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Did WTC 7 fall "too fast"?

Analysis of fire and collapse theories and examination of related evidence.

Did WTC 7 fall "too fast"?

Postby Dr. G » Tue Nov 11, 2008 3:49 am

Did WTC 7 really fall "too fast", .... in fact, was it even faster than a typical CD?

There have been lots of great posts recently on various issues concerning the collapse of WTC 1, 2 & 7. That’s why I was not sure where to put this post, especially with all this overlap of interesting ideas, …. so I decided to simply start a new WTC 7 thread.

E. Yarimer at London University appears to be one of the few engineer/scientists who has studied real building demolitions by explosives. He has at least two papers on this subject, both written back in the 1990s, (and unfortunately hard to find on the web).

Here is a quote from the first (1994) paper:

“The current practice in controlled demolition (CD) by explosives is to pre-weaken the building on most floors, and to blast only a portion of the floors, for example one floor in two, or one floor in three. Even so, the number of charges to be placed in individual boreholes can be large: up to 6000 charges have been used depending on the size of the job. The blast floors will readily disintegrate, but the non-blast floors need the force of the impact in order to break-up. Even on the blast floors, the perimeter walls above the ground floor are usually not charged for safety reasons, and they are expected to break up by impact. The entire process is driven by gravity but the downward velocities are attenuated by the energy absorption at the point of impact, and the motion will accelerate less than a case of free fall; it may even decelerate. A spectacular case of decelerating motion was that of Northaird Point in London in 1985, which came to rest with 10 floors still intact.”

In his second, (1996) paper, Yarimer used electronic and photographic timing devices to study a number of real CDs. One of great interest to the present discussion was the 1995 demolition of a 20-story high-rise known as Sandwell East Tower. This demolition showed - as was observed for some other CDs studied by Yarimer - a latency period of ~ 1.5 seconds before significant bulk motions were detected.

I have taken Yarimer’s data to look at the accelerations for the Sandwell East Tower CD. Some time-drop data for the first 5 seconds are: 0 s, 0 m; 1 s, 0 m; 2 s, 1.8 m; 3.0 s, 10 m; 4.0 s, 22.3 m; 5.0 s, 35.9 m. These data show the collapse was well below free fall. Indeed, Yarimer states in his discussion of this data: “Near time t = 0, the calculated accelerations are influenced by the observed latency, thus lifting the estimate of the upwards reaction force.” It appears that even Yarimer had t(0) problems!

Nevertheless, I have analysed Yarimer’s data (with allowance for the t(0) problem) using the same approach many of us have applied to WTC 7 collapse data. What is most significant is that, even with a time shift of ~ 1.5 seconds, the Sandwell East building fell only about 40 meters in the first 4 seconds of bulk motion with an acceleration of no more than 5 m/s^2. And let’s remember that this was observed for a real-world CD on a 20-story building. Scaling this result to a 47-story, (WTC-7-sized building), I would predict a 50 % collapse to take at least 6 seconds and allowing for a latency period of about 1.5 seconds, a full collapse to take ~ 10 seconds or more.

D. Isobe et al. have carried out finite element calculations on a 20-story steel framed building subjected to a Kobe-wave type of seismic collapse. Isobe found that incremental collapse begins
after an initial 26-second period of vibration during which time plastic hinges are formed and column fractures occur near the ground level of the building. The modelled structure was 50 % collapsed about 10 seconds after the first bulk downward motion, and still only about 35 % collapsed after 14 seconds!

Thus we see experimental and theoretical confirmation that the global collapse of a 20-story building would take at least 10 seconds to partially collapse from deliberate man-made explosive or natural seismic trauma to lower portions of its structure.

One can only wonder what mysterious combination of forces brought down a 47 story building in less than 8 seconds.....
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Re: Did WTC 7 fall "too fast"?

Postby OneWhiteEye » Tue Nov 11, 2008 4:41 am

Very cool, Dr. G.

Is this the same information you had posted some time back at Physorg? I've wanted to dig that post up, but the board's been down (for good) and searching through the 'lofi' version is quite a pain. These papers and the seismic studies are extremely valuable.

More comments after additional time for consideration.

It appears that even Yarimer had t(0) problems!

Hahaha, now I feel better!
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Re: Did WTC 7 fall "too fast"?

