> its an hour, but it is interesting. In it they show some videos of the > collapse and in it is clear that the mast on top of WTC 1? begins to > collapse down into the core of the building well before the rest of the > structure, indicating that the central supports of the building (the key > to its structural integrity) were the first things to go. They attribute > this, and the damage on the ground floor prior to collapse, to bombs in > the basement of the building (and elsewhere) that were triggered after > the impact and that were the cause of the collapse. These bombs cut the > central supports and initiated the collapse.
They also have people > inside the building, and seismic records etc that allegedly corroborate > the idea. One thing that explains something that always worried me about > the collapses but I never knew why, is the fact that the whole thing > took around 10 seconds, in both cases. Given the height of the buildings > ( I didn't take notes when I was watching, so watch it for details - > they even have formulas !) this is basically free fall speed. So the > structure of the building, even the thousands of tonnes of undamaged > hurricane-proof steel and concrete below the collision levels, did > absolutely nothing to impede the collapse speed. It is as if the whole > building just broke into a zillion separate pieces and fell at free fall > speed to the ground. There are three things to consider about this. First, the terminal velocity of an object is dependant on a number of factors, including the shape of the object, the density of the object, and the size of the object (square/cubed law). Each tower was about 500 thousand tons. Assuming these are English tons, and using 68 meters x 68 metric as the footprint, we have 500 tons per meter squared of area by the time the falling structure got to the bottom...which is where air resistance should be slowing things the most. Knowing that the terminal velocity of skydiving humans is 120 mph...when resistance is maximized, it seems reasonable to see that the terminal velocity of this great of a mass/meter squared is far greater. To pick a number, I'd say at least 400 mph, or about 600 fps. In 10 sec, total free fall gives us 320 fps, or 98 mps. Since air resistance goes as v^3, we'd see, roughly, no more than a 10% reduction of the maximum velocity. Second, g forces involved with stopping fallen objects are much higher than g forces required to hold something up. For example, dropping a hammer on a floor generates 1000 g's of force. The g forces are also sheer, they pull the cross beams down, thus magnifying the force. Let's assume a factor of 2 safety margin is involved. (I'm pretty sure a number close to that has been quoted somewhere with regard to the temperature weakening) So, an addition g, on top of gravity is needed to break the bonds. While the building as a whole does have give to bending, steel beam structures do not have significant give with respect to shearing forces. It would only take milliseconds for the falling building to generate the additional 1g force that is needed. 100 decelerations of, say, 5 milliseconds each, would result in an extra half second above free fall. I admit to doing a bit of back of the envelope work here. But, my point is that's a lot more work than the folks who maintain the websites did. They just appealed to untrained intuition. A more careful job could certainly be done. I've referenced numerous professional groups who have analyzed the collapse, and have found it very understandable. So, my question is why trust an appeal to intuition over both rough calculations and the work of a number of professional groups. I know that appeal to authority is not a good argument, but I do tend to trust analysis done by multiple groups of people who are both trained in practice in evaluating structures and their failures more than arm waving by folks who don't have similar track records, training or tools. This is just enhanced by my own back of the envelope work which is much more consistent with the pros. > It is just too strange to contemplate that it was an inside job, that > part of it is the thing I find very hard to accept, the idea that anyone > would think that they could get away with it. But there are a lot of > things that don't seem to add up. And the 911 Commission (which took 411 > days to start, and had an initial budget of 3 million, versus a week > and 50 million for the Challenger disaster) report looks more like a > whitewash to me the more I learn about it. If Gautam were still on the list, he might have been persuaded to talk to the liberal Democrat he's going to school with who compiled this. Why would liberal Democrats want to whitewash Bush? Dan M. _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
