The weakest moment in my March 6 Mohr/Gage debate was explaining the 2.25 seconds of free-fall collapse in Building 7. It may have been a curtain-fall collapse of the outside perimeter, but those are most common in masonry walls. Since there were 58 steel perimeter columns offering structural support to the outside of Building 7, it doesn't seem logical to assert that the interior collapsed and therefore the perimeter walls had no support.
A collapse, yes, absolutely. Totally free fall for 8 or 9 stories with strong steel supports right in the perimeter walls? I'm searching for a short, simple narrative that can really nail this. NIST does not think it's important, they just studied the probable collapse sequence up to its start and then they say gravity took care of the actual collapse from there. The best help I've gotten so far has been from Ryan Mackey, who mentioned that one interior support column eight stories in length remained attached and helped actually drag the perimeter wall down during that 2.25 seconds.
Hi Chris,
I think I gave you some kind of explanation in e-mail, but I can try again.
This is a hard thing to explain, however, simply because it seems pretty obvious from where I'm standing. It's fake controversy -- the argument makes no scientific sense. But to explain it properly one has to get into the mind and understand the thinking of the opposition.
I can't do this. The "free-fall" argument makes no sense. But even without any scientific training, there is also the word of firefighters to consider, notably Chief Nigro. They saw clearly, with their own eyes, how the structure's integrity deteriorated as it burned, enough so that they got well away from it, fearing a collapse, hours before it actually fell. Not even the craziest Truther has suggested there were bombs detonated during this time period. But this is tantamount to accepting that fire can cause structural degradation and, yes, collapse of a steel-framed skyscraper. In essense, they aren't denying the
fact of collapse, they're merely denying the
style of collapse.
Simply put, if a regular collapse wouldn't behave like this, and the mystery saboteurs needed to put a huge excess of silent, priceless nanotech explosives to cause this effect, why did they do it? It's nonsense.
So with that in mind, let me try the second-best approach, namely by walking through a series of thought experiments, each in increasing complexity until we approach the problem.
Here's the first thought experiment. Suppose we have a slender column supporting a significant weight. If you want to play along at home, say we've got a cardboard packing tube, and we've stacked a number of phone books carefully on top of it. As we approach its failure point, we start adding weight very slowly, until it collapses.
How will it collapse? It will probably not just topple over, and it certainly will not topple over if we have the base of it fixed to the floor. Why would it? Toppling requires the tube to remain straight. The only way it topples is if a tiny bit at the bottom bends first, and the rest remains intact. Why would it fail there first, when the stress is basically the same over the whole tube?
What happens instead is the tube will buckle. It will bend in the middle, the phone books will start to fall virtually straight down, and we wind up with a bent tube and pile of books on the floor. This is classic column buckling in action.
Now, suppose we measure the vertical progression of this collapse, using a high-speed camera and a ruler, Mythbusters style. The collapse will have three distinct phases. First, there will be a barely perceptible shortening of the structure as the column bows, then begins to kink -- the phone books will only drop a matter of millimeters during this stage, since the bowing and kinking don't change the effective height of the column very much. This stage will take a little time, maybe a second or two. During this time the tube is
absorbing energy, undergoing plastic strain at the kink, until it yields.
After this, there is the second stage, where the column is kinked. At this point it provides almost no strength whatsoever. Think about it -- once bent, can you straighten up that tube and put weight on it again? Nope. After buckling, its strength is gone forever. So, the books fall. The tube does
not absorb energy during this stage, or at least not enough to notice, because it's already buckled. It has reached its yield point and can't even support itself any longer. Try it: After buckling, it won't even stand up on its own. The upper part just flops around. Thus, strength has gone to zero, so the books fall at "free-fall speed" (really at ~ 1
g of acceleration).
Then the books hit the ground, and stop. Once they encounter something that does have strength -- the floor, in this case -- they no longer fall at 1
g. That's the third stage.
These three stages are
exactly what we see in the WTC 7 perimeter collapse. Deceitfully, the Truthers only consider the second stage.
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Now on to the second thought experiment. "That's just one column," you're thinking, "and we added weight until it collapsed. WTC 7 didn't add weight." That's correct. What really matters, though, is not the amount of weight, but rather the
demand to capacity ratio, or the relationship between weight and the carrying ability of the structure. To gradually build up to collapse, we can either add weight like we did the first time, or we can reduce the strength of our column. Once the buckling point is reached, it behaves the same regardless of how we got there.
For our second thought experiment, instead of just a simple column we're going to set up some buttresses. Beside our cardboard tube I'm going to build three very strong stacks of bricks, all round the cardboard tube at the same distance, spaced just far enough that I can wedge a paper cup between the bricks and the tube. I'll put three cups total, all halfway up the tube. Then I'll start stacking phone books on the tube just like before.
When I do this, I find that now the tube can handle a whole lot more weight than it could before. Why? Well, remember, our tube bent in the middle. I've got these buttresses in place now that keep the tube from bending in the middle. If I want the tube to buckle now, it needs to form a more complicated shape, like an "S" instead of just a "C" shape. That more complicated shape takes more energy. As a result, the buttressed tube is stronger. In structural engineering, we talk about the "effective length" of a column -- a slender column with good bracing is just as strong as a shorter column of the same diameter.
Instead of overloading this column, let's give it the same weight it took to collapse our unbraced column. The tube will handle this weight and stay standing. Or, at least it will until something happens to the paper cups. Say I knock them out, or set them on fire...