Postby Dr. G » Tue Nov 11, 2008 1:51 pm

OWE:

Yes, I mentioned Yarimer's paper briefly before on PhysOrg, but I have since found another paper by this author. The two papers are:

"Factors Affecting the Numerical Modelling of Demolition by Explosives" In Transactions on the Built Environment, Vol 8 (1994)

"The Effect of Rubble Accumulation on the Mechanics of Demolition by Rapid Collapse" In Structures Under Shock and Impact IV. (1996)
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Re: Did WTC 7 fall "too fast"?

Postby Major_Tom » Tue Nov 11, 2008 7:30 pm

OHE pointed out that he had the impression from watching video that the "upper block" lacked structural integrity from early in the collapse.

It seems pretty obvious that the huge "kink" down the middle actually means that the "upper block" consisted of at least 2 upper blocks.


Say we agree that the fall was too fast. What does that really mean? Doesn't it show that the upper block had even less structural intergity than many buildings in actual demolitions?


Note: Aren't crush-down equations very applicable to demolitions? Couldn't one test the usefulness of crush-down equations by studying known demolitions? The idea is often to let gravity do the work, so you get the object moving at a low spot, just like WTC 7. I bet similar general equations would apply to allow someone to predict fall times.

Isn't an extreme loss of the "upper block" structural integrity the only thing that can account for "too fast"? What else could speed up the time of an "upper block" crush?
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Re: Did WTC 7 fall "too fast"?

Postby OneWhiteEye » Wed Nov 12, 2008 10:51 am

My opinion only.

Major_Tom wrote:Say we agree that the fall was too fast. What does that really mean?

Too fast means we don't know the mechanism.

Doesn't it show that the upper block had even less structural intergity than many buildings in actual demolitions?

Yes. By whatever description, that would be the essence of it.

Aren't crush-down equations very applicable to demolitions?

Yes.

Couldn't one test the usefulness of crush-down equations by studying known demolitions?

Yes, and it's something I intend to do. Got a ways to go. There may be too many factors and not enough data available.

I bet similar general equations would apply to allow someone to predict fall times.

Yes. BLGB is pretty successful for the towers, all things considered. I recall shagster at physorg came up with something like 6.6 seconds for a crushup solution for WTC7, but I don't know how he got to that point. But... einsteen's equations have some pretty interesting properties, one is that it's hard to come up with fall times much over 8 seconds for WTC7. Dr. G, with an entirely different approach above, finds ~10 seconds as a more realistic number. I think things are more discovery oriented than predictive, for now.


Isn't an extreme loss of the "upper block" structural integrity the only thing that can account for "too fast"?

Maybe the exterior was dragged down a bit by the already moving interior.
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Re: Did WTC 7 fall "too fast"?

Postby Heiwa » Wed Nov 12, 2008 11:29 am

In CD you intentionally create a lot of local failures, almost simultaneously, in the structure with the intent to bring it down.

If a local failure occurs for any other reason, e.g. a part fails due to a fracture or, why not, thermal expansion (heat), this initial failure may set off a chain reaction due to load transfers and other local failures occur. But each new local failure requires, apart from load transfer, energy and it can only be provided by gravity.

Evidently the chain reaction goes fast, but it stops immediately when you run out of available energy. A number of times I have investigated damaged structures with multiple failures and tried to work backwards to find the failure that initiated all the damages. Not easy.

The WTC7 initial singular failure was apparently induced by heat but how it could propagate from one column to another is not explained by NIST in its WTC7 report. Each primary structure column is virtually independent of other columns even if they are connected by supporting, seccondary structure, beams.

Say there are local failures of secondary structure (beams) around one column and that column fails. The load carried by may drop off or will be transferred to other columns adjacent columns (via the secondary beams). The adjacent columns may become higher stressed ... but fail? There is a fair amount of redundancy, safety factors, in these column/beam structures. They can take certain overload without failing.

And one inner column is only connected to maximum four adjacent columns. That one column fails and that this causes four other columns to fail (total 5) that then causes another eight columns (total 13) is quite unlikely. But if it happens, it takes time and hardly produce free fall drop of the complete upper part as registered on the videos.

