Full-Scale Cyclic Testing of Deep Slender Wide-Flange Steel Beam-Columns under Unidirectional and Bidirectional Lateral Drift Demands

Deep and slender wide-flange steel beam-columns are commonly used in steel special moment frames (SMFs) in North America. These sections provide large moment-of-inertia required to satisfy the design code’s drift limits while keeping the column steel weight at minimum. Due to their low out-of-plane moment-of-inertia and high web and flange slenderness ratios, such sections are vulnerable to local and global geometric instabilities when subjected to lateral drift demands coupled with compressive axial load. Past experimental research on steel beam-columns has mainly focused on small and stocky wideflange sections. To this end, an extensive testing program was conducted at École Polytechnique de Montreal that investigated the cyclic behavior of 10 full-scale deep beam-columns. The column specimens were 600mm deep (i.e., W610 sections). The beam-columns were tested under unidirectional and bidirectional lateral loading protocols coupled with different levels of constant compressive axial load. The test setup involved a 6 degrees-of-freedom control system that was capable of realistically simulating the flexibility of the beam-to-column connection at the top of a first-story beam-column; hence the tests utilized a movable point of inflection during lateral loading as would occur in reality. This paper discusses the main findings from the experimental program and a corroborating finite element analytical study that investigated the hysteretic behavior of 40 different deep wide-flange beam-columns. In particular, the effects of the local and global slenderness ratio, lateral loading protocol, applied axial load ratios, and boundary conditions on the column’s hysteretic behavior were quantified. Furthermore, the effect of bidirectional lateral deformations coupled with compressive axial load on the overall dynamic stability of steel beam-columns is discussed. Finally, out-of-plane forces exerted at the top of the column due to twisting and global buckling about the column’s weak-axis are quantified. The quantification of out-of-plane forces is essential to properly design the lateral bracing of steel beam-columns.