Proposed Backing Bar Detail in Welded Beam-to-Column Connections for Seismic Applications

Abstract

The use of prequalified welded unreinforced flange-welded web (WUF-W) beam-to-column connections in capacity-designed steel moment-resisting frames necessitates the removal of the weld backing bar at the bottom beam flange-to-column flange groove welded portion. Weld root back gouging and fillet weld reinforcement is also necessary to minimize the fracture potential at this location. This paper revisits the current detailing of WUF-W connections in order to propose simplifications in their fabrication process by intentionally keeping a customized beveled backing bar in place without impairing the connection’s ductility under seismic loading. The analysis relies on traditional fracture mechanics applied in three-dimensional continuum finite element models. The modeling approach is validated with welded unreinforced flange-bolted web (WUF-B) pre-Northridge connection tests. The results suggest that field-welded connections with the proposed beveled backing bar reach lateral drift demands of at least 4%–6% rads even when flange groove welds feature a low-toughness E70T-4 weld electrode, which was typical in pre-Northridge WUF-B connections. Fillet weld reinforced backing bars provide inferior connection ductility compared to the proposed beveled backing bar. The simulation results from a series of prequalified WUF-W connections that respect the current seismic design and fabrication requirements, indicate that when the beveled backing bar configuration is intentionally left in place after completing the complete joint penetration groove welds, WUF-W connections can sustain lateral drift demands of at least 6% rads prior to fracture regardless of the panel zone strength, the steel beam depth, and the flange thickness of the respective beam and column. Limitations as well as suggestions for future work are also discussed.