Outside energy had to be introduced for the twin towers to collapse the way they did

Originally posted by -PLB-

Originally posted by IrishWristwatch
And he claimed his analysis disproved intentional demolition, which is absurd. His papers merely show that assistance was not required, not that it was necessarily absent.

He did? Where did he claim that exactly?

BLGB abstract starts with:

Previous analysis of progressive collapse showed that gravity alone suffices to explain the overall collapse of the World Trade Center Towers. However, it remains to be determined whether the recent allegations of controlled demolition have any scientific merit. The present analysis proves that they do not.


All BLGB does is extend the analysis to include secondary effects and do a more in-depth comparision with video and seismic record. In the body of the other papers, the statements are correct in stating CD was unnecessary, but he went over the top with this one. Prior to publication, Benson solicited comments and a couple of people rode his ass about this. He agreed, but he was junior author and was overruled.

It indeed sounds a bit of a silly claim. It is a classic logical fallacy.

Yeah, right? Makes you wonder. The guy is a genius, a titan in the field. Over 400 publications; textbooks! But this...

It's probably worth pointing out that, if you tweak any of these inelastic accretion models to "release slabs" at some point prior to arrival of the primary crush front, it doesn't make much difference until the lead distance is > 20(? can't remember exactly) stories ahead. The crush front, which has acquired velocity already, simply catches up to the "demoed" floors and the collapse degenerates to natural.

There's some unruliness in the definition of X stories ahead, though, which throws a wrench into analytic treatment. If you imagine initiation occurring at, say, story 95, and then instantaneously apply the criteria of an advance front 20 stories ahead, then the new crush front starts 20 stories lower from rest at the same time. Naturally, you must apply the same criteria again and now it's 40 stories lower, etc. In this scenario, destruction propagates instantly to ground and the whole thing is meaningless.

Applying more interesting and useful criteria indicates that detaching floors just ahead of the crush front to "help" it along simply results in those floors being swept up anyway since they start from rest. This means charges all the way down would not only be unnecessary, they'd probably be crushed before they had a chance to do squat.

Likewise, a good fit of natural collapse mechanics to observed dynamics doesn't mean there couldn't be explosives and, no matter how well Bazant fits the collapse, it's not refutation of CD. One has to look to other more qualititative observations for that.

reply to post by IrishWristwatch


I don't see much wrong here. He says "allegations of controlled demolition". First you will have to know what allegations he refers to. A very common one is "the buildings could only have completely collapse if explosives were used". That allegation is proven to be wrong. I don't really like this kind of jumping to conclusions based on a line of text that can be explained in more than one way.

I also think there are some very good arguments why crush up would not happen before crush down. First of all there is the video record showing a largely intact top section for as long as it is not obscured. Secondly there is the fact that failed floors will be between the top and lower section. These failed floors will fall on the lower section. The forces on the upper floor will be lower, it is a simple matter of inertia. Maybe you could explain is a bit more detail why you think that crush up would occur before crush down (or simultaneously).

Originally posted by -PLB-
reply to post by IrishWristwatch

 

I don't see much wrong here. He says "allegations of controlled demolition". First you will have to know what allegations he refers to. A very common one is "the buildings could only have completely collapse if explosives were used". That allegation is proven to be wrong. I don't really like this kind of jumping to conclusions based on a line of text that can be explained in more than one way.

I suppose, if you let him pick which of the allegations he chooses to address (such as the most popular/specious), then it slides. You make a good point, and it softens my attititude towards it and makes me see it in a different light. Still, one of the co-authors agreed with the objection, for what that's worth.

I also think there are some very good arguments why crush up would not happen before crush down.

Yeah, Bazant & Le's reply to James Gourley, for one. That's where he rigorously justifies the one way crush. But that's also my confirmation that it is indeed a narrow window for exclusive crush down. If you look at the figures he provides in that paper (I'll extract those and post later if you want), you can see how very close his own analysis comes to bidirectional crushing. The columns of the upper section are deep into plastic deformation and are riding the downside of the load-displacement response precariously close to demand-to-capacity of 1. The energy required to push them into ultimate yield is a small fraction of that which it took to compress from equilibrium length.

