The impacts of the planes striking the World Trade Center buildings caused fuel vapor explosions and fires. Because of the high combustible load value in the area of the fires, high temperatures developed. The fires spread through the damaged and destroyed building structures. The fire-resistant coatings of load-bearing structural elements were damaged, which seriously decreased the fire resistance of the buildings. The summary effect of the impact, explosion, and fire caused the buildings to collapse.

The World Trade Center buildings had a high fire resistance rating of R240 (4 hours) for the external bearing walls and R180 (3 hours) for all other load-bearing elements. Such times (3 hours and more) guarantee the fire resistance of the building, because firefighting systems should extinguish the fire in that time. The impact and explosion decreased the fire resistance of the damaged elements. The major process responsible for the structural collapse was creep flow of the steel elements. Undamaged load-bearing elements took the strain from the destroyed elements, so the creep flow became more intense and the critical point was achieved in less time than under standard fire resistance test conditions. If certain elements are withstanding an additional load, bearing failure can occur when the temperature of the bearing element reaches 400–420 °C. Because the fire-resistant coating of many structural elements in the impact zone was damaged, the rise of structural temperatures to the above-mentioned values led to the collapse of the buildings.

The Russian Scientific Research Institute for Fire Protection has conducted studies involving the modeling of fire development in the damage zone in buildings after airplane impacts. The main purpose of the research was to obtain information necessary for estimating the necessary fire resistance rating for building structures.

The impact of a Boeing-767 into the World Trade Center was considered as a model situation. It was assumed that the crash would result in a 50 × 10 m opening in the external wall and would create an internal hollow measuring 50 × 50 × 10 m. Assuming that kerosene is spilled on the entire floor area of the damaged zone and flashover occurs quickly, an integral fire development model¹ was used for estimating fire endurance time.

The main system of equations consisted of

• mass conservation equation

• energy conservation equation