Prediction of steel connection failure using computational fracture mechanics

Damage to steel structures during the Northridge earthquake has demonstrated that welded steel structures are more prone to premature fractures that were generally thought. Since these earthquakes, research has been underway to develop more reliable welded moment connections and to establish ductility demands on modern moment frames. A key issue in developing ductile fracture resistant welded connections for seismic design is determining the minimum toughness requirements for the weld and base metal. This research is dedicated to conducting a series of detailed finite element analyses with conventional fracture mechanics and micro-mechanical ductile fracture modeling.; The study summarized focus on toughness demands in welded beam-column connection subassemblies with the “pre-Northridge” single-sided welded beam column connections with varied geometries and weld details. Additionally, two different “post-Northridge” connections are investigated. Two- and three-dimensional analyses are conducted to evaluate crack initiation criterion from flaws in the bottom flange weld. Toughness demands are determined using J-integral and expressed in terms of stress intensity factor K _I and Crack Tip Opening Displacement. K_I and CTOD indices are compared to critical values determined from fracture toughness data for common weld metals.; The analyses confirm the fact that pre-Northridge connections with large weld root flaws and low toughness E70T-4 weld metal will fracture without significant yielding. Further, the analyses demonstrate that using higher toughness materials does not provide sufficient fracture resistance to ensure levels of ductility required for seismic design. Parametric studies of various design connection design parameters examine how appropriate combinations of these will control fracture demands within tolerable levels for notch-toughness rated welds and base metals. In particular, when properly detailed to limit panel zone deformations and beam flange stresses/strains at the column face, the reduced beam section concept is shown to be an effective means of controlling toughness demands in the fracture critical beam flange weld region. Also, simple hand-calculation equations are developed to evaluate the fracture toughness demands without finite element fracture studies and thus facilitate the design of fracture resistance in welded beam-column connections.