Effect of composite action on seismic performance of steel moment connections with reduced beam sections

Many steel buildings suffered from fracturing of beam-to-column connections in the Northridge earthquake and Kobe earthquake. It was also found that most of the connections which failed were due to fracturing at the weld between the bottom flange of the beam and the column flange plate. The stiffness and strength contributed from floor slabs may cause the fracture on the bottom flanges of the connecting beams. In this study, the effect of composite action on the ductility performance of the connections are examined through a series of large size experimental studies of beam-to-column subassemblies which include floor slabs. The beam strength around the connection was reduced according to the seismic strength demand, and it was found that an enlarged plastic zone can be achieved and the deformation capacity can be increased substantially. From experimental studies, it was found that the ratio of positive moment to negative moment strength may be as high as 1.18 which is mainly from the contribution of floor slabs. It was also found that the floor slabs cause the beam sections to become unsymmetrical and induce higher strain on the bottom flanges. Owing to these effects, the fractures of the specimens tested were on the bottom flanges which resembled the failure mode which occurred during the Northridge earthquake. (C) 2001 Elsevier Science Ltd. All rights reserved.