Seismic Response of Composite Moment-Resisting Connections. I: Performance

The 1994 Northridge, California, earthquake precipitated numerous unexpected fractures in welded steel moment-resisting frame connections, mainly in the regions of the bottom girder flanges. This two-part paper reports on research to determine if structural phenomena, primarily the effect of composite concrete floor slabs, contributed substantially to these failures. The results of cyclic experiments of one bare steel and two composite full-scale specimens representing typical pre-Northridge, interior moment-resisting connections are described, along with corroborating analyses. The structural performance of the test specimens is highlighted in this paper. The connections failed either brittlely at the welded interface of the bottom girder flange and the column flange, or with a low cycle fatigue failure emanating from the root of the bottom girder flange access hole. None of the connections achieved adequate levels of cyclic ductility, and the composite connections did not achieve their calculated strengths when subjected to positive bending moment. In the companion paper, the behavior of the connections and the structural phenomena that contributed to the failures are documented.