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Effect of fire and fire following an earthquake on steel reduced beam section moment connections

Date

2013

Authors

Turbert, Collin, author
Mahmoud, Hussam, advisor
Atadero, Rebecca, committee member
Kirkpatrick, Allan, committee member

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Abstract

The main objective of this research is to investigate the behavior of steel frames with reduced beam sections (RBS) during a fire as well as during the combined events of fire following an earthquake (FFE). Historical events and recent disasters have clearly demonstrated that the occurrence of these two events (fire and FFE) within steel framed buildings represents a probable scenario that warrants further investigation. Accurate analytical evaluation of the structural behavior of steel buildings under fire, and to a lesser extent an earthquake, is difficult due to the many complex and uncertain phenomena involved. Detailed numerical modeling of the overall structural system has been shown to provide the most reliable simulation results under current research development. However, detailed analysis is generally computationally expensive and as such not practically applicable. In addition, the nonlinear behavior of entire structures is complex and not fully understood. Therefore, detailed numerical models of the overall structural system often have difficulty capturing local failure modes. This research provides a practical analytical approach to perform accurate numerical evaluation of steel structures under fire and FFE and to closely investigate its characteristic behavior. The approach utilized is to limit the focus on localized compartment fires and investigate the behavior of a single beam-column subassembly within the chosen compartment. By limiting the focus of the study the numerical models can be simplified by utilizing specifically appropriate subassembly models for the analysis. Using the finite element program ABAQUS, two different beam-column subassemblies with RBS were created and analyzed. The subassemblies are representative of actual connections in two steel special moment resisting frames that were designed for the highly seismic Los Angeles region. The frames selected for analysis are an 8-story 4-bay frame and a 16-story 4-bay frame and the selected subassemblies are located at the exterior of the frames at the mid and lower levels, respectively. Both subassemblies were analyzed under fire alone to determine their structural behavior during the event as well as allow for a better understanding of the influence the seismic demand has on the behavior of the connection when exposed to FFE. For the FFE simulations both models were analyzed under a suite of earthquake ground motions followed by a fire simulation. For the fire analysis portion of both simulations (fire alone and FFE) a sequentially coupled thermo-mechanical modeling technique, which includes representative constraint elements to simulate the restraint imposed by the frame is employed. The results of the study highlight the significance of including realistic boundary conditions during fire simulations and points towards the possibility for the occurrence of substantial damage in unprotected steel frames during fire as well as protected steel frames during fire following an earthquake.

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