adoucette, I have had to do a great deal of research to substantiate it, but it appears that you are correct, and the core of the WTC was in fact made of steel and gypsum drywall. This considerably changes my estimation of the sources of the concrete we see in the pictures; the only concrete used in the construction was the concrete in the floor pads. So there is nothing in the core but steel columns and open space; I have to ask, however, what the floors of the elevator lobbies were made from. My impression is that there were separate concrete floors poured there too, but this is nothing but a conjecture. The fact that there were horizontal trusses connecting the vertical columns beneath those floors, whatever they were made of, is documented fact; and they could not have been horizontally diagonally braced, because that would have prevented putting elevator shafts between them. This means that their horizontal strength would have been limited to the strength of the joints between the vertical core columns and the horizontal braces between those columns.

The use of the term, "Vierendeel truss," to describe the design, refers to the fact that rigid beams, the perimeter columns, were connected perpendicularly by horizontal spandrels, which were welded to the perimeter columns; the absence of diagonal reinforcement is the primary characteristic of the Vierendeel truss design. An important point about this design is that to remain rigid, it must have rigid connections; in other words, the flexure at the perpendicular joints must be limited, or the trusses will become unstable and collapse. The resistance of this design to lateral loading (i.e. horizontal loads imposed by wind) is not on the face exposed to the wind, but in the sides parallel to it; in other words, when the wind pushes at the face of the building, the sides that are parallel to the wind resist deformation at the joints between the spandrels and the perimeter columns.

This means that the force of the wind attempting to buckle the building comes on the sides parallel to the direction of the force, not on the core, and not on the face the wind is blowing against or the face opposite that one. Thus, the core can be more lightly built, since it only has to resist the vertical force of gravity. The face perpendicular to the wind only needs to be prevented from buckling inward, and that force can be distributed along the entire height of the core by the horizontal trusses under the floors, just as the sides parallel to the direction of the wind distribute the buckling forces across every joint in their entire height and prevent the bulk of this force from being placed on the core. Rigidity in the floor joints to either the perimeter columns or the core columns is not necessary, and would in fact place more of the force resisting buckling on the core rather than on the parallel sides, defeating the design goal of reducing the lateral stress on the core and imperilling the building, since the core was only designed to take the direct wind pressure, not the additional buckling component; and even some of that pressure would have been relieved by the spandrels across the perpendicular face attempting to compress the spandrels across the parallel faces, as they would have to do to move the perpendicular face against the floors and apply lateral force to the core.

Furthermore, part of the design would have included the idea of the perimeter columns not supporting all of their own weight; this is clear from the existence of the "hat truss" at the top of the building, which transferred some of the weight (vertical force) of the columns onto the core columns, allowing the perimeter columns to be lighter, as long as they could support strong enough joints to preserve the Vierendeel truss' resistance to horizontal forces, since they didn't entirely have to support their own weight.

So the only things the core columns would have had to break in order to buckle inward were the horizontal trusses connecting them to one another and whatever flooring existed inside the "hole" in the main floor pads at each story. And in between floors, they would be free to buckle inward or outward, since they were not supported by any other structure (we'll ignore drywall in the context of concrete and steel; a moment's experiment with a hammer or even your bare fist will convince you that drywall might as well be air in this context).

