63,
Sorry for the length. (once on a roll …
)
It didn't come down solely due to fire. It came down due to bits of another building falling on it, which set it on fire.
Yeah, actually it did come down solely due to fire.
Plus some unrecognized weaknesses in the design, which will now be eliminated from new building design.
The specific weaknesses were: extremely long beams, beams that attached to girders from only one side (generating huge asymmetric side loads when they heated up, side loads that would not have occurred if the beams tied into the girders from both sides), and inadequate anchoring of girders to column seats.
The "inadequate anchoring" refers to the fact that the main girders (& beams) were not welded in place, they were merely held by small bolts. This was just fine, as long as all the geometry of the building was fixed & constrained by the shear studs anchored into the concrete floors.
But once those shear studs failed & the concrete fractured in the fires, suddenly all those critical components were free to slide around. This is an absolute prescription for disaster in any building, but especially in a skyscraper.
I used the analogy before that, with the beams properly anchored into the concrete, the building was solid as a Mercedes. But when the shear studs failed, it was equivalent to taking out about 3/4ths of the bolts in one portion (say the front suspension), and loosening up all the remaining bolts.
You'd change the car from a tight-as-a-drum Mercedes into a rattle trap, that would surprise no mechanic if it fell to pieces as you drove down the road. Even while nobody expects an "as built" Mercedes to simply fall to pieces as you drive.
The damage caused by the collapse of WTC1 onto the building was not a component of the collapse initiation (except, of course, for setting the fires & crushing the water mains that prevented the firefighters from fighting the fire).
The damage on the south & southeast sides of the building played a small role in changing some of the late details of the collapse. (In essence, a 2nd collapse began at the site of the main damage on the south wall). But by that time, the total collapse of the building was inevitable.
As for Tony's comment that "fire has never caused the collapse of a steel framed structure", it is rare.
It is certainly not "unknown", and people here have given numerous examples.
Sight & Sound Theater, PA
McCormack Place, Chicago
Kadar Toy Factory, Thailand
And others.
And here is one more: WTC5. VERY interesting.
http://www.youtube.com/watch?v=jeYPm8XzC3g&t=52m30s
It'll jump right to the pertinent point in the video.
If it doesn't jump right to the point on "fire only collapse of WTC5", simply slide the slider to 52 minutes, 30 seconds.
[Tony, C7, care to comment on this video?]
BTW, most steel buildings that do collapse (and they do every year, killing firefighters with terrible regularity), do so when they are cooling down. It makes it spooky to be intentionally cooling down the fire by squirting water on it. Especially if you're standing on top of it, or have a bunch of the building above your head.
WTC5 was unusual because it appears to have collapsed during its heating phase. That means that it happened very quickly.
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BTW, truthers started with the patently false meme that "steel buildings have never collapsed simply due to fires". When it was pointed out to them that this is simply wrong, they modified it to "steel framed
high-rise buildings have never collapsed simply due to fires."
The determinant of collapse is simply stress in the affected member. The beams & girders don't know, & don't care about how many stories are above them. They simply respond to their LOCAL stress.
Beams & girders generally carry only the load of their own floor. Approximately equal stresses on all floors.
But columns are the "load paths" that carry the accumulating loads of all the floors above. Low stresses on top floors, very high stresses on lower floors.
(I know I'm stating the obvious, but it's to focus on my point.)
Most times, when steel framed buildings collapse, it's the connectors, trusses, beams & girders that fail first. This is because they tend to be lightweight (higher surface area to volume ratio) compared to the columns in tall buildings, and therefore heat up quicker than the more massive columns. And they fail FIRST.
1) If fires can cause the collapse of beams & girders in a single story design, then they can absolutely cause the collapse in those same beams & girders when they are in a multi-story design. Because, as we said earlier, the beams & girders & their connections to the columns carry only the weight of their own floor, which (for a given design) is the same in a 1 story or 100 story building.
All other things being equal, connectors, trusses, beams & girders in high rises are exactly as likely to fail in high rise fires as they are in single story fires.
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It turns out that few or no floors above some particular column at some particular height puts light loads into that column. But many floors above that same column puts high loads into the column. But engineers compensate for the extra loads by adding material. In the end, the stresses in high rise columns are just about identical to the stresses in single story buildings. And this is true from the top of the building to the bottom.
The one advantage that the lower columns in high rises have over the columns in low rise buildings is that they are more massive & take longer to heat up. And fires are dynamic, transient events.
However this one advantage is completely negated by the fact that the columns depend completely on the integrity of the beams & trusses to keep them in close alignment in order for the columns to do their job.
Lose the beams, lose the geometry, lose the columns, lose the building.
Finally, the huge disadvantage that the lower columns in high rise buildings have is that, if anything changes (damage, buckling floors, changed geometry, airplane impact, etc.) then, with all the weight being carried, the loads (and stresses) can increase enormously.
Highly stressed components fail much, much, much faster in fires than equivalent low stressed components.
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The reason that collapse happens so rarely is that skyscrapers are very, very expensive buildings, and fires & fire protection is taken very, very seriously. Engineers & building owners spend lots of extra money to make sure that they do survive fires, that they don't spend on smaller, less expensive buildings.
Truthers addition of "high rise" or "skyscraper" to the conditions of their "first time ever" argument is not only irrelevant, it is from an engineering POV completely ass backward.
What a surprise, eh…?
tom
PS. Then truthers add "total collapse" to the growing list of qualifiers to account for events like the Windsor Towers.
At this point, real engineers merely laugh & walk away.
Christ, I've got to take my own advice in that last sentence…!!
Fires don't go back to burned out areas and reheat steel because the fuel has been exhausted.