It is true he reduced capacity in the overlying story by 15% to account for fire damage, but one has to admit that's a pretty arbitrary figure. Rerun it with (e.g.) 23% reduction and it crushes up simultaneously according to his own analysis. What's more, even when leaving it at 15%, there is the particular way in which he sets up the initial conditions for the problem. Unlike the mental picture most people have of the abstraction of a free drop of one story through empty space, the space wasn't empty. He assumes a linear momentum distribution through the failing story such that at full compaction it's already moving at half the velocity of the descending upper block. The problem with that is that most of the mass of a story is concentrated near the floor assemblies, so this overestimates the momentum of the failing story at "collision". Again, because the margin of safety against crush up is so small by his own analysis, this difference could tip the scales.

I pursued this point on my own using different methods; discrete and continuous, analytical and numeric. In all cases, there was a strong propensity towards bidirectional crush, and I couldn't figure out why. I began to suspect Bazant was wrong. So I dissected B&L in gory detail and found the difference between his models and mine, and that was the already-in-motion interstitial story. It 'protects' the upper layer by plowing ahead. Once I adjusted the models (the discrete has to be jerry-rigged to accept this), my results favored crush down, too.

But that's the abstract. On to the real.

First of all there is the video record showing a largely intact top section for as long as it is not obscured.

This is true for WTC2, which had a considerably larger upper section, but even that did not fare as well as you probably think. WTC1 - upper block trashed fairly early on. Proving this is going be a sizeable endeavour, digging up pictures and frames. I will, but the forum I've been linking to is down right now (naturally) or at least the domain name is not getting resolved, and that's the best organized repository I have for these resources.

The quick shot is this: consider the core remnants, and what it takes for the tall spire of WTC1 and the wide squat core section of WTC2 to have survived the passage of a largely intact upper block. Please remember to factor in the hat truss.

More, but this forum limits my post size quite severely.

Originally posted by IrishWristwatch
Yeah, Bazant & Le's reply to James Gourley, for one. That's where he rigorously justifies the one way crush. But that's also my confirmation that it is indeed a narrow window for exclusive crush down. If you look at the figures he provides in that paper (I'll extract those and post later if you want), you can see how very close his own analysis comes to bidirectional crushing. The columns of the upper section are deep into plastic deformation and are riding the downside of the load-displacement response precariously close to demand-to-capacity of 1. The energy required to push them into ultimate yield is a small fraction of that which it took to compress from equilibrium length.

It is true he reduced capacity in the overlying story by 15% to account for fire damage, but one has to admit that's a pretty arbitrary figure. Rerun it with (e.g.) 23% reduction and it crushes up simultaneously according to his own analysis. What's more, even when leaving it at 15%, there is the particular way in which he sets up the initial conditions for the problem. Unlike the mental picture most people have of the abstraction of a free drop of one story through empty space, the space wasn't empty. He assumes a linear momentum distribution through the failing story such that at full compaction it's already moving at half the velocity of the descending upper block. The problem with that is that most of the mass of a story is concentrated near the floor assemblies, so this overestimates the momentum of the failing story at "collision". Again, because the margin of safety against crush up is so small by his own analysis, this difference could tip the scales.

I pursued this point on my own using different methods; discrete and continuous, analytical and numeric. In all cases, there was a strong propensity towards bidirectional crush, and I couldn't figure out why. I began to suspect Bazant was wrong. So I dissected B&L in gory detail and found the difference between his models and mine, and that was the already-in-motion interstitial story. It 'protects' the upper layer by plowing ahead. Once I adjusted the models (the discrete has to be jerry-rigged to accept this), my results favored crush down, too.

I don't completely follow you here. Are you talking about crush up only after initiation, or also crush up at a futher stage of the collapse? The way I see it there is a difference between the two. I can understand how after initiation at least a couple of floors are crushed in the top. But once the collapse is several floors underway, it seems to me that crush up will stop. The debris front consisting of failed floors is taking all the hits.

Maybe this is just a matter of me mixing up Bazants model with what would actually happen. I must say I don't know the ins and outs of Bazants model that well.

This is true for WTC2, which had a considerably larger upper section, but even that did not fare as well as you probably think. WTC1 - upper block trashed fairly early on. Proving this is going be a sizeable endeavour, digging up pictures and frames. I will, but the forum I've been linking to is down right now (naturally) or at least the domain name is not getting resolved, and that's the best organized repository I have for these resources.

