At the time the buildings were erected, no one conceived of a situation where anyone would *intentionally* fly an airplane into the building, let alone with the sole purpose of destroying the building. They *were* designed to withstand the impact of a Boeing 707, the largest passenger jet of the time, but they hadn't considered the resultant events that would follow. An impact, in and of itself, is relatively insignificant. Nevertheless, they hadn't planned on someone *intentionally* flying into the building.
Accidents are accidents, but this was no accident. But how would an intentional strike differ in terms of physics from an accident? The hijackers purposely picked the largest planes they could get their hands on, and ensured they were *fully* loaded with fuel for a trans-continental flight. In an accident, the plane, *more than likely* is *not* aimed at the center of the building.
The likelihood of the mass of the aircraft reaching the core is less significant in an accident. Also, by the alignment of the air traffic control routes among the three major airports in the area, it's highly unlikely a plane would intentionally cross Manhattan Island. Controllers would notify the pilot to alter course to avoid the buildings. So, in an accident, it is far more likely for the plane to clip the building, rather than plow into it. In such a case, the structure of the buildings was more than sufficient to withstand the impact and resulting fire.
What doomed the buildings was the huge amounts of burning jet fuel streaming down the elevator shafts. In an accident, a collision with the building would be most likely to occur when the plane was landing, not taking off or during transitional flight, so the quantity of fuel on board would be significantly less than what these planes had available. The interesting thing is that in a building of similar height built today, that core would almost certainly be super-high-strength concrete. Enough of the plane's inertia would've been lost during transition though the shell that anything that reached the core wouldn't have penetrated the core. So a lot less of the plane would've made it to the core than with the twin towers.
If you were to compare the destruction of the building using, say, a cruise missile, although much smaller
in size, the missile is designed to penetrate the shell, and then detonate in the core. The terrorists didn't have such technology available, so they had to choose something of sufficient mass to penetrate the shell *and* core, and since they had no explosives available, they had to choose something with a sufficient quantity of flammable materials to generate the intended result. A Boeing 767, fully loaded for a trans-continental flight (obviously)met those requirements. The plane itself never acutally detonated, it disintegrated, in the process, releasing highly flammable liquids, which were ignited by heat generated by the engines, the friction of the impact with the building, and the severed electrical systems within the building. And much of the fuel was atomized on impact, creating a highly explosive mist within the building envelope. What didn't atomize ran down through the core, spreading the "accellerant" through enough floors to affect the structural integrity.
Now then, now that we have the physics of things that burn, let's talk about some other factors that I believe contributed to the fall of Towers 1 and 2. Let's talk about building codes, shall we? In general, building codes are a pretty recent development in the construction industry. (So is the Americans With Disabilities Act, but that's another post.)
Building codes came about in earnest in the U.S. after the massive fire in the MGM Grand Casino and Hotel in Las Vegas, NV in 1980. Of all of the fatalities, the vast majority of them were in the hotel tower due to smoke inhalation, not from actually being burned to death (though it was a spectacular fireball indeed--it cleared a distance of two football fields in less than eight seconds and burned gamblers to a perfect crisp with their hands still clutching the levers on their slot machines). The mechanical and elevator shafts in the hotel tower channeled the smoke right up to each floor, thereby asphyxiating the hotel's guests on floors that the fireball never touched. Since then, the primary goal of building codes is to first save lives and allow people to safely exit the building; second, to save the structure so that the non-loadbearing elements can be rebuilt; and third, to save contents and possessions if possible. Sprinkler systems help do this, but it's a secondary line of defense. Structural integrity and egress (exit) paths are the first line of defense.
The World Trade Center was built in 1971-1972, eight years before building codes were in wide implementation and enforcement. Hence, I don't know that any spray-on fireproofing (an image of which is forthcoming in tomorrow's Detail of the Week) was applied to the structure. Without fireproofing, the immense heat of the fire could very easily have deformed the columns and allowed the building to collapse. Also, much of the Twin Towers' structure was along the exterior skin in order to free up the inside of the building for office space. When the exterior skin was ruptured by the plane, the building lost a great deal of its structural integrity. The main innards of the Towers, called the "core", which consists of the elevators, the stairs, toilets, and the mechanical and electrical shafts and rooms, take up the very center. See the photo below. (I know it's not easy to see, but I've marked where the stairs are in the plan.)
The WTC's tenants used these stairs as regular circulation instead of taking the elevators one or two floors. Given that due to the size of each floor, as many as 300 people could work on each floor at the same time, that's a lot of people to try to funnel down the stairs. Imagine trying to funnel 300 people per floor times dozens of floors down the stairs...while fully loaded firefighters were coming up the stairs. Then imagine some of the people coming down the stairs having to carry some of their fellow coworkers because the person gives out 30 floors down and still has another 20 or 30 to go to get to the street. Two normal people need at least 5'-0" of width to pass each other going in opposite directions, but let's add that one person is carrying a person while the person going in the opposite direction is wearing a full jacket, a 60-lb pack, an axe, and a breathing apparatus.
Need a little more width than 5'-0", don't you?
Also, elevators often shut down when there's a fire. They go to the bottom of their shafts and pop their doors open to show the firefighters that no one is trapped inside. However, in tall hospitals (like Pomme de Terre), at least one elevator will be operable in a fire so that staff can evacuate incapacitated patients if they're on the floor where the fire is located. It sounds like the Twin Towers could have used a few emergency-operating elevators. These same elevators can be used by firefighters to get up to the floor where the fire is burning. Perhaps the fighters couldn't use them though, because the planes had severed the power supply and shorted them out. I'm guessing about this part, don't know for sure.
Furthermore, human nature played a part in the disaster. Some tenants reported being told to go back in the building after the planes had been burning for a while because "everything was okay and the fire was under control". Many of us--surely all of us--were so shocked to see what was happening that a part of us just couldn't fathom it, couldn't accept it, and wanted everything to be okay. Let's face it, none of us were prepared for the immensity of 9/11, and we really had no plan to deal with it. So the tragedy of 9/11 was due to the condition of a purposeful air strike, an absence of code and fire safety compliance at the time of construction, and a lack of a plan to handle the humans leaving the building.
The design of the Twin Towers, plus the luck of the planes colliding so high up on the buildings (in the top quarter of the tower), allowed the floor slabs to pancake on top of each other and fall fairly cleanly onto each other. Imagine how much worse it could have been if they'd toppled over from the middle and dropped floors 55 through 110 onto neighboring buildings. Still frightening though, is that what fireproofing that was in the building was acheived through the use of asbestos, a carcinogen so potent that I was told as an intern if I so much as peeked into an asbestos-clad ceiling they'd have to evacuate the building. I've heard some reports that they did asbestos abatement on the towers in the 90s, but I have to wonder if some tiny friable fibers of 220 stories of asbestos are still floating around Lower Manhattan.
Okay, I acknowledge that this was a lot of info, and I'm sure other WAD readers out there will have something to add, contribute, or dispute. But Wandering Author asked me for an opinion, and given my extent of knowledge, that's what I have to say.
