Advanced Finite Element Analyses of Moment Resisting Connections for Improving Seismic Performance and Exploring Effects of Residual Stress and Fire Damag

This dissertation research deals with rigorous finite element (FE) analysis and experimental investigation for exploring failure mechanisms of welded steel moment connections (WSMCs) and developing seismic performance enhancement techniques for moment resisting steel buildings. Two types of moment resisting connections in steel buildings were studied, viz. eight bolt stiffened extended end-plate (8ES) connection and welded unreinforced flange bolted/welded web (WUF-B/WUF-W) connection. The AISC prequalified eight bolt stiffened extended end plate (8ES) connection showed good performance in energy dissipation and ductility in many experimental studies under seismic loading. However, fracture of beam flange or stiffener or weld between beam flange/end plate and stiffener, leading to early strength degradation or brittle failure of the connection has also been observed. Hence, the 8ES connection was analyzed using advanced nonlinear finite element (FE) modeling to explore these failure responses. Based on the simulation responses, seismic performance enhancement techniques were developed and analytically evaluated. The modified 8ES connection has been experimentally validated to demonstrate excellent seismic performance.;The aftermath of the 1994 Northridge Earthquake led to significant modifications in the design of moment resisting connections. Although the post-Northridge modified connections showed improved performance in terms of ductility and energy dissipation capacity, lowcycle fatigue failures have been repeatedly observed at the weld toe or access hole of the welded unreinforced flange bolted web (WUF-B) connections. Weld induced residual stresses are considered to be one of the leading contributors to the low-cycle fatigue failures observed in the post-Northridge connections. An experimental study was performed on WUF-B connection to study the influence of welding procedure on the failure mechanism of the connections. Two specimens of WUF-B connections were tested under constant amplitude cyclic loading history and accumulation of axial strain near the welded joint was observed for both the specimens. However, the rate and magnitude of strain accumulation was different for different welding sequence and hence, the fatigue lives of the specimens were different. Two specimens used in the experimental investigation were simulated, using advanced thermo-mechanical finite element modeling techniques. Numerical techniques, incorporating the W-shape manufacturing processes and real time weld sequences, were developed for simulating initial and welding residual stresses. The analysis model was validated based on the measured temperature history during welding and strain data from the experimental study, and initial residual stress data from literature. The simulation responses of the WUF-W connection demonstrated that under low-cycle fatigue loading despite the welding residual stress relaxation, the axial strain at the weld toe continued to grow with cycle (strain ratcheting) to induce brittle fracture similar to what was observed in the experiments. The critical locations of high stress and strain concentrations are also depicted by the simulated contour plots. These novel numerical techniques can be implemented to optimize weld sequence and heat treatment techniques for enhancing seismic or fatigue life of beam-column connections.;Finally, implications of fire damage to moment resisting steel building under seismic loading have been explored. It is not known how a rehabilitated fire damaged building would behave under a seismic event. Properties of structural steel become heterogeneous because of different peak temperature exposure at different locations of the building. Depending on the rate of cooling or water quenching steel properties will have a much wider range compared to those before fire exposure. Finite element analysis was performed to shed light on the performance of the fire damaged building under simulated seismic loading. The seismic analysis results demonstrated that the lateral drift demands were significantly influenced by the post-fire strength degradation and heterogeneity of structural steel. It was observed that nearly all of the earthquake-induced lateral displacement occurs at the story level where the fire exposed compartments are located. This may lead to single story mechanism commonly known as soft story mechanism at the fire exposed story level which may lead to catastrophic failure of the structure.