The quick shot is this: consider the core remnants, and what it takes for the tall spire of WTC1 and the wide squat core section of WTC2 to have survived the passage of a largely intact upper block. Please remember to factor in the hat truss.

More, but this forum limits my post size quite severely.

Agreed, the top section would be heavily damage, as proven by the spire. Although I do not regard heavily damaged as the same as crushed up. Crushed up to me means it is compacted to a small size, all the air has been push out. Heavily damaged however can also mean it has fallen apart and/or has large holes in it.


Anyway, I think we both agree that this matter is not that important. Whether the top was crushed before the bottom does not really change the outcome.

Continuing...

The pictures I posted a couple of pages ago are pretty handy.



The lowest pair of rectangles depict the position of the lowest tip of the north face upper perimeter section at initiation and at the current frame shown in the Sauret video. As you can see, it's sliding outside of the lower perimeter. Needless to say, that necessitates the shearing of floor connections in that area. Long after the entire mass has passed and is below, the NW corner of the lower section can be observed up to the 98th floor. This in itself isn't proof of dissociation of the upper block as a whole, but...

Remember the top is tilting to the south?



Ouch. The lower core, geometrically speaking, has to be impaling the floors on the north side. The floors will not win that battle guaranteed. Even if they drop as rubble around the core, well that's the north side top disintegrating, QED.

Now, with the inward bowing on the opposite face, it is reasonable to expect the upper tucked inside the lower much as what happened of the east face of WTC2, so that is likely shearing the floor connections on the lower section and there might well be a semi-intact wedge driving down the interior of the south side, for a time.

WTC2 had ~15 more stories to work with, and greater tip angle, so naturally the block there hung on longer. But, it you pay attention to the core remnant height, you'll see it didn't get very far largely intact. The top remant was also out pretty far over the side quickly, that negated the core/hat truss engagement which was inevitable in WTC1.

There are more compelling presentations to be made, but still no forum access.

Going back to the theoretical end, remember two things. The crush up is really going down along with everything else, obviously it doesn't go up (excluding fracture from cleavage, etc). Practically speaking, the collapse is pretty far along before the top crush is expected to conclude, so you WILL see an intact upper part riding down (but you aren't seeing an intact leading edge, are you?). The other thing is that I'm not saying the upper blocks crushed completely - not only do the hat truss and upper mechanical floors present considerable rigidity, but I can still be correct about bidirectional crushing even if it only proceeds partway.

Make sense?

Tomorrow I'll put together a more compelling case if you like.

Originally posted by -PLB-
I don't completely follow you here. Are you talking about crush up only after initiation, or also crush up at a futher stage of the collapse?

Initial.

The way I see it there is a difference between the two. I can understand how after initiation at least a couple of floors are crushed in the top. But once the collapse is several floors underway, it seems to me that crush up will stop. The debris front consisting of failed floors is taking all the hits.

The debris cushion is... well, I don't think there was much compaction going on early, I think most of the "sedimentary" layers we hear about came from the lower floors and the huge KE dissipation when it hit full compaction at the bottom. It's just my opinion. If you look at WTC2 from the north, you can see the rivers of debris from above flowing out around the core and NE corner perimeter. In that one isolated case, it looks like liquid.

Agreed, the top section would be heavily damage, as proven by the spire. Although I do not regard heavily damaged as the same as crushed up. Crushed up to me means it is compacted to a small size, all the air has been push out. Heavily damaged however can also mean it has fallen apart and/or has large holes in it.

Ah, it seems some of our difference is in semantics. I do consider any destruction in the real collapse to be crushing. Remove that verbal distinction, and we're probably close to being on the same page. I just don't think there was a hell of a lot of compaction in either direction early on. I base this on the spatial heterogenity in the observable part of the crush front expulsions, both vertically and horizontally. It looks very loose early on. Later, more effective perimeter containment and funneling, and a big whoooom when it hits the basement. That probably packed some **** real nicely.

Anyway, I think we both agree that this matter is not that important. Whether the top was crushed before the bottom does not really change the outcome.

Nope. It all hits the ground at some point.

reply to post by IrishWristwatch


I think we more or less agree, although I give the NIST report a bit more credit than you do. It is so far the best explanation I heard for the things we could observe. What is your take on sagging trusses pulling in the columns? How you do explain the inward bowing?

