Seismic response modification factor for moment resisting frames retrofitted by weakening beam

Plastic hinges are developed at both ends of the beams in special moment resisting frames(SMRFs) under seismic excitation. The limited length of such hinges in short span framestriggers rupture. Therefore, a lower limit is specified by seismic codes for the span-to-depthratio. This specification limits the application of special moment resisting frames in shortspantall tubular frame buildings. In this paper a new approach is proposed to resolve thisproblem by using a shear nonlinear fuse instead of the flexural hinges at the beam ends.Accordingly, the middle of the beam that has a negligible moment under seismic loading isweakened to form a shear fuse. The effectiveness of the proposed approach has been shownin a nine-story SMRF. The seismic response modification factors (R) of the original andretrofitted buildings are quantified using the methodology of FEMA P695. The methodologyrequires estimation of the seismic collapse margin ratio (CMR) of the proposed system.

Keywords:

Special moment resisting frames , Shear plastic hinge , FEMA P695 , R factor , SAC building