Sunday, April 30, 2006

The WTC Towers could have survived if they met the NYC Fire Code

If the WTC Towers had been built the way they were supposed to be they would not have fallen (according to the original designs from to the Port Authority's response to the public's fears during the debates before construction began).

The WTC Towers were supposed to have a fire suppression system that was to be able to handle the fire caused by a direct collision by a Boeing 707 airliner.

Since the Empire State Building had already been hit twice by airplanes (a small biplane and a B29 bomber) the prediction was that the WTC towers would have a 100% chance of being hit by a large aircraft. The Boeing 707 was the largest airliner at the time the WTC Towers were being designed.

The Boeing 707 airliner carries 10,000 gallons of Jet Fuel while the Boeing 757 airliners that struck the WTC Towers carry 24,000 gallons of Jet Fuel. Both airliners have a top speed of 600-650mph and weigh about the same. The structure of a Boeing 707 is significantly denser than a Boeing 757 due to the use of older technology for the airframe that used heavy machined aluminum structures in the body and wings. The damage caused by the impacts would therefore be similar.

The ammount of fuel that contributed to the fires that ultimately caused the failure of the remaining structure was probably less than 25% of the total fuel in the Boeing 757's or 6,000 gallons. The rest of the fuel passed through the WTC Towers and out the opposite side and formed a fireball outside of the buildings.

The Jet Fuel was atomized by the impact forces spraying fuel out of the cracks in the tanks and by turbulence effects as the mixture of fuel, air and the parts of the 767 passed through the WTC Towers and out the other side.

The fuel that remained in the WTC Towers was the portion of the spray that was able to wet the surfaces of objects as the blast passed through and the portion of the spray that remained suspended in the air afterwards. The fuel mist was probably too rich to burn until more air had a chance to flow back into the buildings through the broken windows, therefor there was no fuel explosion within the WTC Towers. The backdraft flame that is visible on the video of the WTC Towers causes a very soft wavefront that does not cause any explosive damage unless it is confined by solid walls (which were not there by then).

At that point, the sprinkler system should have been fully activated and the sprinkler spray should have been able to keep the temperature in the burning floors low enough so the structural steel would not have been weakened.

The WTC Towers were designed before Engineers understood how fireballs form in high speed collisions so they did not expect the fuel to pass through the towers and out the other side. The original plans presumed that all 10,000 gallons of fuel from a 707 would stay in the Towers and the sprinkler system in the plans was sized to handle the expected fire. When the Towers were bombed, it was discovered that the original sprinkler design had been deleted before the Towers were built and later a sub-standard sprinkler system had been retrofitted.

According to my calculations, a sprinkler system with a flow rate of 350-1500 Gallons Per Minute would have been able to completely absorb the total Heat of Combustion of 6,000 gallons of Jet Fuel burning at a rate of 25% per hour.

Jet Fuel is predominantly Decane with a small percentage of Naptha. The Decane/Naptha mix has a density between 0.75 and 0.8 times the density of water and a total Heat of Combustion of between 31,300 and 33,550 Kilo Calories per Gallon.

Water at 20 degrees celcius (68 farenhight) has a total Heat of Vaporization of 2,347 Kilo Calories per Gallon so a minimum of 15 gallons of water is required to absorb the total Heat of Combustion of 1 gallon of Jet Fuel.

If 6,000 gallons of Jet Fuel burned in 1 hour the required sprinkler flow would be 1500 gallons per minute total per floor. If only 25% of the fuel burned each hour the required flow would be about 375-450 gallons per minute total per floor. These flow rates are easilly achieved by commercial/industrial sprinkler systems.

Adequate sprinklers would have kept temperature on the burning floors of the WTC Towers below 250 degrees centigrade, well below the 800 degree centigrade failure temperature for the structural steel. At that temperature, the steel would not fail even if all of the cladding had been blown off.

The most authoritative source for information on sprinkler systems and suppressing liquid fuel based fires is Factory Mutual Insurance Co. .