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Fire Structure Interface and Thermal Response of the World Trade Center Towers. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.


pdf icon Fire Structure Interface and Thermal Response of the World Trade Center Towers. Federal Building and Fire Safety Investigation of the World Trade Center Disaster. (24171 K)
Prasad, K. R.; Baum, H. R.

NIST NCSTAR 1-5G; 338 p. September 2005.

Keywords:

World Trade Center; high rise buildings; building collapse; disasters; fire safety; fire investigations; terrorists; terrorism; structures; thermal response; flameproofing; radiative heat transfer; thickness; columns; beams; concrete slabs

Abstract:

The collapse of the World Trade Center (WTC) towers on September 11, 2001, resulting from a combination of aircraft impact damage and subsequent fires, was studied as part of the Federal Building and Fire Safety Investigation of the WTC Disaster. This report documents the evolving thermal state of the structure on the focus floors of each tower. A methodology was developed to couple the thermal response of the towers to the fire dynamic simulations. Heat transfer to sub-grid scale structural elements was computed using a simple radiative transport model that assumes the compartment can be locally divided into a hot, sooty upper layer and a cool relatively clear lower layer. Properties of the two layers were extracted from temporal averages of the results of the fire simulations. The model predictions were found to compare favorably with measurements from a series of large-scale experiments. Exploratory studies were conducted to estimate the role of fireproofing thickness and sensitivity of the results to input parameters. The methodology was subsequently used extensively in National Institute of Standards and Technology's (NIST's) WTC Investigation to predict the thermally induced structural response to spatially and temporally developing fires. Finite element models were constructed for the steel structural elements and the fireproofing that covers the floor trusses, core beams, perimeter and core columns as well as the concrete slab on floors 92-99 of the North Tower and floors 78-83 of the South Tower. Structural and fireproofing damage due to aircraft impact on the various floors of each tower was incorporated into the models. The three dimensional time dependent thermal response of the structural elements was generated in a format that is consistent with the structural models and the finite element analysis software. Four global simulations, two each for WTC 1 and WTC 2 are reported for prediction of thermally induced structural response and collapse of the towers. Based on the results of the global simulations, fireproofing thickness and fireproofing damage due to aircraft impact was identified as the single most important parameter that had the largest effect on steel temperature and on the thermally induced structural response.