Baiscally what you’re saying is that you “doubt EXTREMELY MUCH” that it happened as is widely accepted (which seems to leave open the possibility that it happened as is widely accepted), but you’ve got absolutely nothing in the way of what you actually believe happened. Does that about sum it up?
Let me just throw this question your way once again:
[QUOTE=Valgard]
So I understand that you believe that information is somehow critical but why do you think it is? What is the basis for your belief if it’s not education or experience in the subject?
[/QUOTE]
I was educated as a structural engineer and I’ve tried to describe as simply as possible in a couple of lines of text (as opposed to a few years of college courses) how basic structural analysis goes. I am telling you that what you think is critical information is not, certainly not in the manner that you think it’s used. I am not claiming to be An Authority but I’m certainly qualified to discuss the basics.
So psikeyhackr, your point is that since the impact force didn’t topple the buildings, that the fires could not have been responsible? Huh?
Can someone explain to me again how an unevenly burning fire can simultaneously weaken all the steel above it, eventually causing a ‘sudden symmetrical collapse’, which the building below could not bear, as opposed to a gradual one, which common sense tells me should have dissipated the energy created, as it met the stronger structure beneath it?
Did the different weights of the areas above each impact zone have a recognisable difference on the collapse times?
It doesn’t have to “simultaneously weaken all the steel above it”, just enough of the steel that provide support. In one section, not “all” sections.
And I don’t know what you’re looking at that states that the collapse was “symmetrical”, but the YouTube link provided a few posts ago clearly shows that the collapse began in one corner - watch the top of the tower start to lean into the corner section that originally lost structual integrity.
How much strength does your “common sense” tell you is needed to “dissapate the energy” of 400,000,000 pounds of WTC dropping on it?
Look, the video clearly shows a giant fire in one corner of the building. Then that corner starts to sag ever so slightly. And then everything above the fire starts to fall, and keeps falling.
If you watch other videos, the fall clearly starts at the crash sites, and the upper floors begin to fall into the floors below. This is very different from controlled demolitions where the lower floors start to fall and it looks like a more or less intact building falls into a hole in the ground.
In order to achieve this effect, you’d have to have the floors where the planes hit wired for demolition, then leave them burning for a long time, then trigger the collapse later and hope the fires and crash didn’t destroy your demolition equipment. And you’d have to have demolition equipment throughout the building, unless you were able to predict the exact floors the planes were going to crash into. And wouldn’t it be embarrassing if your demo charges on the other floors went off at the wrong time?
The building fell more or less symmetrically because gravity operates in only one direction–straight down. And all the other forces acting on the buildings that day were very weak compared to the force of gravity pulling those hundreds of tons of concrete and steel straight down. A building that large will collapse like a house of cards because the forces holding the concrete and steel together are very weak compared to the mass of concrete and steel they have to hold together. As I said earlier, our intuition about the strength of concrete and steel are pretty good on the scale we’re used to. But scale that up by a factor of 100, and rigid steel and concrete bend like taffy and crumple like paper. A building 100 times taller is 100 times weaker.
I do not think that means what you think it means.
South Tower remained standing for 56 minutes; one report from floor 105 said that interior portions had already been falling at least 22 minutes earlier.
North Tower remained standing for 102 minutes; NYPD helicopter units had reported the upper floors leaning eight minutes earlier.
I don’t know if anyone is saying that that’s what happened (fire simultaneously weakening all the structural steel) but it’s not really necessary for there to be a collapse.
As steel gets hotter it loses strength. IIRC at 1000F it’s lost about 50% strength. So you’ve got a huge load (ten floors of a massive office building) that is held up by a number of beams and columns. Now sever a large number of those structural members by slamming a large jet into them at 500+ mph. Buildings are overdesigned so that might not cause the floor to collapse, depending on exactly what gets knocked out.
Next there’s something called the P-Delta effect; roughly speaking a column that is loaded along its own axis has a certain strength, however if you offset that load just a tiny bit (say by having the column deformed perpendicular to its own axis) then the compressive load the column is carrying also generates a bending moment (P is the compressive force, delta is the deflection, so the moment is P*delta) which acts to stress the column to deform even more in the direction of delta (so the structural members that were bent but not cut are now subject to loads that make them want to deform further, thus increasing the moment, thus increasing the deformation, etc) - this starts nudging those supports towards failure.
