This post was triggered because of this comment at Taking a Closer Look: Hard Science and the Collapse of the World Trade Center
Jon P of IL, USA writes: Brandon Christensen made a post citing the logistical enormity of rigging a Detroit building for demolition, as evidence to refute the strong case for demolition at the WTC. Not content with Mr Heller proving scientifically that this was a demolition, Brandon Christensen wants us to use conjecture to explain how it was all set up! Mr Christensen: Mr Heller has put forward a sound theory based on hard evidence. Instead of ignoring that theory, why don't you directly address it? Explain to us all, for example, how 110 floors 'pancaking' into each other, offer such insignificant resistance as compared to freefall in air.This Jon P. was not the only commenter on this subject of pancaking. After reading about it for the fourth time, I thought I might inform some of the readers of that web page of the other side of the story, from someone who has actually had some experience in such matters as structural design and the effect of high temperatures on the strength of steel.
Posted Jul 31, 2006
One has to understand first and foremost that almost all of us hated word problems in math class, and that vast majority of us who did hate them forgot how to do them as soon as we could get away with it. So, most of us live in a world where we can't put these things together very well, and we thus can fall for some argument that SOUNDS logical and informed and puts the pieces together in a wonderful gestalt, when it may not be after all. Some of us, however, kept running into word problems and eventually became good enough at them to become designers and engineers - because that field is the place where word problems are solved, for real...
Here is the comment I tried to post and am still not sure if it "took":
.. Where to start...
I will get to "pancaking", but I have to give you a lot of background first. Please bear with me...
WTC1 and WTC2 were unique buildings, in all the world. I have heard that WTC7 had the same structural design, but I can't swear to that. I will not go into the WTC7 collapse, as I have not put any thought into it. My apologies.
To begin:
WTC1 and WTC2 were not built like we are used to seeing steel buildings built. The cross members were not steel "wide flange" beams (WF beams), often referred to as "I-beams" (which are something not often used in buildings at all!). Normal skyscrapers are designed with WF beams as vertical columns and WF beams also used as main horizontal cross beams and for most secondary horizontal cross beams.
The WTC buildings were not designed that normal way. Their outer SKIN (believe it or not!) was the main component of their load bearing structure. Vertical members, whether WF beams or the outer skin, must be braced every so many feet in order to not be flimsy. They can only be used in their full strength design capacity if they are braced properly and frequently enough.
The outer skin of the WTC buildings WAS braced at proper intervals, but the braces were only what are known as steel joists, which are spidery webs made up of much weaker elements, usually just angle iron and round bars, welded together. Joists are almost always designed to hold up only flooring and the "live load" that a given floor supports, such as filing cabinets, desks and people. This live load is usually only 200 pounds per square foot as a maximum. In normal steel structure design, that figure would have been given a multiplier "safety factor" of about 3. I assume that the WTC architects used a similar safety factor in their non-standard structural design.
The two ends of a joist are usually just plates - laid horizontally - with a few bolt holes to keep the joist from slipping.
In the WTC buildings, the joists were serving the double functions of holding up the live loads PLUS being the bracing for the outer skin. This was never done anywhere else that I have been able to determine so far. The only thing bracing the outer skin, in order to keep it from getting too flimsy were those bolted connections and the horizontal strength of the joists. BUT: joists are normally not designed for horizontal forces - only for vertical forces.
In the area of the jetliner impact, the joists were damaged to some unknown degree, with some of them certainly rendered completely useless. The outer skin in this area would have had its flimsiness factor (called the "radius of gyration") affected negatively, making parts of the outer skin in that area weaker than they were designed to be.
A short slender support can withstand a certain amount of weight (load). But if the height of the slender support is doubled (from 10 feet to 20 feet, for example), the amount of load it can support is reduced by about 4 times, more or less. The slender support begins to buckle under the same load, simply by making it taller. When one of the joists bracing the outer skin was "taken out", the outer skin's load bearing capacity in that area (if considered just by itself) therefore was reduced by about 75%. If only one joist was damaged, the skin could withstand that loss of bracing, because the adjacent, properly braced, outer skin members would take up some of the load.
In the impact and fire in the WTC towers, three factors combined with the weird structural design to cause the upper structure to fall. One factor was this loss of bracing. One was the fact that some of the outer skin was also removed, making the other outer skin panels have to take up the load. Those panels did manage to support the upper floors quite well - not counting the third factor. The third factor was the fire and the effect it had on the steel.
Contrary to what some people on the web are touting, steel does not have to melt in order to fail. As the steel temperature increases above 900 degrees Fahrenheit, the steel starts to fairly rapidly lose its strength. If memory serves, at about 1500 F, the strength has dropped to about 40% of its room temperature strength.
How hot did the steel get? Tests done on airliner structures, in fires from jet fuel, showed the temperatures getting to about 1500 F (again, if my memory is accurate). I looked this up a couple of years ago, and could find the link if asked.
