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the explosion was perfectly, even eerily, situated to cripple the massive complex.
When originally constructed, the two towers were not provided with automatic fire sprinkler protection. However, such protection was installed as a retrofit circa 1990, and automatic sprinklers covered nearly 100 percent of WTC 1 and WTC 2 at the time of the September 11 attacks. In addition, each building had standpipes running through each of its three stairways. A 1.5-inch hose line and a cabinet containing two air pressurized water (APW) extinguishers were also present at each floor in each stairway.
The primary water supply was provided by a dedicated fire yard main that looped around most of the complex. This yard main was supplied directly from the municipal water supply. Two remotely located high pressure, multi-stage, 750-gallons per minute (gpm) electrical fire pumps took suction from the New York City municipal water supply and produced the required operating pressures for the yard main.
Each tower had three electrical fire pumps that provided additional pressure for the standpipes. One pump, located on the 7th floor, received the discharge from the yard main fire pumps and moved it up to the 41st floor, where a second 750-gpm fire pump pushed it up to a third pump on the 75th floor. Each fire pump produced sufficient pressure to supply water to the pump two stages up from it in the event that any one pump should fail. Several 5,000-gallon storage tanks, filled from the domestic water system, provided a secondary water supply. Tanks on the 41st, 75th, and 110th floors provided water directly to a standpipe system. A tank on the 20th floor supplied water directly to the yard main. Numerous Fire Department of New York (FDNY) connections were located around the complex to allow the fire department to boost water pressure in the buildings.
Both the NIST calculations and interviews with survivors and firefighters indicated that the aircraft impacts severed the water pipes that carried the water to the sprinkler systems. The sprinklers were not operating on the principal fire floors.
However, there were ample sources of the water in the stairwells. The water pipes ran vertically within the stairwells. Moreover, there would have been copious water from the broken restroom supply lines and from the water tanks that supplied the initial water for the sprinklers. Thus, it is not surprising that evacuating occupants encountered a lot of water.
Even if the automatic sprinklers had been operational, the sprinkler systems—which were installed in accordance with the prevailing fire safety code—were designed to suppress a fire that covered as much as 1,500 square feet on a given floor. This amount of coverage is capable of controlling almost all fires that are likely to occur in an office building. On Sept. 11, 2001, the jet-fuel ignited fires quickly spread over most of the 40,000 square feet on several floors in each tower. This created infernos that could not have been suppressed even by an undamaged sprinkler system, much less one that had been appreciably degraded.