Full-Scale Testing and Design of Special Truss Moment Frames for High-Seismic Areas

The US design code provisions for steel special truss moment frames (STMFs) were formulated based on research work carried out in the 1990s with double-angle sections as truss members. To provide the higher capacity needed for STMFs in high-seismic zones, stronger members using double channels are required. With much stronger sections than double-angle sections, the heavy welding near the plastic-hinging regions can induce unfavorable restraint. Engineers often remove the $X$-diagonal web members in the special segments to meet architectural requirements, which leads to multiple Vierendeel panels with one or two intermediate vertical members (IVMs) in the special segments (SS). Although IVMs can significantly increase the strength of the special segments, such overstrength is not considered in the current code provisions. These practical concerns were investigated by a series of component tests and tests on two full-scale STMF specimens. Current code provisions prescribe an equation to compute the expected vertical shear strength, $V_{ne}$, of SS for designing all nonyielding members. However, the current $V_{ne}$ equation considerably overestimates the capacity of SS using heavy sections. A new $V_{ne}$ equation addressing this issue and including the contribution of IVMs is developed. Other recommended details for enhancing the seismic performance of STMFs and plastic-hinge models of chord members are also proposed.