So now, pending comment from someone who has good structural engineering knowledge, I think we can understand the distribution of the forces in the normal (undamaged) building:
a. The perimeter columns and spandrels form a Vierendeel truss that resists deformation because of the rigid attachments between the perimeter columns and the spandrels, thus resisting forces that run along the spandrels.
b. Forces that are perpendicular to both elements of the Vierendeel truss, in other words pressing inward or outward, are distributed between the ends of the spandrels and the floor diaphragms, and this prevents buckling inward or outward.
c. The perimeter columns and spandrels are assisted in holding up their own weight by distribution of some of that weight to the core columns via the hat truss at the top of the building.
d. The core columns hold up their own weight, the vertical weight of the floors, and some of the weight of the perimeter column/spandrel Vierendeel truss system.
e. The perimeter column/spandrel Vierendeel truss system prevents lateral loads from having too much influence on the needed core column sizes, reducing their size and therefore weight to only that required to support the vertical load. It is in turn reduced in size and weight itself because it is partly supported by the hat truss and therefore by the core columns.
f. This entire design maximizes the available usable space within the building, both by eliminating the regular columns that are necessary all over the floor in a traditional design, and by eliminating the need for diagonal trusses which would obstruct open areas.
g. There are certain weaknesses to this design:
i. The attachments between the spandrels and the perimeter columns; however, there are so many such attachments that the force is so distributed that no one joint takes more than a very small fraction of it.
ii. The hat truss. If the hat truss fails, the perimeter columns will not be able to hold up their own weight.
iii. The internal rigidity of the core columns to prevent them from buckling horizontally. This is helped by the existence of the horizontal trusses connecting them presumably at some or all floors, and perhaps by the flooring (depending on what it was), but without solid concrete between the columns between floors, nothing prevents them from buckling inward; not only that, but the trusses between them cannot prevent them from buckling outward, and the tensile strength of steel is considerably less than its compression yield strength, so the natural direction it will fail is outward, since inward is constrained.

Now, we begin to see what happened when the plane hit the building. First, severed perimeter columns' weight below the point they were severed was no longer supported directly by the hat truss; instead, this weight (at least that beyond what the perimeter columns themselves could bear) would be distributed outward to unsevered columns, and thence upward to the hat truss and downward across the other perimeter columns, and some would remain on the bottoms of the severed columns. This would change the loading on the hat truss, and also change the loading on the core columns, and finally change the loading on the other perimeter columns. The additional force would not be a direct function of the percentage of the columns cut, either; the more columns cut, the more weight would have to be redistributed. Remember also that the entire weight of the severed perimeter columns would come on the hat truss, instead of it partly being on the lower portions of those columns. So we have an increase in the total amount the core columns have to support both from the redistribution of forces from below the severed points, and from above them as well. Obviously, the lower on the building the perimeter columns are severed, the more extra force there will be above the severed columns, and the less there will be redistributed through the spandrels.

Severing perimeter columns does not affect the lateral strength of the building perpendicular to the severed columns and their spandrels; it affects the lateral strength of the building parallel to the plane they make; this is obvious if you examine the way the forces are distributed in the undamaged building. In other words, if the north face is damaged, a north or south wind isn't the problem; it's the east or west wind that is the problem, since the north and south faces resist such winds.

Severing core columns does not affect the lateral strength either; it only affects the vertical strength. However, we must also note that in the event that all of the core columns fail on one floor, the entire perimeter column structure added to the hat truss and the upper segment of the core and all the floors between the point at which the core fails and the top of the building can no longer support its own weight. It is not designed to; it is designed to be held up by the hat truss, allowing it to be much lighter than it otherwise would have to be! This point is essential: if the core fails, the perimeter columns cannot hold up both their own weight and that of the core and all floors between the failure and the top.

Now, one other point is salient: the antenna mounting on 1 WTC. We know from sources already presented that the antenna was mounted on the hat truss, which formed its only support.

OK, now how does this affect the dynamics of the two collapses?

In the collapse of 2 WTC, we note first that the top tilted, and second that the collapse occurred much sooner after the impact of the plane, and third that the damage was much lower down, and fourth that the damage involved corners of both the core and perimeter column structures.

