The 9/11 Forum

Sorry, yes, I meant to quote you.

It wasn't a flippant question, by the way. If the block was destroyed by any means you wish to consider, how much jolt could we expect to see transmitted through a non-rigid body to the roofline? This is why I never expected to see a jolt in the first place, the block does not look like it survives, much. That there may be some speed bumps is another issue entirely. The block definitely got skewered by the spire, which was taller than initially believed from early pictures and comprised of quite a few core columns. I don't see anything that I'd call intact passing through that. So, if the block (hypothetically) were toast by that point, it would be on its way to being toast over these early displacements.

The upper part C, floors 97 - roof line, or parts of it, is definitely toasted in the first 3+ seconds, because, if it didn't toast but remained intact, as per BLGB, and dropped and compressed lower part A, 12 or 13 floors of part A, i.e. down to floor 84, would then have been destroyed. But it seems most of part A below floor, say 92, is still intact, when part C is toasted.

Using the roof line as reference point, you must recall that the hat truss extends down to four floors below the roof line. So what is toasted is the upper part C, floor 97 up to the bottom of the hat truss, while the latter drops down as recorded on videos into the toasted upper part C.

Part A has then not yet started to be toasted. It comes a little later.

The great amounts of smoke being ejected sideways and upwards from part C is clear evidence that C is toasted (and is not dropping down intact one-way crushing part A as per Bazant & Co).

OneWhiteEye wrote:It wasn't a flippant question, by the way. If the block was destroyed by any means you wish to consider, how much jolt could we expect to see transmitted through a non-rigid body to the roofline? This is why I never expected to see a jolt in the first place, the block does not look like it survives, much. That there may be some speed bumps is another issue entirely. The block definitely got skewered by the spire, which was taller than initially believed from early pictures and comprised of quite a few core columns. I don't see anything that I'd call intact passing through that. So, if the block (hypothetically) were toast by that point, it would be on its way to being toast over these early displacements.

The upper block can't get to be toast in a natural way without a jolt, as the forces required to cause the breakup were much greater than the static 1g load for it also. The upper block was certainly stiff enough to transmit any deceleration and velocity loss, which a jolt would cause, to the roof. If you need to see a calculation there I can do that.

No deceleration and velocity loss seen at the roof means something artificial was allowing/causing the breakup to take less force than was required.

It really is that simple.

The fact that the upper block was toast before doing much damage to the lower block then creates an additional problem for the NIST/Bazant explanation since now they have no pile driver, as Heiwa has pointed out.

The NIST/Bazant explanation has serious problems in several areas in the case of WTC 1 and probably WTC 2, although it is more difficult to make good observations on. Besides the lack of deceleration and velocity loss and their pile driver breaking up, the NIST report does not show stresses anywhere near high enough to cause overload of the east and west perimeter walls and remaining north wall of WTC 1. They simply make the assertion that the instability propagated around the building after the south wall failed.

David B. Benson wrote:Floors 90--93 were type 9. Above that all type 1 until floor 106.

Floor 92 was the lowest floor with observed fires.

Major_TomNCSTAR1-2A Appendix G, of course.

T_Szamboti wrote:The upper block can't get to be toast in a natural way without a jolt, ...

Only requires removing truss seats. A mere 50 MJ is enough to remove all of the truss seats on one floor.

If you need to see a calculation there I can do that.

To do so you will have to make many assumptions for which there is no evidence to justify.

It really is that simple.

False. I attempted to explain to you the role of the pronounced tilt and the partial floor collapses. The only required artificiality was flying an airliner into the tower.

The fact that the upper block was toast before doing much damage to the lower block ...

You have no evidence of this.

... the NIST report does not show stresses anywhere near high enough to cause overload of the east and west perimeter walls and remaining north wall of WTC 1.

South wall either. FEA stopped 2 minutes before collapse commenced.

They simply make the assertion that the instability propagated around the building after the south wall failed.

No! Please do learn not to make re-interpretations of what is written in the report.

Well, its 10,000 pages long and I haven't read it all. If they did make such an assertion, kindly give the citation.

David B. Benson wrote:Only requires removing truss seats. A mere 50 MJ is enough to remove all of the truss seats on one floor.

Sounds easy! How many truss seats are there. And how do you remove them? I understand each truss has two seats (one at each end!). Which one do you start with?

Exactly, Heiwa. Please explain the mechanism by which it happens. Physics cannot be reduced to scalar functions alone. This is very important for you and Dr G to realize.