Originally posted by -PLB-
How you do explain the inward bowing?

Oh good finally a discussion on the initiation.

There was no inward bowing, it was simply the aluminum cladding bowing. There is no evidence the trusses bowed, and no evidence even IF they did it would cause complete collapse of the whole tower.

There would have been a gap between the aluminum and the steel to prevent galvanic corrosion, when the aluminum got hot it bowed inwards.

A truss sagging from heat cannot also create a pulling force. The very reason it would sag is because it has nowhere else to go. If it could move the columns it would have pushed them out, not pulled them in, because when things heat up they expand. The reason they would sag is because they cannot push the columns out, and that expansion has to go somewhere, so it goes down.

You can clearly see the cladding bowing in towards the steel in this pic...

reply to post by ANOK


I already seen this nonsense. The displacement measured from images was magnitudes greater than the space between the columns and cladding.

Originally posted by -PLB-

I already seen this nonsense. The displacement measured from images was magnitudes greater than the space between the columns and cladding.

Again an empty claim with no evidence. How much space was there between the steel and the aluminum?

Are you just lazy, or are you just making things up? Hard to tell when you provide no evidence for your claims.

Sagging trusses cannot put a pulling force on columns, why did you not address that? Bowing cladding, or not, that was the pertinent point you should have addressed.

Originally posted by -PLB-
reply to post by IrishWristwatch

 

I think we more or less agree, although I give the NIST report a bit more credit than you do. It is so far the best explanation I heard for the things we could observe. What is your take on sagging trusses pulling in the columns? How you do explain the inward bowing?

I can't explain the IB, I admit it. But, doesn't mean I have to like their explanation, even if it is the best of what's been proposed.

The problem I have with the sag is primarily the moment imposed on the floor connections at the core and perimeter. There are 5/8" bolts and welds to the gusset plates on the perimeter. Neither are particularly known for ductility. When you consider the geometry involved, a caternary sag in the floor assemblies would require a maximum vertical deflection of about nine feet. That in itself isn't so bad, but now consider the angle the floor makes as it approaches the wall. There are two extremes: the floor joins at the caternary angle, which would be steepest at the wall, or the floor (somehow, how? magic?) joins more or less orthogonally at the wall as it always did, requiring the caternary sag to exist starting only some distance from the wall. Of course, the real situation could be anywhere between the two extremes.

Suppose it is more like the first. Then we must postulate the angle brackets can endure rotation of a pretty severe angle, yet be strong enough to pull in the perimeters which, obviously, are a good deal stronger. Now, the perimeters would experience force transverse to their orientation, which is indeed the direction which affords the least resistance, but just imagine the tension on those bolts! How much rotation is realistically expected before ductile failure in typical steel assembly? Not much.

Don't take my word for it. See Assessment of Progressive Collapse in Multi-Storey Buildings - Izzuddin, Vlassis, Elghazouli, Nethercot, wherein you'll find:

Importantly, the rotational ductility supply offered by typical steel and composite connections of between 4º to 6º (70mrad to 100mrad) is inadequate for the development of full tensile catenary action, and therefore reliance should be placed mainly on bending and compressive arching resistance for the provision of robustness under column removal scenarios.

Now, stack that up next to the angle required to maintain caternary sag all the way to the wall and produce 55" IB. Those are some mighty strong and atypically ductile connections! Makes you wonder why they failed at all!

----

(Tired of the post editor telling me I have 500 characters left, then I paste 50 characters in and the char count goes to zero and half of the post I typed disappears in the edit box with no undo. How do you guys deal with this? Of course, my forum has been down for about 12 hours now, so I can only complain so much.)

Continued next post.

(continued)

So, let's constrain the connections to remain at less than 10 degrees rotation. One of two conditions has to be true: either the sag angle becomes rapidly more severe as you move away from the wall, or the wall experiences a bending moment and yields. As to the latter, not only is that ridiculously improbable, given the perimeter strength relative to the connections, it would also produce distortion on the face (wrinkling) as the perimeter is pulled in above the connection and pushed out below it (torque). On top of those opposing forces strong enough to twist the perimeter-out-of-plane, there must be superposed tension force sufficient to cause IB. This last scenario is kind of a no-starter, anyway, because the wrinkle at each floor would be easily seen on the exterior, and wasn't.

