Experimental seismic study of pressurized fire sprinkler piping subsystems
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A fire extinguishing sprinkler piping subsystem not only accounts for a significant portion of typical investment in building construction, but also represents one of the key components that ensures the functionality and safety of a building. However, recent earthquake events have demonstrated the vulnerability and sometimes poor performance of fire extinguishing sprinkler piping subsystems, which can cause a wide range of damage resulting in substantial property loss, loss of building functionality, as well as posing a significant hazard in potential fire spread and loss of life. Limited research has been conducted on sprinkler piping subsystem under seismic loading and information obtained from previous studies is not sufficient to fully describe their dynamic response and failure mechanisms. In order to better understand the seismic behavior of fire suppression systems and their interaction with other structural members and nonstructural subsystems, experimental and numerical studies were conducted as part of George E. Brown, Jr., Network for Earthquake Engineering Simulation - Nonstructural Grand Challenge Project (NEES - NGC). Two test series were carried out in the Structural Engineering and Earthquake Simulation Laboratory (SEESL) at the State University of New York in Buffalo. In the first series, a total of 48 tee joint components for sprinkler piping systems with nominal diameters from 3⁄4" to 6'' and made of various materials and joint types (black iron with threaded joints, chlorinated polyvinyl chloride (CPVC) with cement joints, and steel with groove-fit connections) were tested under reverse cyclic loading to determine their rotational capacities at which leakage and/or fracture occurred. The failure mechanisms observed in the piping joints were identified and the ATC-58 framework was applied to develop a seismic fragility database for pressurized fire suppression sprinkler joints. The fragility curves used joint rotation as the demand parameter. Structural analysis models of sprinkler piping systems would be required to generate fragility curves in terms of more global demand parameters, such as floor accelerations. Subsequently, two-story, full-scale (11 ft. × 29 ft.) fire extinguishing sprinkler piping subsystems were tested on the University at Buffalo Nonstructural Component Simulator (UB-NCS). A total of three specimens with different materials and joint arrangements were tested with various level of bracing systems under dynamic loading. All three fully braced specimens performed well under a Maximum Considered Earthquake (MCE) level of loading, validating current code-based requirements for bracing system design. However, the unbraced systems, which are typically installed in low to moderate seismic regions or could exist in older construction, did not perform as well as the fully braced systems. Damage to sprinkler heads, failures of vertical hangers, as well as a branch line fracture, were observed during the tests. A number of hysteresis models were introduced to simulate the nonlinear moment-rotation behavior of tee joint components made of various materials and joint types. The proposed hysteresis models were capable of capturing the strength degradation, change of stiffness during unloading, as well as energy dissipation. As a result, nonlinear rotational springs using the calibrated analytical models were used to model full-scale fire sprinkler piping subsystems. To validate the numerical model, simulations based on the UB-NCS seismic tests were conducted. Nonlinear response-history dynamic analyses were performed to predict the seismic test results. Results obtained from the numerical simulations showed close agreement with the experimental results in terms of displacement, acceleration, and moment—rotation relation at piping joints. Finally, a hypothetical acute care facility equipped with full-scale fire sprinkler systems was selected as an example of the use of the numerical model to develop seismic fragility curves for sprinkler piping systems with floor accelerations as the demand parameter. For this purpose, Incremental Dynamic Analyses (IDA) were conducted, and fragility curves associated with various performance objectives in terms of pipe leakage were developed. This study focused only on the failure of joints and did not consider other failure mechanisms of sprinkler piping systems, including impact with ceilings and other surrounding structural and nonstructural components.