Experimental methods for understanding the behavior and residual capacity of bolts and steel bolted connections under impulsive loads

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Sanborn, Marc J.
Stewart, Lauren K.
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Explosive threats continue to be a primary tactic of terrorist organizations in today’s international security environment, often impacting civil structures and infrastructure. When structures are subjected to blast events, the potential for progressive collapse of the structure due to the loss of load bearing members is a significant concern. A more thorough understanding of the behavior of bolted steel connections during and after extreme loading events is necessary to be able to predict and prevent progressive collapse of the structures. Connections ensure ductility and transfer loads to surviving structural elements after an extreme loading event. The research presented in this thesis developed an experimental method and experimental test program to investigate the behavior of structural bolts during and after an impulsive event. An experimental residual capacity test system was developed to subject bolts and bolted connections to an impulsive shear event and then test the residual static capacity of the bolt in-situ after the event. In the test system, impulsive loads were generated through the use of a high speed hydraulic actuator to accelerate a flyer mass to a desired impact velocity. A modular system was designed to be installed within the impulsive setup to apply a quasi-static load after the impulsive event. The residual capacity experimental test system was used to investigate three fundamental aspects of bolted steel connections under impulsive loads: 1) the response and behavior of structural bolts during an impulsive event; 2) the residual capacity of structural bolts after an impulsive event; and 3) the response and behavior of clamping force and friction during and after an impulsive event in slip-critical connections. The response and behavior of structural bolts during an impulsive event was investigated by subjecting structural bolts to varying impulsive loads. Impulsive loads were generated in the intermediate strain rate range, typical of the structural response strain rate from a blast load. A linear relationship between impact energy and residual permanent shear strain was observed. Impulsively damaged structural bolts were then quasi-statically tested to determine their residual capacity. Structural bolts were found to exhibit reduced ductility but equivalent strength to virgin bolts. A response model to predict the permanent residual strain and resulting residual capacity for a bolt subjected to an impulsive shear load was developed. The model identifies four response zones based on the impact energy applied to the bolt. In the investigation of slip-critical connections, it was observed that only 20 to 30% of the initial impact energy was transferred to the structural bolt in bearing due to energy dissipation through friction. Finally, a numerical study was conducted to investigate how well high fidelity physics based models capture the behavior observed in the experimental tests. Representative simulations were conducted and results compared to the experimental data. In general, the models predict the overall impulsive, residual capacity, and slip-critical behavior observed in experimental tests, but have several shortcomings regarding the accuracy of the results due to lack of experimentally validated material parameters.
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