Investigation of Composite Slab Effect on Seismic Performance of Steel Moment Connections

In the 1994 Northridge earthquake, connection damage initiated from the beam bottom flange was prevalent. Increased strain demand in the beam bottom flange due to the presence of concrete slab and resulting composite action was speculated to be one of the critical causes of the prevalent bottom flange fracture. Past experimental studies available in the literature after the 1994 Northridge earthquake were reviewed first in this paper. The review showed that different observations and recommendations were made for the slab effect on seismic performance among researchers depending upon the details of beam-to-column connections and concrete slab. In order to augment the test database and emphasize possible negative effect of a composite slab on seismic performance, full-scale cyclic seismic testing was also conducted for three welded steel moment connections. The pre-Northridge (PN) type connection specimen designed with a low degree of composite action (15% fully composite) exhibited a significant upward shift of the neutral axis under a positive (sagging) moment, thus experiencing larger strains in the bottom flange, and showed a poor seismic performance because of the bottom flange fracture at 3% story drift. The specimen designed by using reduced beam section (RBS) connection and with the details of minimized composite action exhibited a superior seismic performance without experiencing any fracture or concrete crushing, close to that of bare steel counterpart. Based on the review of previous experimental studies and test results of this study, it is clearly shown that welded seismic steel moment connections and concrete floor should be designed and constructed to minimize the composite action, if possible, especially when shallow beams and conventional connections are involved.