Ductile crack initiation in welded connections with highly dissipative panel zones

Shear yielding enables beam-to-column connections in capacity-designed steel moment frames to dissipate energy and remain stable at lateral drifts up to 4% rad, as long as ductile fracture does not compromise their hysteretic behavior at lower drift levels. This paper characterizes the fracture potential of welded unreinforced flange-welded web (WUF-W) connections designed with such an alternative design concept that defies the current status quo of pre-qualified welded connections. The analysis relies on state-of-the-art micromechanics based fracture models along with detailed mechanical testing and scanning electron microscopy of samples extracted from WUF-W connections exhibiting ductile fracture originating at the kink points of their panel zone at deformation demands characteristic of earthquakes with a low probability of occurrence. The analysis results demonstrate that while the origin of ductile fracture is near the complete joint penetration (CJP) weld toe of the beam flange-to-column face joint, the overall hysteretic behavior of the connections meet the pre-qualification limits established in current seismic standards. Leaving a customized beveled backing bar in place after completing the CJP welds does not elevate the risk of ductile fracture at this location, thus enabling a more straightforward fabrication process than the current practice. The simulation of hypothetical extreme earthquake loading scenarios demonstrates that, the examined WUF-W connections are not prone to ductile fracture till at least 6% rad while achieving instability-free hysteretic response. • The ductile fracture initiation of welded connections (WUF-W) with highly dissipative panel zones is investigated. • Rigorous fracture modeling including multiple tests for micromechanics-based models is conducted. • WUF-W connections with inelastic panel zones are resilient to ductile fracture. • Simplified weld details can be used to alleviate the fracture potential in welded connections.