Earthquake resistance frames with combination of rigid and semi-rigid connections

• The concept of the earthquake resistance hybrid steel frame which is a steel frame with combination of rigid and semi-rigid connections is introduced.
• The most effective pattern for placement of semi-rigid connections in the hybrid frame is identified.
• A reliability analysis, utilizing the collapse margin ratio presented in FEMA P695 is performed.
• The proposed frame system performed superior to conventional SMFs when subjected to earthquake excitation.

The concept of the hybrid steel frame system, as it pertains to mixtures of fully-rigid and semi-rigid steel connections used in 20-story SAC frames, is presented herein. Several different patterns and locations of semi-rigid connection replacements within the frame were examined in order to identify hybrid frames with the best seismic performance. The effective connection stiffness was identified by performing a parametric study on the initial stiffness of the semi-rigid connections. Then, the cyclic behavior of the connections with the most effective stiffness was obtained, using nonlinear finite element analysis. Inelastic dynamic analyses were conducted on the proposed selected frames by subjecting them to Los Angeles earthquake records characterized as those with 2% and 10% probabilities of exceedance in fifty years. The maximum story drift for the hybrid frames was determined and compared with the “life safety” and “collapse prevention” performance limits, as recommended by FEMA 356 [12]. The story drift and member forces for the proposed hybrid frames were compared with those of conventional SAC frames without semi-rigid connections. Finally, a reliability analysis, utilizing the collapse margin ratio presented in FEMA P695, was performed to quantify and compare the collapse performance of the proposed hybrid frames and conventional rigid frames.