Fire Induced Thermal and Structural Response of the World Trade Center Towers
Kuldeep R. Prasad, Anthony P. Hamins, Therese P. McAllister, John L. Gross
Over the past several years, there has been a resurgence of interest in studying the response of building structures to fires. Typically, the thermal loading for structural analysis of a building subject to fire is obtained from a standard time-temperature curve or by assuming a spatially uniform enclosure temperature. This decouples the structural analysis from the fire simulations and as a result the structural response to spatially and temporally evolving fires cannot be predicted. Simulations of the effects of severe fires on the structural integrity of buildings requires a close coupling between the gas phase energy release and transport phenomena and the stress analysis in the load bearing materials. A methodology has been developed for coupling CFD simulations of fire growth with finite element models for thermal and structural analysis. A simple radiative transport model that assumes the compartment is divided locally into a hot, soot laden upper layer and a cool, relatively clear lower layer is employed to predict radiative fluxes incident on sub-grid scale structural members. Thermal responses for realistic fire simulations of various steel structural components on floors of World Trade Center Tower 1 that were subjected to aircraft impact damage and fires are presented. One of the most striking observations that emerges from these results is the wide variation of time-temperature curves at different points in the structure and the lack of resemblance of these curves to the standard time-temperature curve used in furnace tests of structural elements. The thermal response was used to compute the reduction in load carrying capacity of the structural components as a function of time.