Energy is not the only factor to consider. The mechanism of how perimeter walls separate into two sheets and the means by which truss seats are broken are equally important.

You have no mechanism to explain how unbuckled sections of the upper block perimeter kicked out and over the lower perimeter walls. Sufficient energy is only a small part of what is necessary to understand how motion occurs.

DBB, according to your own logic one perimeter sheet slips inside the other during the initial failure. The outer truss seats are especially vulnerable on the sheet that slips to the outside.
The inside perimeter sheet can act as a hammer which strips the outer truss connections from the sheet which fell outwards.

In the case of WTC1 (and 2 n, w and s walls) the upper perimeter walls are seen to fall outside the lower wall on all observable faces.

Therefore, according to your own logic, the upper block is much more easily rendered "toast" in the case of WTC1 (from floor 98 to the hat truss).

Heiwa wrote:How many truss seats are there.

All in the NIST report.

And how do you remove them?

Apply enough force (numbers in the report) over a long enough action distance(scaled disgrams of truss seats in the report.)

I understand each truss has two seats (one at each end!). Which one do you start with?

Wrong. Actual number in the report.

Major_Tom --- Just drop something heavy enough on top, as clearly happened for the center section of the south wall that NIST pieced together.

You are not following. According to your own logic of using the perimeter sheet which slips inwards as a hammer to "peel" the perimeter which slips outwards from it's flooring, the upper block is rendered toast all the easier.

Upper block has no structure without outer walls.

David, to see visual evidence of the destruction of the upper block you have to be able to watch the dynamic process (watch videos),

For you to claim Tony has no visual evidence for the destruction of the upper block is totally hypocritical. You have been supporting the notion of an intact upper block with no visual evidence for some time.

Comments by you concerning visual evidence of the dynamic process should not be taken too seriously. You don't watch videos, remember?

Major_Tom wrote:Upper block has no structure without outer walls.

Which might come from either the upper or lower portions, I don't care!. All I care about is that most of zone C stays up on top most of the way to the bottom for WTC 1.

David, to see visual evidence of the destruction of the upper block you have to be able to watch the dynamic process (watch videos),

I watched on 2001 Sep 11. Someone wanting to make a zone C distruction claim needs to post conclusive stills. You did that for the wall sections which detached early on, well done. As those are too small to bother me, try again with something definitive for the west wall of WTC 1 as I'm prepared to accept that the upper wall went outside the lower wall on that side, although I haven't seen it myself. Presumably you will use the Sauret video. If so, kindly attach frame numbers to the stills.

For you to claim Tony has no visual evidence for the destruction of the upper block is totally hypocritical.

That wasn't, I am quite sure, my statement.

T_Szamboti wrote:If you need to see a calculation there I can do that.

No, thanks, it was more of a "what's your impression of this variation" sort of thing. How it would affect the overall result. More focused on the later time of when the antenna sways back (or just as likely breaks off) than the displacement through the first few floors. With that in mind, you do agree that there was some tilt after initiation, correct? Once some tilt (or off-axis translation) is established, do you think a hypothetical rigid block would dissipate a similar amount of energy per unit displacement as a block in perfect alignment?

Take a normal piece of paper. Grasp it at both ends, in the middle of the edges, then pull it into tension. Pull as hard as you can without crinkling it up where it's being held, i.e., keep it planar while in tension. If you succeed in ripping it apart, it will take a great force but, once it begins to separate, it rips rapidly - in the blink of an eye, literally. This peak force is applied over a small displacement and renders the paper in two.

Now, grasp another sheet at adjacent corners and start pulling. The paper still resists tearing briefly, but a much lower peak force is required to initiate failure. The other big difference is some small force must be maintained through a substantial distance to finish tearing the paper in half. The result is the same, the energy expended probably not (but assume it is for a second).

Pedantic? Somewhat. But highly relevant. In energetic terms, there are different potential barriers between initial and final states of the system in the two cases. The first case has a high spike of a potential barrier through the first small displacement, then nothing. The second has a much smaller spike followed by a very low, nearly constant potential through the remainder of the displacement. If the area under both curves is the same, the same total energy is dissipated through the full displacement.

