Influence of Weld Sequence on the Low-Cycle Fatigue Failure of WUF-B Connections

Abstract

Experiments on post-Northridge welded unreinforced flange bolted web (WUF-B) connections demonstrated a new low-cycle fatigue (LCF) crack initiation mechanism with final rupture occurring either near the weld access hole or the weld regions. Post-Northridge connection research reports and related commentary indicated that the weld and heat-affected zone (HAZ) conditions might be contributing factors in LCF-initiated failures of the modified WUF-B connections. The experimental study reported herein investigated the influence of weld sequence on the fatigue failure of the WUF-B connections. Two exterior WUF-B connections were fabricated using different weld sequences in laying the complete joint penetration welding between the beam and column flanges. These connections were tested under a constant-amplitude displacement-controlled loading protocol until crack initiation. Both specimens failed by cracking at the weld access hole, one in a brittle manner and the other in a ductile manner. Analysis of the recorded data demonstrated the influence of weld sequence on strain responses at the weld toe and weld access hole regions. Accumulation of strain with cycles, which is a phenomenon known as strain ratcheting, was observed near these locations in both tests. Recorded strain responses near the crack locations indicated the cause of earlier failure of one specimen compared with the other. Finally, future research needs in mitigating the influence of welding sequence on fatigue failure of welded steel moment connections are discussed.