlinear one and the equivalent modal damping ratios of the previous chapter. The modal behavior factors are determined from the equivalent modal damping ratios with the aid of the modal damping reduction factors. Empirical expressions for the modal behavior factors as functions of period, deformation/damage and soil types for the seismic design of steel plane moment resisting and braced frames are derived. These expressions are appropriately converted to ones which can be used directly in conjunction with code defined elastic pseudo-acceleration design spectra with 5% damping. The proposed method is illustrated with representative numerical examples that demonstrate its advantages over code-based seismic design methods.