As soon as the cups are gone or fail, the tube will fail, and we get a collapse. This collapse behaves exactly the same as the one in our first thought experiment.
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On to the third trial. What if there's more than one column?
Well, let's set it up. The perimeter of WTC 7 is more like a mesh than a bunch of totally disassociated columns, so the load is shared between them. Let's set up a row of cardboard tubes and put a solid load on top of them -- say, a stack of wooden planks. We'll put in buttresses just like before, one on each side of the columns in the middle, and on three sides of the columns at the ends.
Finally, we put buttresses between adjacent tubes. Even though they're all the same strength, the tubes actually do support each other by keeping each other straight and true.
Now we start knocking out the cups, one by one. We'll leave the ones between tubes in place, just hit the ones on the outside. What happens?
Wherever cups are knocked out, the tubes will start to buckle. When this happens, it means the load is transferred to other tubes -- the buckled one will shorten a tiny bit, but that's all. If you aren't measuring carefully, you can miss it.
When we knock out enough cups so that the
total strength of the tubes isn't enough, then we can't redistribute the load any more. At that point
all of the tubes will buckle, and we'll precipitate a total collapse. It will look quite a lot like the previous experiments -- brief period of buckling, followed by free-fall of our load, followed by the load hitting the ground.
In the actual structure, the load redistribution takes place at the speed of sound in steel, or several thousand meters per second. This is so fast that it may as well be instantaneous, compared to collapses that last a few seconds.
One thing, though, in our little experiment it is possible to get a little bit of tilting, and this may cause our stack of boards to actually fall off, leaving some columns looking much more kinked than others. In the actual WTC 7, however, this can't happen. The load there is not an ideal load concentrated on the top and just balanced there. The actual load is the weight of the columns themselves, which are, again, more of a mesh. It's got nowhere to go, except down.
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The last thought experiment is to note that columns can also buckle at different locations than the middle, if there's something that causes them to be weaker there. For instance, we could cut part of our tube a quarter of the way up. It'll buckle there first instead of at the middle, even though it'll still carry almost as much load as the intact tube before that happens.
Another way is to actually push on the tube. This is the opposite of our buttresses. In structural engineering this is called an "eccentric load," i.e. a load that isn't axial, or all vertical and centered over the column's base. You could have a string tugging on the tube, for instance, and if you did, you'd notice it took significantly less weight for the tube to collapse. The reasoning by now should be simple -- the side load causes the tube to bend, because it's already bent it has less "springiness" to resist buckling, and it absorbs less energy when it buckles.
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Now let's relate it all to WTC 7. We have visual and simulation evidence that demonstrates the core was basically collapsed before the perimeter begins to move. We are then left with the perimeter alone, which is a network of columns all tied together, and made of steel.
The core collapse is like our buttresses failing. When we have solid columns supporting each other, with the floor beams, at every story, the columns have a short effective length and are quite strong. But when the core is gone, they're no longer supported. Their effective length goes up by ten, twenty, maybe even thirty times! After this happens, their strength is
vastly reduced -- reduced so far, in fact, that they now don't even have enough strength to hold themselves up.
The core collapse also introduces eccentric loads. The collapsing material has to go somewhere. It's a big pile in the interior, but it won't make a neat pile. Instead, it flows, and it pushes the perimeter columns outward. This further weakens the perimeter columns, and it makes them more likely to buckle low in the structure, where the eccentric load is.
Additionally, there is one corner of the WTC 7 lower floors that is expected to survive, attached to the perimeter, while the rest falls away. This is because that one corner experiences less fire and is predicted to have more intact connections to the perimeter. Guess what this means? That's right, even more eccentric load.
This last factor explains why the buckle happened how it did. Because the supports are gone and yield strength has dropped below its self-weight, some buckling is inevitable. Because the perimeter columns are tied together, rapid load redistribution bewteen them is expected, so when it buckles it'll buckle pretty much all together, and the last few remaining columns will be pulled down by their buckled neighbors. But it could happen over the entire height, or it could happen over a few floors.
The individual columns in WTC 7 are not tubes, and they have many imperfections that each could start the buckle. The columns are built up from many individual pieces of steel, and welded together. Any one of these welds is a weak point. So we could see a fracture of three welds, say, for an initial buckle of two pieces of steel. Or we could see more.
What we actually see is an eight-story buckle, and this is due to the eccentric loading in the corner. That chunk of remaining floors attached to the perimeter after the core fails literally pulls in eight stories of column.
But not at once. Look at NIST's displacement curve. In the first few seconds, the perimeter only moves slightly. This is the eccentric load pulling inward, shortening the wall slightly, while the wall resists bending but slowly deforms. And then it buckles.
After it buckles, of course we see some "free fall." There is
no strength whatsoever at that point. The very last supports between the top of the perimeter wall and the ground are gone.
After eight stories, the upper part of the columns basically hit the rubble pile, and then they slow down. The collapse continues, of course, but now it's a confusion of rubble piling on rubble, and already-buckled columns breaking and tumbling further. There is resistance because upper and ground are touching, but it's not a stable load path -- it's a big pile of junk.
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And that, frankly, is all there is to it. If you build another structure like WTC 7 and fail its core first, you'll see much the same behavior at the perimeter. "Explosives" is a fairy tale.
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