Question:
How do you pull centralized columns inwards?
Did the fires melt the 47 core columns?
How did the core come straight down?
If the floor trusses are connected at their ends to the external columns AND the internal columns then the pull is towards the centre of the truss, not the centre of the building.
The NIST report is concerned with what initiates the collapse....what causes the catastrophic failure of the structure.
The floor trusses are always going to be the most likely weakest link in the structure because they are lightweight engineered members (In other words instead of using a large steel beam to span from the outer columns to the inner, they use a designed structure of thin steel members some of which are in tension and some which are in compression but as a whole they have the same loading capacity as a single large (and heavy) steel beam) and with any truss design each small member plays a part in providing strength to the truss as a whole. It's possible that if you live in a house with a pitched roof and go and have a look up in your roofspace you will see roof trusses. In the UK the most common type of roof truss is called a 'fink' and on elevation it is basically a triangle of small timber members (the rafters and ceiling joists) with a 'W' arrangement of small timber members filling the space within the triangle. If you damage or remove any of those members you dramatically weaken the truss. A conventional roof would have much larger rafters and ceiling joists and the space within the triagle would be clear. But this makes the roof heavier and more expensive to construct, so trusses are favoured for certain types of construction.
The WTC floor beams were also designed to be lightweight and cheaper than raising and placing huge steel beams.
The sag of the truss is due to the effect of heat on these small steel sections, which is why fire protection is important. However if the fire protection is missing, damaged or destroyed by impact then the trusses become vulnerable.
As the truss sags it pulls on the connections with the external and internal columns. At some point those connections are going to fail or the connection between the columns above and below are going to fail.
This initiates the collapse because, just as with the truss, the building as a whole is an engineered structure which stands because all of the members are working together to defy gravity.
The columns hold up the trusses, but the vertical loads on those columns makes them want to move outwards, just as your pitched roof rafter wants to spread outwards where it meets the external wall but is restrained by the ceiling joists (or bottom chord of the roof truss), and so the restraining member is placed under tension.
The columns of the wtc tower are pulling against the floor trusses and the floor trusses are pushing down on the columns.
When one or other of these fails, then that section of the building is unstable and becomes dependant upon the redundancy (the ability of other members to take additional loading) of the remaining structure to maintain it's integrity.
However, take out enough of the structure, and apply heat to the remainder, and you have a structural failure waiting to happen unless you can remove the heat and repair the damage (or temporary support the structure)
Once the failure is initiated it can only snowball as the already damaged structure is now receiving further damage from the parts of the structure that fail first. So if the structure fails locally at the original area of damage, the structure above this point of failure is going to stress beyond it's ability to work as originally designed, in just the same way that removing a given portion of your external wall will result in the roof collapsing because skyhooks have yet to be invented.
Now that the collapse is initiated the members which made up the structure will, to all intents and purposes, resort back to their original state. In other words, a steel column which was delivered to the contruction site on the back of a lorry will most likely end up being removed from the site of the collapse on the back of another lorry.
phew!
I hope that helps
Someone
Somewhere.