Ok, here's the short answer: Hi rises are required to have fire proofing (heat insulation) on the steel load bearing structure. They usually are required to have a 3 hr fire rating. 3 hrs has been decided upon (by city code) because it gives enough time to get the people evacuated and firefighters on the scene long enough to fight the fires to prevent the collapse/failure of the building. Why would that be required if steel doesn't burn? Because steel will lose enough of it's structural strength to fail when exposed to hi heat for a long enough time. The fires in 7 began when 1 collapsed, around 10:30am. The 3 hr rating would take that to 1-2pm. After that, the insulation ( a passive prevention measure as opposed to sprinklers - an active measure) would no longer be protecting the structural steel sufficently to ensure that it wouldn't collapse. By 5pm, the steel had heated enough for it to fail. Maybe that wasn't very short,but there ya go... Here's a quote from a fire chief that was there: Here is an e-mail from Chief Daniel Nigro Regarding WTC 7: The long-awaited US Government NIST (National Institute of Standards and Technology) report on the collapse of WTC 7 is due to be published at the end of this year (although it has been delayed already a few times [ adding fuel to the conspiracy theorists fires!]). That report should explain the cause and mechanics of the collapse in great detail. Early on the afternoon of September 11th 2001, following the collapse of WTC 1 & 2, I feared a collapse of WTC 7 (as did many on my staff). The reasons are as follows: 1 - Although prior to that day high-rise structures had never collapsed, The collapse of WTC 1 & 2 showed that certain high-rise structures subjected to damage from impact and from fire will collapse. 2. The collapse of WTC 1 damaged portions of the lower floors of WTC 7. 3. WTC 7, we knew, was built on a small number of large columns providing an open Atrium on the lower levels. 4. numerous fires on many floors of WTC 7 burned without sufficient water supply to attack them. For these reasons I made the decision (without consulting the owner, the mayor or anyone else - as ranking fire officer, that decision was my responsibility) to clear a collapse zone surrounding the building and to stop all activity within that zone. Approximately three hours after that order was given, WTC 7 collapsed. Conspiracy theories abound and I believe firmly that all of them are without merit. Regards, Dan Nigro Chief of Department FDNY (retired) http://911guide.googlepages.com/danielnigro
The "official" story is that burning debris from the twin towers hit building 7, thereby causing a fire that resulted in total collapse within hours. Directly between building 7 and the twin towers were buildings 5 and 6. In order for buring debris to get to building 7 (340 yards away) it had to travel directly over/through buildings 5 and 6 to reach building 7. (see the map attached) Buildings 5 and 6 were burned badly and battered, however their steel frames remained intact and standing. Building 7 collapsed neatly within hours. Buildings 5 and 6 were "pulled" by domolition experts days later in an effort to clear ground zero. Make your own mind up!