Just looking at the big structural pieces in the rubble poses questions. We can see big riveted joints of columns/beams where all primary, say 1 - a column, and secondary parts, say 4 beams, have been fractured completely, i.e. to produce that piece of junk, five members were fractured - and the primary one in two locations - above and below the joint, the others just away from the joint! Normally you would expect only one part fractured and the other just deformed - still connected to other structure. Ok, the joint is strong - but to produce complete six fractures requires energy. Strangely enough little deformation is seen at the fractures, i.e. the plastic hinges where the fracture forms.

Plenty of energy was required to produce that junk and I doubt gravity did it.

Because all these pieces of junk were created fast - while the building collapsed. It was the only time potential energy was available. That this energy would be applied to all these parts so exactly and create all these complete fractures (6 around one piece of junk only) is unbelievable.

Back to CD. If you look at the remains of a CD you find that most serious failures/fractures were caused by the charges. The rests of the structure were not further ripped apart - just deformed - by gravity. And some CDs go wrong. You destroy too few primary parts or charges do not go off ... and redundancy produces a sloppy job. The collapse is arrested.

Just by looking at the rubble of WTC7 I would conclude a very sophisticated CD caused those big pieces of junk. One reason why they were shipped off and recycled so fast?
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Re: Did WTC 7 fall "too fast"?

Postby Dr. G » Wed Nov 12, 2008 2:41 pm

Relatively simple energy transfer calculations applied to a crush-up of WTC 7 are able to reproduce collapse times under 8 seconds, so I have no problem with fast collapse times per se. However, as with WTC 1 & 2, collapse initiation is a problem!

For WTC 7, it looks like structural failure starts in the lower core of the building causing the roof to bow/fold near its center. However, in my opinion, this does not mean the entire inside of the building had "lost structural integrity" prior to global collapse. Rather I believe, (as I have said before), once a lower floor failed, the upper block moved down as one relatively intact unit. Such a collapse would mimic a CD no matter what started the initial floor failure.

In fact, I believe the collapse was a lot like the sequence shown in the linked paper by D. Isobe et al: " Numerical Code for Seismic Collapse Analysis of Framed Structures".

www.kz.tsukuba.ac.ip/~isobe/seismic-e.html

The problem is to explain why the collapse of WTC 7 looked like a seismic collapse.

Edit: My apologies, I cannot get the link to Isobe's paper to work, not sure why....
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Re: Did WTC 7 fall "too fast"?

Postby OneWhiteEye » Wed Nov 12, 2008 4:16 pm

Here it is:

http://www.kz.tsukuba.ac.jp/~isobe/seismic-e.html

It was "ip" instead of "jp", very hard to notice with the proportional font.

I say the top had little integrity because we're going to need some high res data just to distinguish between the resistance it afforded - and zero.
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Re: Did WTC 7 fall "too fast"?

Postby Dr. G » Wed Nov 12, 2008 5:26 pm

Thanks OWE!

Here is a better version of the seismic collapse (see Figure 10 of the linked paper) which shows that for the collapse modeled by Isobe, all the action was on the lowest floor:

http://www.kz.tsukuba.ac.jp/~isobe/161.pdf

Interestingly, if you look carefully at Isobe's collapse sequence, the roof shows a kink about 3 seconds into the collapse.
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Re: Did WTC 7 fall "too fast"?

Postby OneWhiteEye » Wed Nov 12, 2008 5:39 pm

Excellent find, Dr. G. Other obligations prevent me now, but I'll read this later today, looks very relevant.
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Re: Did WTC 7 fall "too fast"?

Postby Major_Tom » Wed Nov 12, 2008 9:59 pm

in my opinion, this does not mean the entire inside of the building had "lost structural integrity" prior to global collapse.


Not entire.

When you say "upper block" do you mean that in a geometrical sense or a structural one?

I agree that the top portion is, geometrically, an upper block. Hard to argue.

But a structurally intact upper block? To a degree yes and to a degree no.

Or...

Maybe the exterior was dragged down a bit by the already moving interior.


??
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Re: Did WTC 7 fall "too fast"?

Postby Dr. G » Wed Nov 12, 2008 11:23 pm

Major_Tom:

I have trouble picturing the inside of WTC 7 self-destructing during the moments before global collapse occurred, as described by NIST, that's for sure. So I am not clear what the term "loss of structural integrity" of the upper block of WTC 7 really means. I prefer to think of a collapse mechanism for WTC 7 that is basically a simple sequential floor-by-floor crush-up, starting somewhere between floors 8 and 14.

