Cyclic Behavior of Steel Double-Channel Built-Up Components with a New Lateral-Torsional-Buckling Prevention Detail

Current standards require that, for moment-resisting frames, the strength degradation of a beam–column connection should not reduce flexural strength measured at a drift angle of 0.04 rad to less than 80% of the nominal flexural strength, $M_p$. This requirement is generally sufficient for special moment-resisting frames (SMFs) to prevent collapse due to instability. However, in other seismic force-resisting systems (SFRSs), such as special truss moment frames (STMFs), the chord members within the predefined yielding panel, referred to as a special segment, experience a much larger member rotation. The rotational capacity and ductility of a steel member are controlled by the interaction of three instabilities: flange local buckling (FLB), web local buckling (WLB), and lateral-torsional buckling (LTB). In this study, a new connection detail was developed which uses a center gusset plate and horizontal stitches to prevent global lateral-torsional buckling of double-channel built-up sections, thereby enhancing rotational capacity. For deeper channel sections, web stiffeners can be used to separate WLB and FLB and thus minimize their interaction. Component testing was carried out on members with various sizes of double-channel sections, as well as a reduced beam section (RBS). The test results showed that the new detailing allows double-channel (C310) sections to achieve a member rotation of 0.09 rad (0.065-rad moment frame story drift angle) with more than 80% of the nominal flexural strength. As a result, double-channel built-up sections are able to provide sufficient ductility to prevent the structures they support from deforming into a strength-degrading range under major earthquakes, which makes them a good candidate for STMFs and for potential use in other seismic force-resisting systems.