Tuesday, September 27, 2011
It didn't fall down then...
The February 13, 1975 North Tower Fire has been carefully hidden from you. Here are a few reports concerning it.
This 110-story steel-framed office building suffered a fire on the 11th floor on February 13, 1975. The loss was estimated at over $2,000,000. The building is one of a pair of towers, 412 m in height. The fire started at approximately 11:45 P.M. in a furnished office on the 11th floor and spread through the corridors toward the main open office area. A porter saw flames under the door and sounded the alarm. It was later that the smoke detector in the air-conditioning plenum on the 11th floor was activated. The delay was probably because the air-conditioning system was turned off at night. The building engineers placed the ventilation system in the purge mode, to blow fresh air into the core area and to draw air from all the offices on the 11th floor so as to prevent further smoke spread.
The fire department on arrival found a very intense fire. It was not immediately known that the fire was spreading vertically from floor to floor through openings in the floor slab. These 300-mm x 450-mm (12-in. x 18-in.) openings in the slab provided access for telephone cables. Subsidiary fires on the 9th to the 19th floors were discovered and readily extinguished. The only occupants of the building at the time of fire were cleaning and service personnel. They were evacuated without any fatalities. However, there were 125 firemen involved in fighting this fire and 28 sustained injuries from the intense heat and smoke. The cause of the fire is unknown.
Also, from the New York Times (Saturday 15th February 1975):
Fire Commissioner John T. O’Hagan said yesterday that he would make a vigorous effort to have a sprinkler system installed in the World Trade Center towers as a consequence of the fire that burned for three hours in one of them early yesterday morning. The towers, each 110 stories tall and the highest structures in the city, are owned and operated by the Port Authority of New York and New Jersey, which is not subject to local safety codes. As Commissioner O’Hagan stood in the sooty puddles of the North Tower’s 11th floor hallway, he told reporters that the fire would not have spread as far as it did if sprinklers had been installed there.
The fire spread throughout about half of the offices of the floor and ignited the insulation of telephone cables in a cable shaft that runs vertically between floors. Commissioner O’Hagan said that the absence of fire-stopper material in gaps around the telephone cables had allowed the blaze to spread to other floors within the cable shaft. Inside the shaft, it spread down to the 9th floor and up to the 16th floor, but the blaze did not escape from the shaft out into room or hallways on the other floors.........
Only the 11th floor office area was burned, but extensive water damage occurred on the 9th and 10th floors, and smoke damage extended as far as the 15th floor, the spokesman said. Although there were no direct casualties, 28 of the 150 firemen called to the scene suffered minor injuries.
More from the New York Times (Saturday 14th February 1975):
"It was like fighting a blow torch" according to Captain Harold Kull of Engine Co. 6,........ Flames could be seen pouring out of 11th floor windows on the east side of the building.
So, this was a very serious fire which spread over some 65 per cent of the eleventh floor (the core plus half the office area) in the very same building that supposedly "collapsed" on 9/11 due to a similar, or lesser, fire. This fire also spread to a number of other floors. And although it lasted over 3 hours, it caused no serious structural damage and trusses survived the fires without replacement and supported the building for many, many more years after the fires were put out.
It should be emphasized that the North Tower suffered no serious structural damage from this fire. In particular, no trusses needed to be replaced.
That the 1975 fire was more intense than the 9/11 fires is evident from the fact that it caused the 11th floor east side windows to break and flames could be seen pouring from these broken windows. This indicates a temperature greater than 700°C. In the 9/11 fires the windows were not broken by the heat (only by the aircraft impact) indicating a temperature below 700°C.
So now you know that the WTC towers were well designed and quite capable of surviving a serious fire. I repeat that this was a very hot fire that burnt through the open-plan office area of the eleventh floor and spread up and down the central core area for many floors. This was a serious fire.
Much was learned from the 1975 WTC fire. In particular, the fact that the fire had not been contained to a single floor but spread to many floors, caused much concern. The points of entry of the fire to other floors were identified and the floors of each building were modified to make sure that this would never happen again. For some strange reason, the modifications failed to perform on September 11, 2001 and again the fires spread from floor to floor.