The former scenario, where we avoid moment on the wall by having the floor assembly approach the wall at a less severe angle, has problems of its own. We would have to accept the floor assemblies are weak enough to form a catenary, but strong enough to do so with near-orthogonal connection to the wall, and mediate sufficient force via tension to produce IB. Draw a freebody diagram in your mind and imagine where that tension could possibly come from. I suspect you'll conclude that there is no mechanism by which caternary sag can be translated to predominantly horizontal force without a point interior to the floor assemblies constrained to be held fixed vertically and allowed to slide across horizontally. In fact, all force must be directed downward if the approach is horizontal, just as it is in the original intact configuration. Regular catenary, yes, but not this mutant caternary I propose which maintains mostly horizontal approach to the perimeter while sagging in the interior.

Neither of the scenarios are viable, in my opinion, and neither are intermediate states between the two since it just becomes a superposition of the two problems; no "sweet spot" in between emerges. Have I overlooked anything?

This is why I find the NIST scenario quite dissatisfying. I'm not really under any obligation to produce a better scenario in order to find fault with theirs. However, I'll take a wild ass guess. The core on the side of IB underwent creep buckling and load was transferred to the perimeter via hat truss and outriggers, such that the perimeter became overloaded. The floors sagged as a necessary consequence (effect rather than cause, just along for the ride) and, once the angles became severe, the connection failed under rotation. The floors dropped and collapse began.

Originally posted by ANOK

Originally posted by -PLB-
How you do explain the inward bowing?

Oh good finally a discussion on the initiation.

There was no inward bowing,

femr2.ucoz.com...
femr2.ucoz.com...

...it was simply the aluminum cladding bowing.

How does that happen? How does the cladding bonded to the exterior of the columns displace inward WITHOUT the columns also moving in?

There is no evidence the trusses bowed...

No direct evidence, but if IB occurs, there is that much less span for the floors to occupy. They must deform in some way. I did make a fairly weak argument above against the trusses being a cause of IB, but I think it pretty much has to be one of cause or effect.

...and no evidence even IF they did it would cause complete collapse of the whole tower.

No direct evidence, true, but such can be deduced given proper initial conditions and some engineering mechanics.

There would have been a gap between the aluminum and the steel to prevent galvanic corrosion, when the aluminum got hot it bowed inwards.

Whoa. Just whoa. Fifty plus inches of gap? I'm not even going to touch that.

A truss sagging from heat cannot also create a pulling force.

Simple caternary tension has a force component in both horizontal and vertical dimensions. I don't believe the force of a sag of a floor or two could produce the IB, but it's simply not true that NO horizontal force exists.

The very reason it would sag is because it has nowhere else to go.

True. There would initially be an outwardly directed force, and maybe there was a period where the columns were displaced outward a bit. I don't think so and, again, I don't accept the NIST scenario so won't defend it. If their scenario was true, however, I don't know a priori what sort of initial outward deflection might be expected.

If it could move the columns it would have pushed them out, not pulled them in, because when things heat up they expand.

Initially, yes.

The reason they would sag is because they cannot push the columns out, and that expansion has to go somewhere, so it goes down.

No reason to expect force in compression to be the same as that in tension; in fact, every reason to expect them to be different.

You can clearly see the cladding bowing in towards the steel in this pic...

Hmm. You posted the same view I did. How can we come to such radically different interpretations? Of course the cladding is deforming and detaching, this is the period of initiation. The top is beginning to tilt. Those columns ARE bowed inward, and to suggest otherwise is incredible to me. Shortly after this point in the source video, the columns are seen to fracture and most snap back; then you can directly see the ends of the columns! This is not an optical illusion from cladding, although there were illusions of bowing from smoke discoloration on the cladding of the north tower, easily abated by overlaying vertical lines on the image.

I must say that this is beyond my level of knowledge, so I am afraid I am not of much use here. Some things that come to mind is that the columns were also weakened because of the fires. As for the angle of the trusses, I can imagine that the seats they were bolted to may have bend, or the top chord bend right at the edge of the seat. I don't really have an idea of the forces involved though. I also read an article about trusses contracting when they cooled down again in their deformed state, increasing the pull-in force.