However, if a unique collapse-initiating event, (assuming there was in fact one failure that started the whole process), was located near the lower center of the building, I could imagine such a failure "pulling" the middle of the upper section down a few meters (as observed). But, I would expect any slumping of the roof to occur more or less concurrently with the downward motion of the entire upper block.
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Re: Did WTC 7 fall "too fast"?

Postby Heiwa » Thu Nov 13, 2008 2:05 pm

OneWhiteEye wrote:Here it is:

http://www.kz.tsukuba.ac.jp/~isobe/seismic-e.html

It was "ip" instead of "jp", very hard to notice with the proportional font.

I say the top had little integrity because we're going to need some high res data just to distinguish between the resistance it afforded - and zero.


Good link/paper/model of tower structure collapsing due earth quake! Actually in the model the earth quake forces shear off all 22 or so columns at ground at once - simultanesously, these columns fail simultaesously and the structure above crumbles ... like a CD where the ground columns are blasted apart.

The model how a columns deforms, buckles, plastic hinges develop and then fractures develop and cut through the structural part and how much energy is required is good.

It is very likely that the top structure - just beams/columns (no floors) - will only deform in the rubble, i.e. no new plastic hinges/fractures there because no earth quake forces are applied to that - only gravity. And due to these deformations requiring energy the collapse cannot be free fall.

Has nothing to do with WTC7 where we are told there were some local failures due thermal expansion down below around one column ... and no further explanations. Only that these local failures caused some load redistribution causing all other 46 columns to suddenly, simultaesously fail. But not how!

In reality, of course, it is unlikely that the ground columns in the Japanese model will shear off as they are the strongest columns in the whole structure assuming normal construction. When the earthquake starts ajacent to the structure and the whole ground moves sideways the structure will start to oscialate sideways absorbing energy and normally not collapse at all. This happens frequently in Japan where I happened to live 5 years in the 70's. All furniture moving around, ceiling lamps swinging, etc. Happens regularly.

The worst case is with an earthquake center just below the structure; it will then oscialate vertically which may impose bigger local loads on the structure. Then the risk of collapse is biggest; It is like a WTC1,2 collapses, but the big force is acting on the cellar and not from above by a dropping upper part.
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Re: Did WTC 7 fall "too fast"?

Postby Major_Tom » Thu Nov 13, 2008 6:58 pm

But, I would expect any slumping of the roof to occur more or less concurrently with the downward motion of the entire upper block.


We can think of the upper block geometry as if it is a lattice structure. The lattice points are along all columns with whatever vertical spacings you wish (one point along every floor, weld, whatever).

Now we say that the vertical lattice spacing is much more fixed than the horizontal. This means it is hard to compress or expand a column. They are rather rigid.

This means that the observed contours of the roofline directly reflect the the order in which the columns failed on the 8th to 12th floors.

Because we see the vertical dimension of the lattice as relatively rigid, all the info we can get from the obseved roofline movement can be directly projected onto the 8th floor, say, to tell us the order and speed at which column failures are taking place.

Mapping the roof can be very useful to us.


The rubble pile tells us that the perimeter clearly fell inward and over the internal structure. Though it is not very clear in videos of the fall, the rubble pile contains a distinct and unarguable geometry which tells us that the center acted to pull the perimeter inwards. Of this there can be little doubt.


In summary, it is fair to look at the initiation and collapse as how the individual points on the lattice structure mentioned above move relative to one another. We must view the movement ultimately as the points on the core columns leading adjacent points on the perimeter and pulling them inwards. Even though video doesn't capture this slight pull-in, the rubble pile captures it distinctly.
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Re: Did WTC 7 fall "too fast"?

Postby Major_Tom » Thu Feb 05, 2009 11:05 pm

From the OP,

Thus we see experimental and theoretical confirmation that the global collapse of a 20-story building would take at least 10 seconds to partially collapse from deliberate man-made explosive or natural seismic trauma to lower portions of its structure.

One can only wonder what mysterious combination of forces brought down a 47 story building in less than 8 seconds.....


Especially from the experience with known demos.

So yes, it fell too fast to be either explained by natural collapse or conventional CD.
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