911 Scott Forbes �power-down� story. Possibly False.

Pheonix, what do you think? I've never worked on steel structure near that size, but I have worked on alot of rollbars... the premise is the same, but rollcages are designed to take some pretty extrordinary impacts. On a roll, generally you will have one column that collapses, and the others just bend to compensate... even when you drop almost perfect uside-down, one of the front columns will almost always crush, and the other will just bend to compensate for the side-ways pull.

Not saying that it's impossible for a building like that to collapse in an almost vertical fashion, but it's not too likely considering the damage and where the fires were centralized. Again, Pheonix, what's your take on this?

I recollect seeing an estimate that each floor was something like 3,000,000 million pounds apiece (someone correct me if wrong)

The basics of construction were a core area that contained elevators and other building services THE SPINE if you will allow. The horizontal beams supporting the open floor area connected to the exoskeleton formed by the outer columns at the exterior, the SHOULDER or RIBS if you will allow. This basic design allowed more rentable square footage to lease and a more flexible open floor plan for the occupants, both features being desirable at time of constuction and now for a successful project.

Now imagine you are a stuctural engineer concerned with deadweight load and wind forces, you will end up with a design that is rigid in the verticle plane to support the load while at the same time being somewhat flexible in the horizontal plane for wind load, bend but don't break a criticle joint thats overloaded, the buildings frame then becomes a system to evenly distribute the force among its many parts - never overloading any one part. Of course a safety factor is included to prevent collapse due to unforseen loading, this vareis from 150% up over 200% and better.

Knowing what I know of buildings I was amazed and impressed that the WTC towers lasted as long as they did that day. The designers should be commended, as I watched the aftermath of the first hit and saw how many criticle exterior columns were lost I waited with trepidation and baited breath for the inevitable collapse to happen. It was no surprise to me.

You now that heat makes steel lose its strength through loss of temper well below its melting point.

Imagine a somewhat rigid core of heavy steel and concrete combined with an exterior structure of steel colomns that is held in place by flanges designed for verticle load but a much smaller horizontal stress load.

Now damage or destroy 25% of the exterior columns and damage (guess) 10% of the core with more than 20 floors of weight above, add some heat and what you get is three bad things happening at the same time.

The intact floor framing above the fire expands outward, applying massive force to the more rigid inner core while weaking the load bearing exterior colomn system, this area is above the aircraft damage. Remember 20 floors times 3,000,000 pounds equals 60,000,000 pounds of deadweight transfering much of the force to the interior.

Now lets imagine ourselves on the floors immediatly below the strike area, debris from multiple floors collapsing is resting on an intact floor overloading the horizontal beams of that floor and substantialy bending those beams while the weld flanges are intact the exterior columns in this area are literally pulled inwards towards the core many inchs aplying even more force to the already overstressed core. At this time the exterior is not able to tranfer any more stress from the core. The concrete reaches its compression threshold and turns to powder (remember mass and energy are an interchageble medium) due to the extreme overload condition.

The resulting failure is an unstoppable process that would break all weld plate connections of the floor beams to the exterior exoskeleton as the weight accumulated on the progressive collapse. The core already in a super overload condition would appear to instantainously disintegrate as energy of the overload was converted to heat in its death throws of finding a way to disipate the tremendous energy stored in the overload.

This is what you saw on television.



[Edited on 30-4-2004 by Phoenix]