But I am open for alternative theories. Thing is that I have never encountered one. It does very much seem like the inward bowing resulted in collapse initiation. What I can think of alternatively is that the mean reason for failure was Euler buckling, and the sagging trusses just gave that small pull to make it start. Although I think this was also investigated in the NIST report, I can't recall the exact conclusions.

Originally posted by -PLB-
I must say that this is beyond my level of knowledge, so I am afraid I am not of much use here.

Me, too, really. The argument I put forth is only qualitative and, though I've filled in some gaps myself, parts of it draw on the work of others, including a large scale simulation that studies the imposed moment at the perimeter. If I were expected to carry it forward, it would not only have to be a paying job but my employer would have to send me to school for a few years first.

Some things that come to mind is that the columns were also weakened because of the fires.

No doubt. Not to be difficult, but the connections are right there, too. Widespread area heating couldn't possibly exclude the regions of the connections, and given overall mass and surface to volume ratios of the connection components versus the columns, they are either going to be hotter if heating is occurring or cooler if the opposite, or in thermal equilibrium with the columns. If cooling, this implies they were previously at a temperature equal to or most likely higher than the columns, yet they held. Just sayin'.

As for the angle of the trusses, I can imagine that the seats they were bolted to may have bend, or the top chord bend right at the edge of the seat. I don't really have an idea of the forces involved though.

It is possible.

I also read an article about trusses contracting when they cooled down again in their deformed state, increasing the pull-in force.

Also true.

But I am open for alternative theories. Thing is that I have never encountered one. It does very much seem like the inward bowing resulted in collapse initiation. What I can think of alternatively is that the mean reason for failure was Euler buckling, and the sagging trusses just gave that small pull to make it start. Although I think this was also investigated in the NIST report, I can't recall the exact conclusions.

I agree that it need not be an either/or situation. The core could be unloading and the perimeter taking it up. There can be heating and weakening of the perimeter. A simple horizontal perturbation might be sufficient to make the buckling commence.

reply to post by IrishWristwatch


I think the bottom-line is that ignorance or incredulity is not an argument to support a conspiracy theory. Even when nobody can explain every detail of what exactly happened, it is no reason to conclude it must have been controlled demolition. That is a major logical fallacy, which is similar to the one creationists use. You can't explain why creature y has feature x, therefore creationism is true. It is a false dichotomy.

Originally posted by ANOK

Oh good finally a discussion on the initiation.

There was no inward bowing, it was simply the aluminum cladding bowing.

LOL.

So you're saying that the aluminum cladding bowed inward, but the steel columns didn't, even though they were inside the cladding - and therfore in the direction of the observed bowing????

Brilliant, Mr Physics....

Originally posted by IrishWristwatch

However, I'll take a wild ass guess. The core on the side of IB underwent creep buckling and load was transferred to the perimeter via hat truss and outriggers, such that the perimeter became overloaded.

IIRC, NIST agrees with this, or perhaps it was Bazant. It was also over 3 stories, so the resistance to out of plane buckling is lowered by 9.(?) And don't forget that ext columns were heated too, so it could very likely be all 4 factors : overload, heated columns, 3 story distance, and some pull in.

The floors sagged as a necessary consequence (effect rather than cause, just along for the ride) and, once the angles became severe, the connection failed under rotation. The floors dropped and collapse began.

very reasonable.

Originally posted by IrishWristwatch

True. There would initially be an outwardly directed force, and maybe there was a period where the columns were displaced outward a bit.

NIST reported that this occured in their fea analysis, so it's not unreasonable. ENIK did a similar fea, IIRC, over at Greg's forum and reported similar results?

How can we come to such radically different interpretations?

Cuz he's trolling.

Shortly after this point in the source video, the columns are seen to fracture and most snap back; then you can directly see the ends of the columns!

Actually, I'm not sure that what you're seeing is the ext columns.

At first glance, it looks more like it is the cladding only snapping back. Have MT and/or femr looked at it and confirmed that those particular ext columns ended flush with the ends of the cladding? I would find it unusual it that were the case.

I would also expect permanent, plastic deformation of those column ends in any case, cuz they would have to be deflected what........ 7-10' by the time they snap?