An analytical study is presented on the thermomechanical response of steel moment-frame beam-columns connections during post-earthquake fire exposure, considering the residual post-earthquake stress/strain field in the damaged assembly, the fragility of spray-applied fire-resistive material (SFRM) in moment-frame beam hinge regions, and the thermal degradation and distortion of heat-affected steel. The impact of temperature-induced moment-frame connection softening on global (building) response during post-earthquake fire exposure is investigated in order to addresses the potential vulnerability for sidesway instability.
A case study is developed for a ten-story steel moment-frame test structure, representative of a typical office building located near coastal California. Forty-four nonlinear multi-degree-of-freedom dynamic response analyses are performed to characterize force and deformation demands in the test structure, and anticipated damage to SFRM insulation, for the maximum considered seismic hazard. A high-fidelity thermomechanical submodel is developed for a representative moment-frame beam-column assembly in the test structure to track transient heat flow and moment-rotation response during two compartment fire simulations. The moment-rotation response data from the submodel analyses is then used to calibrate computationally efficient moment-frame beam-column connection models for numerical simulation of global sidesway response during post-earthquake fire exposure. The global response simulations utilize a multi-step procedure to develop the post-earthquake damaged state of the building, and to model the subsequent thermal degradation of the structural system during fire exposure.
Six conceptual post-earthquake fire scenarios are investigated in order to provide insight into behavioral response, and to develop representative case studies for evaluating the efficacy of current building code provisions for the multi-hazard scenario. Parametric sensitivity analyses are utilized to identify potential vulnerabilities related to the extent of thermal degradation in the heat-affected beam hinge regions, as well as temperature-induced softening of the moment-frame columns.
Earthquake-induced spalling of SFRM insulation in moment-frame beam hinge regions is shown to significantly increase heat penetration in the vicinity of the damage sites, leading to considerable thermal degradation of the heat-affected steel and softening of moment-rotation response for the beam-column assembly. Secant rotational stiffness and flexural capacity for the moment-frame beam-column assembly investigated in the study were reduced by 20-50% and 20-30%, respectively. The extent of mechanical restraint against thermal expansion, provided by the adjoining structure, was shown to have a significant effect on the deformation response of the beam-column assembly under the action of the residual post-earthquake force system.
The ten-story steel moment-frame test structure investigated in the study remained stable during the post-earthquake fire simulations with only small fluctuations in lateral displacement. This was attributed to the robust moment-frame design, which was driven by stringent drift control requirements for seismic loading, as well as the considerable rotational stiffness retained in the heat-affected beam hinge regions. The extent of thermal degradation in moment-frame beam hinge regions and moment-frame columns was shown to have a significant effect on drift response in the building during post-earthquake fire exposure, particularly for the case of a multi-floor fire scenario where loss of rotational restraint along successive floors effectively elongates the story height in the sidesway frame.
|Commitee:||Lamont, Susan, Ricles, James, Sause, Richard|
|School Location:||United States -- Pennsylvania|
|Source:||DAI-B 73/10(E), Dissertation Abstracts International|
|Keywords:||Finite element heat transfer analysis, Fire, Post-earthquake fire, Spray-applied fire-resistive material, Steel moment-frame beam-column connections, Thermomechanical finite element analysis|
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