The "yield" strength of A-36 structural steel beams used in standard designs is 66,000 psi. A little bit above that stress level, the steel will actually start to fail at room temperature. Yet, in most states the engineers and architects pretend the steel is much weaker, giving an effective safety factor. The level that is used is 18,000 psi. So, steel that is actually 66,000 psi strength is used as if it is only 18,000 psi. The resultant safety factor is 66,000 divided by 18,000, or 3.67. That is at room temperature.
Factor in the 60% reduction in steel strength at 1500 degrees, and the safety factor becomes 40% of that 3.67, which amounts to 1.46. This is still strong enough to hold up the total design load.
But, when you also factor in that some of the structure was damaged, that 1.46 gets to be VERY precarious. It may AVERAGE 1.46 where the structure is still intact and fully braced. But in the damaged areas, the story was different. While the steel managed to hold up the upper floors for a while - while it was still fairly cool - it was only a matter of time before the steel was thoroughly heated.
Now, a standard design is what I thought the WTC towers WERE on that morning. But even then, with the amount of flames I saw, combined with the structural damage from the impact, I was pretty certain that the upper floors would collapse down into the damaged area. I envisioned those upper structures possibly then leaning over and falling off. That was even with standard design parameters. When I later on heard of the design, I could easily see why the total collapses happened.
Now, on to the "pancaking":
To understand this, you have to get out of your head your conception of steel buildings that you have had your whole life. Those standard "steel skeleton" structures are not what we are talking about here, as I have mentioned. The statements here and elsewhere on the web are correct: No tall steel buildings have ever collapsed due to fire. That is a fact of history. If any HAD collapsed, design standards would have been made more stringent.
The WF beams used as cross-beams would have prevented the WTC buildings from collapsing - BECAUSE the pancaking could not have occurred.
But since the WTC buildings had only the steel joists - which were doubling as bracing AND as load-bearing members for people and desks - in place of WF beams, there was not enough integrity to resist the pancaking from getting started. WF beams design would have had MUCH stronger connections between the horizontal members and the vertical members (in this case, the outer skin panels). The mass of and the strength of the WF beams in the area just below the impact would have been able to withstand the collapse of the upper floors. That is IMHO. The integrity would have been so much greater than the few bolts tying the outer skin to the joists.
Consider how hard you would impact if you fell 10 or 20 feet - the distance that first fall was for the upper floors. A 10- or 20-foot fall can kill a human. The G-forces due to the accumulated speed build-up are pretty high - which is why we don't jump down from second-story windows.
Now, take the mass of 15 or 30 some-odd stories and drop that down about 10 or 20 feet. And do that onto thin outer skin panels and spidery steel joists. The first level of steel joists doesn't stand much chance; the mass just blows through them like a hot knife through butter.
And the outer skin, besides being too weak to resist the impact of the falling mass, also loses its bracing connections and becomes even weaker.
Those bolts on those connections are still ASTM A-490 high-strength bolts, so they don't break easily. But they do. And as they break, they make a HELL of a noise. Trust me on that one; I have heard breaking bolts before, and it sounds like gunshots.
Those first joists slow the falling mass a bit, but not enough. When the mass hits the NEXT level of joists, there is even MORE impact. It has, after all, just fallen another 10 feet, after having a running start. At about the second level the die is cast and the pancaking commences.
The outer skin panels act like the walls of a duct, funneling most of the debris straight down, which is also what gravity is trying to do. After all, the center of the Earth's mass IS straight down. So, these two factors try to keep all the mass right down the middle as it all falls.
But only so much structure and debris can fit inside the duct walls (the outer skin panels). Pretty quickly, it all starts exerting some sizable outward pressure on the skin panels. This not only adds stress on the bolted joist connections, causing them to break even more readily, but it also pushes the panels outward, allowing the debris to start cascading in a fountain-looking pattern.
But it all also exerts pressure INWARD, against the central core. In short order the increasing force - as more mass tries to cram into a fixed duct space - crushes the core inward. As you watch in a video the collapse of the towers, note that there is a central "cloud pillar" that remains "standing" as the outer portions proceed downward. The core was a bit behind the outer portions in collapsing, as I view it happening. I attribute this to the need for greater force to collapse the core inward, as opposed to the force needed to "pop" the outer skin from its bracing connections.
Both the outer skin and the core formed the walls of the "chute" or "duct" down which the debris fell. The core simply resisted longer - but a few stories further down, the force was even too much for the core. It wasn't designed to withstand crushing pressure from the outside of it; it was only designed to withstand vertical forces and stresses arising from the wind on the outer skin.
That is about it.
I am a very, very skeptical person about our government's administration, no matter if it is a Republican or a Democrat in office. Both have perpetrated some heinous things upon the world. And they may have been in some way responsible for 9/11.
But they did not do a controlled demolition of the two main WTC towers. It is all explainable by the unique design of the buildings and the confluence of factors that came together on that morning. As much as I want to blame Bush, Cheney and the neocons, I can't. Every element of their collapse makes sense, without resorting to this particular conspiracy allegation.
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