Response from structural engineer
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As the originally quoted boyfriend in this thread, maybe I can offer some clarification of the comments made regarding my (personal)conclusions about the collapse. The original synopsis was fairly right on.
Regarding what happens to steel under extreme temperature - the previous post regarding crystalization of steel is right on. Steel is a crystaline structure made up mostly of iron molecules (mixed with carbon and some other things to create modern structural steel). Under normal conditions this crystaline structure is fairly densely packed (body-centered cubic for any chemists out there). As steel is heated (above approx. 2000 deg F, but I am not sure about the exact temperature), this crystal structure changes to a "looser" arrangement (face-centered cubic). The net result is that the "strength" of the steel degrades to about half of it's original value. In other words, once the steel crystalizes, its load capacity gets cut in half (approximately).
Regarding the difference between a fire caused by jet fuel and a "normal fire" - Buildings are normally designed to to remain standing during a fire. Or, they are at least designed to remain standing long enough to fully evacuate. Simply stated, take the occupancy of the structure and the rate at which people can exit via emergency routes and you get a time required to empty the building. As a designer, you have to make sure that the building will stay up that long (or longer). The point is that the "design fire" is made up of things that noramlly burn in a building. These are things like paper, drywall, furniture, carpet, etc. They are not things like jet fuel. Jet fuel is composed primarily of hydro-carbons (don't quote me on that one, I'm not a chemist or a fuel expert). The point is that jet fuel burns 2-3 times hotter than one would normally expect a building to burn. The result is that the steel changes phase faster, its strength degrades faster, and the additional forces created by the thermal expansion of the steel columns cause them to experience additional load faster.
Add to this the fact that the fire-resistant coating applied to the steel may have been mechanically damaged (i.e. ripped off) when the impact occured, and the fact that the fire-suppression system in the building was almost certainly damaged at the floors wherethe fire was actualy happening, and you get a building that loses its vertical load carrying capacity a lot faster than anticipated. Add to that the fact that some portion of the vertical load carrying system was already destroyed by having a jet liner crash into it and it's not surprising that the building collapsed prior to full evacuation (at least one of the 4 evacuation stairwells was totally destroyed, and the other 3 may have been on fire or full of smoke slowing evacuation further).
This is a tragedy. The loss of life involved is horrendous. The anger and hate that caused this attack in the first place makes me sick. The anger and hate that I suspect will ensue as a result (leading to more death - sickly illogical) scare me and sadden me. But, if we can momentarily accept that it did happen, and that we must mourn and respond in our own ways, and we can step back to look at how the building actually behaved, it is my initial and personal opinion that the structure behaved very well. It stayed up under the force of the impact of a large passenger jet even after losing some substantial portion of its structural system. It then stayed up for approx. another hour while a fire 2-3 times hotter than would normally be expected in such a building degraded the strength of the remaining structure. All things considered, the building behaved remarkably well!
Please do not interpret this as insensitivity to the terrible loss of life that occured. But, despite the sorrow and rage, consider what would have been required to make a building that would still be standing under those conditions. Can such a building be designed? Yes. Would it be economical to build? No. Would it be a functional to work in? No. Would it serve the purposes required of such a structure? No. And could someone, somewhere still figure out some way to bring it down that the designer had not considered? Yes.
Design and construction of a building like the WTC is an incredible accomplishment. That it performed how it did under that conditions it was subjected to is even more amazing. Much the same response is heard by designers after a major earthquake. Why not build them stronger?! Why did this happen?! Give me any design, and I can give you an event, natural or unatural that will fail that design.
So, where and how does the designer choose that line? Is it acceptable for you to go to work everyday in a 1 story concrete bunker with no windows so that your chances of being killed by a terrorist controlled plane full of jet fuel will be minimized? How would you responded Monday morning if someone had suggested that?
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As the originally quoted boyfriend in this thread, maybe I can offer some clarification of the comments made regarding my (personal)conclusions about the collapse. The original synopsis was fairly right on.
Regarding what happens to steel under extreme temperature - the previous post regarding crystalization of steel is right on. Steel is a crystaline structure made up mostly of iron molecules (mixed with carbon and some other things to create modern structural steel). Under normal conditions this crystaline structure is fairly densely packed (body-centered cubic for any chemists out there). As steel is heated (above approx. 2000 deg F, but I am not sure about the exact temperature), this crystal structure changes to a "looser" arrangement (face-centered cubic). The net result is that the "strength" of the steel degrades to about half of it's original value. In other words, once the steel crystalizes, its load capacity gets cut in half (approximately).
Regarding the difference between a fire caused by jet fuel and a "normal fire" - Buildings are normally designed to to remain standing during a fire. Or, they are at least designed to remain standing long enough to fully evacuate. Simply stated, take the occupancy of the structure and the rate at which people can exit via emergency routes and you get a time required to empty the building. As a designer, you have to make sure that the building will stay up that long (or longer). The point is that the "design fire" is made up of things that noramlly burn in a building. These are things like paper, drywall, furniture, carpet, etc. They are not things like jet fuel. Jet fuel is composed primarily of hydro-carbons (don't quote me on that one, I'm not a chemist or a fuel expert). The point is that jet fuel burns 2-3 times hotter than one would normally expect a building to burn. The result is that the steel changes phase faster, its strength degrades faster, and the additional forces created by the thermal expansion of the steel columns cause them to experience additional load faster.
Add to this the fact that the fire-resistant coating applied to the steel may have been mechanically damaged (i.e. ripped off) when the impact occured, and the fact that the fire-suppression system in the building was almost certainly damaged at the floors wherethe fire was actualy happening, and you get a building that loses its vertical load carrying capacity a lot faster than anticipated. Add to that the fact that some portion of the vertical load carrying system was already destroyed by having a jet liner crash into it and it's not surprising that the building collapsed prior to full evacuation (at least one of the 4 evacuation stairwells was totally destroyed, and the other 3 may have been on fire or full of smoke slowing evacuation further).
This is a tragedy. The loss of life involved is horrendous. The anger and hate that caused this attack in the first place makes me sick. The anger and hate that I suspect will ensue as a result (leading to more death - sickly illogical) scare me and sadden me. But, if we can momentarily accept that it did happen, and that we must mourn and respond in our own ways, and we can step back to look at how the building actually behaved, it is my initial and personal opinion that the structure behaved very well. It stayed up under the force of the impact of a large passenger jet even after losing some substantial portion of its structural system. It then stayed up for approx. another hour while a fire 2-3 times hotter than would normally be expected in such a building degraded the strength of the remaining structure. All things considered, the building behaved remarkably well!
Please do not interpret this as insensitivity to the terrible loss of life that occured. But, despite the sorrow and rage, consider what would have been required to make a building that would still be standing under those conditions. Can such a building be designed? Yes. Would it be economical to build? No. Would it be a functional to work in? No. Would it serve the purposes required of such a structure? No. And could someone, somewhere still figure out some way to bring it down that the designer had not considered? Yes.
Design and construction of a building like the WTC is an incredible accomplishment. That it performed how it did under that conditions it was subjected to is even more amazing. Much the same response is heard by designers after a major earthquake. Why not build them stronger?! Why did this happen?! Give me any design, and I can give you an event, natural or unatural that will fail that design.
So, where and how does the designer choose that line? Is it acceptable for you to go to work everyday in a 1 story concrete bunker with no windows so that your chances of being killed by a terrorist controlled plane full of jet fuel will be minimized? How would you responded Monday morning if someone had suggested that?