The fact that the damage was lower down was the primary cause of the shorter time to collapse; this meant that the force on the core was much greater, due to the extra length of perimeter column and spandrel loaded on the core (because of the severing of the perimeter columns at the impact site). In addition, the corner core columns were larger, and thus took more of the force, than the other core columns, and one of the four would have been more heavily damaged or even destroyed in the off-center 2 WTC impact, as opposed to the more centered 1 WTC impact. And finally, we see that in this failure, the side facing the impact and side away from the impact are not the direction the top leans when the tower collapses; instead, the tower leans toward the side of the tower that the impact was off-centered toward. This is confirmation of the fact that the lateral stress created by the failure of the core columns on that side could not be supported by those same core columns, and when the top started to lean, it caused complete failure of the core, which allowed the top of the building to begin to fall; once this had happened, the remaining core had to support the force of the entire structure above that point falling on it, at some 3 or 4m/s, which resulted in a load scores or even hundreds of times beyond its design criteria. The results are obvious, as is the fact that the most damaged side collapsed. Once the top was falling, the complete collapse was inevitable, because it so severely overloaded the remaining structure; and as each floor hit the next, both the one above and the one below would be destroyed, and the remaining weight of both would descend on the next floor below that one.

In the 1 WTC collapse, much more time was required for the extra weight of the damaged perimeter column section to overcome even the reduced compression yield strength of the core columns. The core failed first, as we can tell from the evidence of the antenna. Once the core had failed, the hat truss went with it; and all the weight of the floors, plus their own weight, came on the perimeter columns; and this was well beyond their designed capacity, so they buckled, and of course they did so most dramatically at the site of the impact, where they were the most overloaded. Once they had done so, the floors were free to fall, and when they did, they smashed all the building below them, accumulating both mass and velocity as they went, just as in the 2 WTC collapse.

The evidence that 2 WTC collapsed on its own is present in the fact it failed first, and the fact that it failed according to just what one would expect after thinking the design constraints over, and the facts that its initial failure was asymmetric, and the impact was asymmetric, and the failure occurred first where the impact would have done the most damage. Had this been a "controlled demolition," the demolition explosives would have had to have been at the impact site; this is impossible, since no such demolition explosives could have survived the crash, nor could they have been placed in the middle of a fire.

The evidence that 1 WTC collapsed on its own is more subtle, and requires more careful examination. What we see in this case is that the core went first, as the evidence of the antenna shows. We cannot see where the core failed, because we cannot see inside the building. However, we can extract some clues from the antenna, and from the behavior of the fire. First of all, statements that the radio tower begins to fall first are inaccurate; I invite you to view this, which claims that the tower falls in frame 6 but the facade does not. In fact, marking the positions of the top edge of the facade and the two light-colored protrusions on the right side of the tower, I was able to determine that the appearance of the antenna falling first is an illusion created by the smoke; since the smoke doesn't fall, one's eye assumes the facade isn't either, but actually, it is, if you measure it. Furthermore, the tower rotates as it falls, and we can (by describing its arc) determine the end of the lever arm it is rotating on. If we do this, we find that the end of that lever arm is at about the same floor that we see show the first signs of collapse, that is, the floor where the billowing clouds of dust and/or smoke are first seen. Note that you have to go beyond the last frame from the above link; the antenna is still falling straight down at that link's "frame 12." Get a good player and go frame-by-frame until you see the antenna start to rotate, then measure the lever arm for yourself. Note that its endpoint has to be moving downward; this will help you get it right. Look here for the full video in MPEG format, and here for the source for both links; it's the first video listed in the North Tower section.

What we see is in fact not symmetric at all; it only appears that way. The evidence of the radio tower is that its symmetry began to decay about the time that three or four floors had collapsed, and the process was well underway; however, by that time, the masses of the floors and the perimeter columns were moving sufficiently fast that nothing was going to stop them, and since they formed the majority of the mass, a small asymmetry in the core collapse, particularly above the place where the ongoing collapse was happening, as it clearly must have been, wasn't going to deflect them enough to matter. Remember also that the only thing that could have tipped the antenna over was existing core underneath the core section the antenna rested on. Thus we see that the core had failed only on one floor, but that failure was enough to allow the top of the building to start downward, and once it was moving nothing could stand in its way.

