Seismic response of composite moment-resisting connections. II: Behavior

This two-part paper presents research to determine the effect of structural phenomena, especially the presence of composite concrete floor slabs, on the behavior of the type of fully-restrained, steel moment-resisting frame connections that failed, primarily near the bottom girder flanges, during the 1994 Northridge, California, earthquake. In a companion paper, the results of three experiments, including one bare steel specimen and two with composite floor slabs, are outlined, along with corroborating analyses. In this paper, the structural phenomena that affected the behavior of these connections are documented. The results indicate that the strains near the bottom flange of the specimens with a slab are several times larger than those near the top flange, and that strain concentrations and a triaxial state of tensile stress precipitate failure in these connection topologies. These results help to explain the predominance of bottom flange failures, and they indicate that the potentially high strain levels leading to fracture and the inherently unsymmetrical topology of composite steel frame connections should be considered in the development of connection design provisions.