If it were instead a pan hanging from the lower edge of a vertical sheet , and a mass were dropped into the pan, the dissipated KE would come in the form of a velocity loss occuring within the displacement interval starting at the point of contact between the mass and pan. Assume the mass is just sufficient to 'break' the paper in tension when dropped from a particular height. In the first case, there is a sharp reduction in velocity, to almost zero, followed by freefall thereafter. In the second, there is an equally brief but much smaller jolt, because the tension is localized to one edge, then failure of the sheet occurs at the edge and the small resisting force of tearing across the width remains until the paper is torn in half.

While the paper is being torn in the latter case, the resisting force is sustained, small and possibly near constant. At no point does the resisting force exceed that of the weight of the falling mass. It is still accelerating, but slightly under freefall. Assuming the total energy dissipated is the same in both cases, by the end of travel in the second case, the first case will have caught up - being in freefall over most of that displacement. Over the entire interval, though, it will be behind due to the velocity reduction of its massive initial jolt. The second case would decelerate briefly, too, but lose much less KE over that brief interval.

There are two different phenomena at work here. The first is reduction in peak force required for initiation, the second is what energy is dissipated during progression.

In reality, it's unlikely that the two cases would dissipate the same total energy, as previously assumed. It's so much easier to tear paper from the edge, no one ever does otherwise! Once there is an edge defect in the sheet, the failure can propagate more easily than when it isn't present. In fact, paper is a very bad example now but consider a crack in your windshield. The distribution of stress about the defect causes the defect to continue propagating in some (rigid) brittle materials with no further loading. Seffen mentions propagation of defects in non-brittle material (synthetic pipeline and latex balloon), indicating the phenomena has broad application. Does it have application here, with steel and concrete, beams and columns, welds and bolts?

Yes. It has extremely broad applicability. It's why you use the sharp edge of the axe and not the flat side. It's catalysis, to use a chemical analogy. Lowering the potential energy barrier allows the reaction to proceed where it may not otherwise, or to proceed at greater (initial or total) rate subsequently.

Without questioning the issue of axial impacts in the first five or so stories, once there is a tilt, and once there is debris build-up, what sort of jolts would you expect? The antenna tip angle change occurs after there is pronounced asymmetry and so much junk piled up (regardless of what made it that way) that it seems very unrealistic to require the same measure of energy dissipation as the ideal as-built configuration. Bazant does this both for simplicity and to establish bounds, BLGB adds in bunch of other sinks.

I totally agree that a simple proportion of unbalanced forces could produce the flip back seen in the mast. Frankly, though, that is a stark admission that asymmetry exists and negates the premise that the huge jolts expected in the ideal case also apply by this time. Now, you're looking at the case of a pile of junk separating an intact lower from a non-aligned upper, both with ragged, bent-up non-planar ends. I claim it wouldn't stand if you just set it stationary like that, but that's my lay opinion. There's no way it will dissipate the same KE through structural resistance.

There may be ways, through (largely velocity dependent) dissipative forces, to dissipate a tremendous amount of KE in ways other than structural resistance. Momentum transfer becomes increasingly insignificant as collapse progesses. But there no longer is anything that's going to provide coherent, immediate peak resistance seen in the ideal axial strike between two intact bodies.

Thus I see the antenna flip-back as quite significant. Either the rigid portion of the upper block is reduced in size and mass, or it is encountering resistance significant enough to do a jolt on the full block not seen in the earlier displacement where it should be more prominent. I suspect it is as Major_Tom says, or perhaps the contact of the hat truss with what would eventually be identified as the initial remaining spire.

David B. Benson wrote:

Major_Tom wrote:Upper block has no structure without outer walls.

Which might come from either the upper or lower portions, I don't care!.

Ahh, but I think you should care a great deal. Please review the statement of d'Alembert's principle. What story-averaged vertical force would a load cell register at ground level in a crush-up? Less than, more than, or the same as the static load for the entire mass? Alternately, imagine a bus experiencing a high-speed collision with another bus, such that they crumple over a meter or two of displacement each. How does the decelerating force on a passenger in the back of the bus compare to that on the driver?

All I care about is that most of zone C stays up on top most of the way to the bottom for WTC 1.

I'll take it up with you on the B&L thread, that's precisely the subject, though not soon I'm afraid because that's where it will get rigorous. Not that it's off topic here because it's very much on topic.

OneWhiteEye wrote: Alternately, imagine a bus experiencing a high-speed collision with another bus, such that they crumple over a meter or two of displacement each. How does the decelerating force on a passenger in the back of the bus compare to that on the driver?

If the bus frame is stiff it can be nearly the same. It all depends on how much deflection there is between the driver and the back of the bus.