Out-of-Plane Skew Effects on the Cyclic Performance of Column-Tree Steel Moment Connections

This study investigates the influence of out-of-plane beam skew on the cyclic performance of column-tree steel moment connections. Utilizing validated finite element (FE) models against experimental data, the cyclic responses of various configurations were evaluated under the AISC cyclic loading protocol up to a story drift ratio of 0.05 rad. Skew angles of 0°, 10°, 20°, and 30° were examined across three representative beam depths. The results demonstrate that all configurations satisfy the AISC 341 acceptance criteria for Special Moment Frames (SMFs), maintaining at least 80% of the plastic moment capacity (0.8 Mp) up to a 0.04 rad story drift ratio. However, the introduction of beam skew resulted in a gradual reduction in energy dissipation capacity, with the total dissipated energy decreasing by 2.9–8.9% at a 30° skew. Notably, the inelastic energy component was more sensitive to the skew than the frictional components, exhibiting a maximum reduction of 15.4%. While out-of-plane skew disrupted the symmetry of stress triaxiality and plastic strain at the beam-to-column interface, the overall fracture susceptibility was not significantly exacerbated up to 30°. Furthermore, column twisting remained within a negligible range (below 0.5°), and its impact on global stability was limited. Despite the general stability, a premature bolted splice failure was observed in deep beam configurations at a 30° skew during the 0.05 rad drift cycles. Based on these findings, it is concluded that column-tree connections with an out-of-plane skew up to 30° are viable; however, a design limit of 20° is recommended for deep beam configurations to ensure structural integrity under extreme cyclic demands.