Seismic assessment of beam-to-column joints for a non-conforming MRF existing structure

As widely investigated in literature, the local behavior of the joints strongly influences the whole performance of the steel structures; thus, both in the design and assessment of the moment resisting frame (MRF) the joint performance should be properly accounted for. The present work is focused on local assessment and retrofit the beam-to-column joints of an existing non-conforming steel building. Therefore, the monotonic and cyclic behaviors of internal beam-to-column joints are investigated by means of finite element analyses (FEAs). The main peculiarities of these joints are the continuity of beams and hollow square columns interrupted at each level and connected both by bolts and fillet welds to the tapered flanges of the beams. On-site surveys allowed fully characterizing the geometry of the joints. The joint assemblies were sub-structured from the moment resisting frames and analyzed against both the vertical loads and seismic actions. The results showed that these types of joints exhibit very poor seismic behavior, concentering most of the damage within the welds. Therefore, different local strengthening interventions were designed and numerically checked. The comparison between the response of the unreinforced and the various strengthening joints is described, and the best solutions in terms of both cyclic behavior and technological feasibility are subsequently identified.