Had 1 WTC been demolished, there would have needed to be explosives on the floor where the failure occurred; but the evidence of the antenna and the video says that that floor was one where fire occurred previously, which again has the explosives in the middle of the fire, or being set by people in the middle of the fire. Again, this is impossible. We also know from the antenna that the core failed at only one point; otherwise, what did the antenna and the core under it hit that caused it to begin to rotate?

Another interesting detail from that first video shows that a section of perimeter columns on the right doesn't fall immediately; this proves that the floors were disconnected from it on that side for at least several floors. Since the floors were bolted to the perimeter columns, it therefore follows that either the bolts or the flanges they were attached to had failed and the floor slabs had fallen without taking that perimeter column section with them. If those bolts or flanges failed to stop the falling floors, what else could? And why would anyone doubt that the floors could simply be stripped off the insides of the perimeter columns? The evidence is right there in front of you: the core supported the floors, not the perimeter.

Let's see where this analysis takes us.

A High School Physics Calculation that sheds light on the FEMA Fairy Tale


From http://reopen911.org/heller.htm (from an article by David Heller)




I'm pretty sure that I've never seen this calculation. However, anybody with high school physics can carry out this calculation, though they might want to do it on a computer, as I doubt there's any mathematical shortcuts.

Of course, if somebody can find this calculation on Hoffman's web site, please post a link, and save yourself or others the trouble of doing this calculation.

To conceptualize what Heller is talking about: imagine cutting each tower into slices 1 floor thick, from the collapse zone on down. (Above the collapse zone, there is no need to do this.) Further, imagine shrinking the height of each floor down to an infinitesimally small value. Pretend that a set of giant claws are holding each floor in place, at it's proper height, but that each claw releases it's floor a split second before the mass falling on top of it hits it. Thus, there is no upward force being considered due to columns, diagonal supports, etc. AT ALL.

So basically, the algorithm is as follows (Assuming collapse initiation at floor 73):

Take the mass of floors 73 through 111 (call this m(Sum(39)), and calculate the downward momentum after falling through a single floor. You certainly can easily calculate the downward velocity at impact and the time interval required for this first impact to occur. Call them v0 and t0

At height = 72 floors, by conservation of momentum, the total downward momentum a split second before impact:

p(m(Sum(39))) ( p is standard nomenclature for momentum )

must equal the total downward momentum after impact:

p(m(Sum(39))) + p(m(Floor 72))

But this is just

p(m(Sum(40))) (i.e, the momentum of the top 40 floors)

(Note that one of the assumptions of this calculation is that the masses "stick together", i.e., they do not bounce. The collision is inelastic. This is favorable to the FEMA Fairy Tale, plus makes the math easy :-) )

Since you know p(m(Sum(40))), and since you know m(Sum(40)), you very easily can determine the new downward velocity of the combined mass right after this first impact (of course, it will be less than the downward velocity of m(Sum(39)) just before impact)

Using these values, you basically iterate the process sketched out above.

You end up with a set of time intervals, t0, t1, t2, ... t71 such that when you add them up, you get the total time of collapse.

Well, ain't it amazing, but apparently when Hoffman did this calculation, it just so happens that you get almost exactly what was observed. Thus, the statement that "the buildings collapsed almost as though the frames didn't exist" is correct.


Note that BYU Physics Professor Steven E. Jones , in Why Indeed Did the WTC Buildings Collapse? has stated that,

QUOTE

"Jim Hoffman, a professional scientist published in several peer-reviewed scientific journals, took a long look at all of this. He calculated that even if the structure itself offered no resistance, that is to say, even if the 110 floors of each tower were hovering in mid-air, the "pancake" theory would still have taken a minimum of 15.5 seconds to reach the ground. So, even if the building essentially didn't exist, if it provided no resistance at all to the collapse, just the floors hitting each other and causing each other to decelerate would've taken 15.5 seconds to reach the ground. "