This is quite simple, and I don't see why you have difficulty with it (barring the fact that you clearly have no experience whatsoever of structural design).
1. Buildings are designed to accommodate dead (i.e. self weight) and live (people, furniture, wind, etc) loadings. These design values are then subject to safety factors based on credible risks, set out in various design codes and standards.
2. Design of framed buildings such as the tower is complex; I do not intend to discuss in any depth the various jointing and connection techniques however a joint - welded or bolted - will only be designed to take specific loadings. These loadings will be for specific directions.
3. Lest anyone doubt how complex this all is, then remember the case of the Citicorp building. If you have no idea about Citicorp without having to Google, then do not trouble this board with any claims of structural expertise.
4. It may be helpful if you consider the complete tower structure - floors, inner core, and outer facade - as acting together as a large girder, or space frame (you must be familiar with both of these). Damage to or loss of one element can and will have an effect on the overall stability of the "girder".
5. The towers were built with spare structural capacity, however a significant part of this was compromised in the initial impact. Further damage was cause dby the fires. Although the designers claim that the design was built to accommodate an aircraft impact, no calculations have ever been produced to show the extent of this (I refer you again to Citicorp); there were (a) no applicable design codes or guidance at the time and (b) limited computer modelling techniques available at the time.
6. The fire weakened the floor trusses, causing sag. This in turn led to deflection of the outer structural envelope (or facade). The steel could not accommodate the required loadings at this point (a buckled structural member will be weaker, even before we consider the impact of buckling on joints and risk of their failure). The hat trusses probably served to redistribute loads, but ultimately exceeded design capacity and failed.
7. At this point, failure of the supporting structure for the upper part of the building is inevitable and what is frankly a massive amount of material begins to move downwards at a 9.8ms/-2. The momentum and mass are substantial.
8. The structure below is not intact, because the hat trusses are no longer doing their work and the bracing effect of the upper structure has been lost. It is overly simplistic to suggest that this portion of the building is sound, a point usually overlooked by "alternative" theories.
9. The steel joints, etc. are not designed to accommodate the loadings imposed by the impact of this massive mass and momentum. They are deisgned to accommodate normal loadings, which will be many magnitudes less. They will fail; there is absolutely no doubt about this, from a structural perspective. The time involved with be absolutely minimal. Although not a NIST document, Greening's paper (again you should be familiar with this) gives you a very basic idea of the kind of issues we're talking about.
10. At this point the collapse becomes progressive and self-perpetuating.
Let me give you a simple analogy (not my own, I hasten to add, but a rather a very good, simple way of looking at the problem posted elsewhere).
If you put a brick on your head, there will be no problems. You will be able to walk around (subject to balance), suffer no injuries, and so on. The additional dead lead of the brick (together with minimal live load for wind, etc. on it's faces) is well within the "design" load of your skeleton.
If we drop that brick from just 0.5 metres (far less than the floor-ceiling height of wtc) then you will suffer major head injuries. If we drop it 2.5 metres, you will suffer severe head and spinal injuries. Realistically, you will die.
Now as far as I can see, the ol' canard you're attempting to pull out of the hat is the one about the resistence of each floor sufficiently slowing down the collapse in order to markedly influence total collapse time.
I have to tell you that the sheer mass and momentum of the upper (mobile) structure is such that it's going to make bugger-all difference. We're talking about tiny fractions of a second each floor, not seconds.
This is what we, as trained professionals, would expect. Number crunching is irrelevant.
Now if you want to prove differently, don't demand that other people do your work for you. Go and find out how each joint was formed. Calculate the design loadings, then look at the imposed loadings from the collapse. Calculate the length of time to failure. THEN come back and tell us if there's an issue or not.
And this, I believe, is where YOU have a problem. You don't understand structures in any competent manner. Hell I work on tall structures every day of the working week and I have to get a team of real experts from Arup do the number crunching for me on a tall buildings project, so what hope has a lay person got?
Intead you try to claim that NIST have been remiss in not calculating something wholly irrelevant.
You cherry pick facts and soundbites, other (wholly irrelevant) cases such as Windsor. Tell me, NB, do you really know about the Citicorp Building without looking it up on Google? Have you ever heard of Ronan Point? How much do you understand about the actual performance of fires without going to Wiki?
Have you read the Sheffield University research papers? Were you even aware that Sheffield University (it's in the UK, btw)has a highly respected fire engineering unit?
Did you know that Edinburgh University (that's in the UK too) had published a paper suggesting through fire modelling that the trusses would have failed even withouth the aircraft impact? Likewise have you seen the Arup papers which seperately came to the same conclusion?
When considering the susceptibility of steel buildings to fire, were you aware that every single building standards/regulatory code in the West (and I suspect elsewhere) had identified the problem for at least 20 years (when I started training) and probably a lot longer? Were you aware that steel firms such as Corus publish extensive advice on this?
Do you know how we protect steel against fire? Are you aware of the different systems available and fire ratings? Hell, do you even know what intumescent means without looking it up on Google?
Have you looked at the various engineering media reports on the collapse (NCE would be a good start, but I suspect you've never heard of that either) in order to try and understand how we as an industry have viewed and understood the collapse.
I can go on all day with a list of architectural, structural, and fire engineering issues which you have to understand before you can even begin to comment on the NIST report with any degree of confidence. Each of these disciplines requires between 5 and 7 years of a university education, with intensive study across a whole range of specialist topics. This is then followed by practical, on-the-job training.
So with the deepest respect, don't read a few general web sites and then come back and start chucking about structural theories or "common sense".
