Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Initial Model for Fires in the World Trade Center Towers (NISTIR 6879)



Ronald G. Rehm, William M. Pitts, Howard R. Baum, D D. Evans, Kuldeep R. Prasad, Kevin B. McGrattan, Glenn P. Forney


Based on preliminary assumptions and analysis, mathematical models have been used to estimate the behavior of the fires in the twin towers of the World Trade Center (WTC) on September 11, 2001. The hijacked-plane collision with each tower produced significant structural damage, generated a spectacular external fireball, and started burning within the tower. The fuel consumed by the fireball was lost as an ignition source, but produced a pressure pulse that broke windows and changed the ventilation for the fires. The subsequent fire in each tower generated a quasi-steady, wind-blown smoke plume. The fire and smoke behavior were simulated using the Fire Dynamics Simulator (FDS). Comparison of the observed plume trajectory with the simulated one allowed us to estimate that the rate of energy supplied by the fire to the plume was of the order of magnitude of a gigawatt (GW). The rate of energy supplied to the plume, plus the energy-loss rate, determine the total heat release rate (HRR), the most important single parameter for each tower fire. Two bounding scenarios for the interior damage and fuel distributions were assumed for the north tower. For each scenario, the simulated visible fire and smoke behavior outside the tower were compared with the photographic and video records to determine which scenario seemed more appropriate. The simulations for the two scenarios also provided estimates of the likely thermal environment within each tower.
NIST Interagency/Internal Report (NISTIR) - 6879
Report Number


World Trade Center, computational fluid dynamics, fire simulation, mathematical models


Rehm, R. , Pitts, W. , Baum, H. , Evans, D. , Prasad, K. , McGrattan, K. and Forney, G. (2002), Initial Model for Fires in the World Trade Center Towers (NISTIR 6879), NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed July 23, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created May 1, 2002, Updated November 10, 2018