Multi-resolution analysis of the SAC steel frames with RBS connections under fire

Vulnerability of steel-framed buildings in many recent fire events has motivated structural engineers to understand their performance under such hazard. Numerical finite element models have been relied upon heavily to evaluate fire behavior of steel structures. Previous studies utilized either isolated models of connections or line-element based models of frames to assess structural response under fire. This paper aims to combine both modeling approaches to evaluate the performance of three SAC steel moment-resisting frames, designed for LA region. These frames are of particular importance since they have been the focus of many previous research studies under earthquake excitations. All moment-resisting connections in the frames are redesigned to reflect post-Northridge design where reduced beams section connections are typically employed. A multi-resolution modeling technique is devised where detailed 3-D representation of the connection is seamlessly integrated with line elements to create full frame models. This technique allows for investigating the global response of the frames under various fire scenarios while accurately evaluating the local behavior of the RBS connections. The results indicate that the beams and columns do not satisfy the fire design limit states in accordance with Appendix 4 of the AISC 360-16 Specification. The RBS connection exhibits significant local buckling developed in the flange and web of the reduced section. The axial force and bending moment assessment at the location of the reduced section and beam-to-column interface reveal very large demand on the RBS connections that needs to be addressed during the structural fire design process. The results support better understanding of the response of steel moment-resisting frames with RBS connections under fire loads.