Reply #291: Sagging, etc.

[Kevin Fenton]

"Are these percentages or the load bearing capacity of the entire building or a single floor?"
The percentages are for the whole impact area. The percentage of the gravity load-bearing capacity of a single floor that was destroyed would be less; for example the maximum number of perimeter columns destroyed on one floor is 18, whereas 35 columns were severed in the North Tower across all floors.

"I assume that you are getting these percentages from the NIST Global Analysis"
The perimeter numbers are from everywhere, the core numbers are from NIST's base case simulation - the final report wasn't drafted using the base case, but the more severe case.

The asymmetric nature of the damage obviously aggravated the problem, but my understanding of the hat truss indicates that, given the relatively minor extent of the damage compared to the whole structure, this wouldn't have been a problem.

There's no way the towers' structures were simply overwhelmed - they were too strong for that. Most people try to get past this by suggesting some sort of weak point and NIST goes for the floors. I don't buy it for various reasons. For example, the duration of the fires (102 and 56 minutes) doesn't seem long enough - lots of buildings burn for hours without falling over. Also, the recovered steel samples indicate temperatures significantly lower than NIST claims, although NIST basically ignored this (i.e. when they had a steel sample that had demonstrably been at a lower temperature than their model indicated, they just tweaked the model locally to reflect this, they didn't make a full adjustment to the model).

"What specifically is wrong with the NIST model?"
I'm more or less happy with their base case model (except the aircraft speed for United 175), but the towers didn't collapse in the base case (or the less severe case), so in the final report NIST used a different case (the "more severe case"), where almost all of the input numbers were altered. For example, to increase the damage to the core columns the aircraft speed was increased, the aircraft's failure strain increased and the tower's failure strain decreased. Also, to increase the effect of the fires, the amount of combustibles was increased by 20% - an aggregate rise of over 200 tons on all the fire floors.

"How do we know they don't pancake?"
AFAIK two highrise buildings have pancaked in history. One of them (L'Ambiance Plaza - but I'm probably spelling it wrong) was not finished, so the various steel elements weren't actually bolted or welded together - no wonder it pancaked. The other, a tower block in London, only suffered a partial collapse and was largely intact afterwards.

"nothing even close to having thousands of gallons of jet juel explode inside."
American 11 was carrying 10,000 gallons of jet fuel when in hit the North Tower. United 175 was carrying 9,100 gallons when it hit the South Tower. Say 20% of the fuel fireballed outside (less for American 11, more for United 175), 15% fireballed inside. The rest spilled and burned up in a few minutes. Then there were just office fires.
Nothing anywhere near this big (or as big as WTC 7) has ever been destroyed by controlled demolition.

Seven
If the damage from the debris from the North Tower was that bad, how come it stood for nearly 7 hours?