So now you’ve got a structural system that has had a number of supports cut (plane impact), others weakened by impact, P-delta effects and the like, and given that the whole thing didn’t give way at that point it’s because the total load they’re holding was being redistributed amongst the remaining supports, which means that parts of the system are now carrying loads in excess of what they were designed for. There are various safety factors used when designing these things, precisely because you occasionally gets loads larger than predicted or strengths lower than predicted, but this event is beyond what you design for.
So everything left that is actually holding up the floor is being overstressed, and now the fire heats it up. At some point the heated steel in various locations is going to fail, and when that happens all that load has to be redistributed to the nearby structure, overloading it even more, those members fail and so on. Keep in mind the staggering loads involved here. When the critical point is reached the whole floor may collapse in an instant - have you ever done the trick of standing on an empty coke can? It holds your weight just fine until someone taps it gently with their toe, that induces local buckling and the can is crushed flat instantly. It doesn’t gradually fail in one spot so that you tip over like a falling tree, it just squashes into a disc all at once.
OK, so ten floors of skyscraper suddenly drops one floor, call it twelve feet. That’s a massive impact load on the floor below (which was also already damaged by impact, explosion, fire) - I did some extremely back-of-the-envelope calculations just now; if you drop something 12 feet and have it come to a complete stop in 1 foot you get an acceleration of about 10g at some point. I’m not saying those numbers are perfectly accurate but the point is that the impact load when one floor gave way could have been several times the static load of those upper floors. That would crush the next floor down and the whole mess would continue to drop onto each floor below.
It might certainly have been different if one floor had slowly and gently failed, gradually lowering the floors above it into place and avoiding the disastrous impact loading, but that’s not what happened.
This is no time for a well thought out, intelligent, easy to understand and polite response!
Hope you’re satisfied, Valgard. You’ve ruined* a perfectly good conspiracy thread! I call for your immediate banning!
*I’m certain (unfortunately) that you haven’t solved or stopped a thing actually. But if one can’t be swayed by your logical and reasoned response, I don’t think it’s going to happen at all.
Wasn’t there a 9/11 thread about three 9/11 threads ago where exactly the point about steel and its heat failure level was discussed, and some idiot fuel-truck driver in Oakland crashed into a support for the freeway, causing a fire that weakened the supports and caused a collapse after 45 minutes of burning fuel? I seem to recall that a live example of steel support failure due to fire seemed almost heaven sent for that thread.
Well, let’s just say that some of us take our debating very seriously. 
A little…too convenient, perhaps? :dubious:
There was and the various CT sites immediately dismissed it as being a totally different situation than the WTC.
The fact that hot metal is weaker than cold metal is not exactly an amazing new concept - blacksmiths have known this for thousands of years, for example. There are plenty of studies out there showing the exact curves of yield strength vs temperature; it was an important point when I was taking steel design courses, the LRFD codebook had a section on it, the value of fireproofing was emphasized (that’s all the crud you see sprayed onto structural steel, shotcrete or something like that).
And of course it has to be re-emphasized that it’s completely unnecessary for steel to actually melt for it to fail; when the capacity of a beam drops below the load it’s carrying, it’s going to fail. It just has to be “hot” not “a puddle of liquid”. Plus there’s the effects of thermal expansion which can introduce new stresses in unusual directions, change the geometry of the structure, etc.
There’s also the 2007 furniture store fire in Charleston, which I alluded to earlier. The 42,000 square foot steel frame flashed over and collapsed after about 45 minutes; I’ve seen at lest one twoofer dismiss this as “nothing”.
I certainly appreciate Valgard’s detailed response above, but I’m still curious to know what we would have seen had the core been sabotaged a third of the way down, allowing the weight above to ‘lurch’, rather than slowly - well, not really slowly! - collapse.
Am I to take it then that if it was possible to gradually drop increasing weight on a structurally sound building, eventually the entire thing would ‘crush down’, and the extra material would not just topple over the sides as the debris built up? Is there a formula for how to work out this ‘collapse point’?
Say an ordinary office fire had gotten out of control on the higher floors; how much longer would either tower have lasted under those circumstances? Obviously something like that must have been considered when the towers were being built.
It was. What was not considered was the possibility of the fireproofing being blown away, 21 tons of kerosene, and a fire department that was largely unable to do anything about the fires.
And don’t forget about structural beams being demolished by a 767.
Those structures would (presumably) still be in place in a run-of-the-mill office fire, and able to lend help in supporting the weight of the upper floors. (Think back to the weight redistribution that ** Valgard** was talking about.)
Any response CPists?