I just came across this:
"live loads on these [perimeter] columns can be increased more than 2000% before failure occurs."
--John Skilling, in Engineering News Record, 4/2/1964
I found it here: stj911.org...
NIST apparently reported earlier in its WTC investigation (according to Wayne Trumpman in a paper authored by him) that the factor of safety of the
perimeter columns, how many more times they could take design loads until failure, was 5, or that they could take 500% more. Keep this in mind.
John Skilling is quoted above putting it to 20, but this was before construction was underway. I'm assuming that these are the most common exterior
columns, or possibly the corner columns, because it's hard for me to fathom the lower-most "tree" columns or the biggest outer columns at the base
to be able to carry 2000% the load of the entire structure above them.
There were 240 perimeter columns on each floor, though, so the loads divided between them would no doubt greatly diminish the role each of them would
have to play individually, provided that the columns are of reasonable size/strength and etc., and if any of this is unreasonable it's just because
the columns we're talking about here were so massive by comparison to other exterior columns commonly used:
The Windsor Tower, for example, had these for exterior support:
The outer beams in the Windsor were also not fire-proofed
, and I don't think I have to post pictures of how bad that fire was, or state once
again that it lasted for a whole day, instead of > two hours. They just bent and warped, as happens, and nothing 'fell apart' until after a whole
day of extremely intense fire, and in a structure in which the steel was very minimal and weak, supporting a lot of concrete in various ways.
And you can see that the Windsor Tower exterior beams were spaced at about the same distance (if not larger!) as the WTC perimeter columns. Just look
at how tightly the WTC exterior columns were grouped in the photo above from Ground Zero. They were linked by spandrel plates in groups of three.
So, big columns, a lot
of them, very tightly packed on the outer "skins" of these massive buildings.
Imagine all the contents of a typical floor in these office buildings (the WTC towers). Keep in mind that the core stucture is taking the majority of
the gravity loads. The perimeter columns also carry some, but now what the perimeter is to be handling collectively is divided by 240 of these things
for each individual column's expected load, right? Is it therefore hard to believe the word of a man that was involved in this project, in 1964, that
the outer columns were being designed be able to hold 20 times their expected live loads individually?
A FoS rating of 5 is much lower, and more common in buildings that require precaution beyond a typical building, but that still theoretically requires
that an equivalent of 4/5 of all the perimeter columns on a given floor 100% lose their ability to support their loads (unlike buckling/etc. when only
a PORTION of the load-bearing capability is lost), before a floor's worth of perimeter structure collectively loses its ability to support it loads
(or is reduced to being able to only hold 100% of its design loads; we'll assume it automatically fails at