Explaining the Supposedly "Rapid" Collapse of WTC Twin Towers
By Mark Ferran, BSEE SCL JD MCL
The NIST reports did not purport to discuss nor explain anything that happened after the tops began visibly tipping which was the time of "collapse initiation" (other than overloading of the flor slab supports in the FAQ previously quoted (click here)). It is unfortunate that NIST engineers did not make any significant effort to use their data, computers and educations to address the many inane conjectures about how the collapses progressed after the tipping began. NIST could have saved a large amount of the National GDP from being consumed by people obsessed with the physics of failing columns and falling buildings.
Alan Higgins is technically correct that NIST reports do not explain "why all the steel broke" "from one side of the building to the other" (after collapse initiation) due to the tipping. The sudden failure of columns from one side to the other was rapid, almost instantaneous, due to a well-understood principle called "leverage" which is taught in engineering college textbooks in the context of the domino failure of steel supports. Engineer Thomas Eager, mentioned but did not "explain" in detail this domino collapse affect in his essay titled: "Why Did the World Trade Center Collapse? Science, Engineering, and Speculation" as follows:
"Nearly every large building has a redundant design that allows for loss of one primary structural member, such as a column. However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down."
Click Here for Eager Article
In 2006 I explained in a pedagogical manner the basics of this well-known "domino" failure affect as follows:
[I recommend viewing these text diagrams in Courier or other single-width font]
----- Original Message -----
From: Mark Ferran
To:
Sent: Wednesday, August 09, 2006 4:21 PM
Subject: Collapsing buildings
My quick explanation of the affect of Gravity upon a falling top-of-a-building explains/outlines the mechanisms of the collapse and "fast" descent of the collapsing towers.
Responding to comment made elsewhere that "because the fires were not symmetrical, deformation should have resulted in an asymmetrical collapse." [It did in each tower.] This mistakenly assumed that there was a “symmetrical collapse”. Once the columns on one side of the building fail, they cause the failure of the next nearest columns, and so on. This can occur in less than a second, and it will appear from outside that all columns failed at the same "time" or that there was perfect symmetry, but that is not quite correct.
Imagine that the simplified figure below is looking from the side at one floor of the core with five massive columns holding up the massive core of the building above.
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I I I I I
Take away the first column on one side (e.g., by airplane impact)
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And the building might still stand (and it did), but now there is more weight (load) on the second column, and that "more weight" is not merely the same amount of the weight that the first (missing) column used to support, because there is LEVERAGE. Some of the weight that the third, fourth, and fifth columns were carrying is now being carried by the second column, due to LEVERAGE. The cantilever (unsupported end) multiplies the force added to the next nearest column. Because iron is flexible, the fulcrum point and muliplication factor of the leverage is NOT easily calculated, but it is real and postive.
If fire causes expansion (causing buckling, or shortening), distortion (Bucking, shortening) or weakens the second column (strength failure),
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I I I
then all the weight of the missing first and second columns is shifted to the remaining three, but the third column absorbs ALL of this shifted weight, PLUS a leverage factor, and the last column may be carrying less of the load than before, or may even be in tension (not supporting any weight of the building at all, but pulling down on it). The fourth column may be in tension or in compression like the last column. The point is, that the moment that the second column fails, the entire load of the building might be pulsed down upon the next (third) column.
At that moment, perhaps a fraction of a second after the second column failed, the third column will be crushed (because it may be carrying the entire load of the building above it);
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I I
and a fraction of a second after that, the load will shift to the fourth column, and the fourth column will be crushed.
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I
When you get to the last column standing, two things can happen: 1) because there is no "leverage" acting on it, it might survive long enough to momentarily act like a pivot or a hinge (as was seen in the tipping of [both WTC tower tops]) [The longer the last column stays intacts, the move visible the tipping will become]
or 2), because the weight is bearing down or has shifted too erratically, it may fail completely a moment after the fourth column failed (in that case, the whole building core above will appear to drop approximately straight down (as in the second tower).
The illusion of "symmetry" is preserved by the large scale of the falling building, but the only symmetry is in the shape of the building.
The same principles of progressive failure of adjacent columns due to shifting loads apply in three-dimensions, where you have a distribution of columns in an area, but the mathematics of load re-distribution is more complex.
If you ever bent a hollow pipe [like a car antenna], although it will bend to a degree, once it is forced past a critical angle/tension, it fails and folds suddenly, unlike the behavior of a solid-steel rod.
If the steel columns of the WTC had been fabricated of
solid (non-hollow) steel columns 1368 tall, instead of hollow box beams bolted together standing end on end, the collapse might have been a bit slower, or the building might not have collapsed at all, but steel has a dollar cost and a significant weight, and it is an established practice to use hollow or I-beam steel instead of solid steel columns/beams.
This is true in both human construction and in nature, where many plants have hollow stems.
For the upper block "to drop instead of tipping further", the intact steel in the back perimiter wall (which caused the tipping) had to fail, which it did after the tipping was significant and noticable. The "steel" of the back wall was not one solid sheet of steel wall. It was a fabric of 3-column sections of beams bolted together, which was designed mainly to support a vertical load. It was not designed to resist sharp bending of the wall due to the enormous weigth (moment) of the descending top block.
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