Effect of Beam Tension on Cyclic Performance of WUF-W Steel Moment Connection

The effect of axial force on the flexural strength of steel members, not connections, has been extensively researched for nonseismic loading conditions. To address the latter for seismic applications, four nominally identical beam-column subassemblies with a welded unreinforced flange-welded web (WUF-W) moment connection were cyclically tested. The specimen design included built-up I-shape beams ($d_b=\mathrm{1,118}\mathrm{mm}$) and built-up I-shape columns ($d_c=\mathrm{1,219}\mathrm{mm}$). Because the member sizes and weights significantly exceeded those permitted by current standards for use as a prequalified WUF-W connection, all except one specimen had flared beam flanges with the goal of protecting the beam flange complete-joint-penetration CJP groove welds. A constant axial tension force in the beam (16% of nominal beam yield strength) was applied to three specimens, whereas one flared specimen did not have axial tension. When tension was present, testing showed that beam local buckling drastically diminished. A higher flexural strength was developed in the plastic hinge, indicating that the $C_{pr}$ value in current standards for capacity design is nonconservative when tension is present. Fracture in the beam web instead of flange CJP welds dominated the failure mode because of a change in the load transfer mechanism. More stringent beam web welding requirements than those specified in current standards were shown to improve connection performance.