Seismic Stability of Steel Moment Frames with Inelastic Panel Zones: Connection Behavior, Design Requirements and System-level Effects

The current status quo in prequalified beam-to-column connections pronounces flexural yielding in steel beams with either elastic or limited inelastic behavior within the beam-to-column web panel. Consequently, inelastic local buckling near the beam ends may occur at modest lateral drift demands depending on the cross-sectional slenderness of the steel beam itself. As such, flexural strength deterioration along with the destabilizing effects of gravity may control the overall seismic stability of steel moment resisting frames (MRFs) during earthquakes with a low probability of occurrence. Besides, steel beam repairs are likely in the aftermath of a design basis earthquake. To address the above challenges, this paper presents an alternative concept for welded connections that defies the current paradigm and instead promotes shear yielding within the panel zone, thus enabling a very stable hysteretic response even at lateral drift demands beyond 5% rad. Full-scale experiments along with coordinated computational studies demonstrate that fracture initiation due to ultra-low-cycle fatigue at the kink locations of the panel zone is not a concern even when beveled weld backing bars are intentionally left in place after the completion of complete joint penetration welds at the bottom beam flange-to-column flange joints conditioned that current weld and fabrication practice is respected. The paper also presents a set of design requirements concerning the panel zone strength and the stability of welded connections with inelastic panel zones when deep and slender steel columns are used in capacity designed steel MRFs. Finally, the paper presents selected results from high-resolution computational studies that substantiate the potential of such an alternative design concept to reduce the residual story drift ratios along the height of steel MRFs and the need for structural repairs in its connections. Prospects of ongoing research work are also discussed.