Evaluation of 3D Steel Moment Frames under Earthquake Excitations. II: Reliability and Redundancy

In NEHRP 2003, a new mechanism-based reliability/redundancy factor, (rho), is proposed to replace the current (rho)factor in building codes. A multiplier of (rho)=1.3 for design lateral force is recommended for structures deemed being lack of redundancy. The effects of detailed structural configurations, nonlinear structural behaviors, and uncertainty in demand and capacity, however, have not been considered in this factor. To examine the adequacy of this new factor, a systematic investigation on the above effects is carried out. It focuses on buildings with 5×5 bays and nine different configurations of special moment resisting frames. The nonlinear structural response behaviors and uncertainty in demand and capacity based on a three-dimensional (3D) incremental dynamic analysis are properly considered. Structural reliability and redundancy are determined. The biaxial spectral acceleration is found to be a more effective predictor of system demand than drift and therefore used as the response measure in the analysis of reliability and redundancy. A uniform-risk redundancy factor, RR, for design lateral force is proposed to provide equal reliability for buildings of different degrees of redundancy. This RR factor is compared with the (rho) factor and areas for improvement of the latter are pointed out.