Failure mode in WTC towers

[Norseman]

Ryan, the moments which the tower columns would have experienced, due to gravity loads were so low they could be ignored. It is no different than your steel creeps forever but essentially it can be ignored it is so low. I am saying that horizontal beams supporting the top of the frame NB showed would eliminate the bending deflections his diagram shows in the columns. You know darn right well that is true.

Like what the beams where unable to do here, just to much snow:

(Crop taken from this picture. Original context.)

The above is what NB have tired to tell you all the time with this figure:

 

[Heiwa]

I think what you really need to consider is that an avalanche is a perfect example of a system in resting on the point of equilibrium. It only takes a very small input of energy to push it out of equilibrium. This is what the fires did on 9/11.

Regardless of the density of the snow, if the lower portion can no longer support the upper you get an avalanche. You're trying to draw analogies using density that simply don't fit. It really doesn't matter how dense the upper section was relative to the lower, what matters is that the point of equilibrium was passed and the static very quickly changed to dynamic. In the dynamic state the system was doomed.

You have not understood. The PE of the upper block is too small and in the wrong locations to affect the structure (or its strain energy, SE) below that is also located in many strategic positions. And if the fire or buckling initiated the collapse is not proven. It is only assumed. And that the PE finds the SE is unlikely in an alleged gravity driven collapse. You have not read http://heiwaco.tripoc.com/nist.htm . Most PE should just drop to the ground without affecting the SE below.

 

[Heiwa]

Wrong again Heiwa. The initiation mechanism of many snow avalanches are a very good example of progressive collapse. Like what you see in the picture below:
[qimg]http://www.internationalskeptics.com/forums/imagehosting/thum_1814147ae3fc54c02d.jpg[/qimg]
Source

A local weight increase fractures the snow crystals in the weak layer, the load is transferred to the next crystal that also fractures and so on, collapsing the weak layer. Just like what happened on floor 98 in WTC 1 when the columns of the south exterior wall no longer were able to carry their weight and failed. And of course snow avalanches can also be triggered by temperature increases weakening the structural strength of the snow.

Whether a weak snow layer collapse will develop into an avalanche or not is dependent on the slope angle and the friction force. A skier can trigger a whole mountainside in this way. Think about it Heiwa, a less than 100 kg skier can trigger a several million kg snow avalanche with just one small step.

And to much snow on the roof can collapse it, look at those steel beams and columns:
http://capretzer.com/images/forensics/snowLoadCollapse.jpg
(Original context)

(Notice that even in this one story building, the columns have spray on fire proofing. But heat weakening was certainly not the problem in this case.)

My point is that a snow avalanche and its PE is a perfect description of a gravity driven collapse of a system. Applying the same principles to WTC1 I find that its collapse was not gravity driven! Extra energy, not the PE, was used to demolish the building.

 

[beachnut]

My point is that a snow avalanche and its PE is a perfect description of a gravity driven collapse of a system. Applying the same principles to WTC1 I find that its collapse was not gravity driven! Extra energy, not the PE, was used to demolish the building.

You are wrong. You need to gather some more information, lots of knowledge, some logic, some rational thinking and try to do your non-study again. Your paper and post is wrong.

No extra energy was found; unless you can produce evidence your point is wrong.

 

[Tony Szamboti]

In reality, the bending moments are small enough to not be a serious issue (except in VERY eccentric shear connections). It is however relevant to my point about how the column spliced will break in bending due to axial failure. But then, he started off by saying that there would be zero bending moments in the column, which is ********.

Of course, if you break the connections of the adjacent horizontal beams then and only then is your diagram accurate. You never said that until now and that isn't what you were portraying initially and the tower columns had horizontal beams in both axes. Your initial post was attempting to explain how the core column welds could break due to bending moments applied to the columns. I simply showed this wasn't true, due to the opposing horizontal beams providing a restraining force and counteracting moment alleviating the column of having to withstand the bending moment caused by the floor deflection.

Without yielding or breaking of the adjacent horizontal beam connections, the columns in your diagram would not have deflected due to bending moments. I don't believe I ever used the word zero, more like insignificant and maybe essentially zero, which is true.

Maybe some of you guys on JREF can maintain some credibility. Although I was amazed at some of the riciculous comments here on this, you did finally behave in a professional manner by admitting it.

 

[X]

My point is that a snow avalanche and its PE is a perfect description of a gravity driven collapse of a system. Applying the same principles to WTC1 I find that its collapse was not gravity driven! Extra energy, not the PE, was used to demolish the building.

I'd say that the bolded portion is half-right.
But it wasn't "extra energy", it was kinetic energy. You know, energy from motion instead of energy stored.
Of course, once you realize that, you realize it was gravity driven.

 

[Heiwa]

I'd say that the bolded portion is half-right.
But it wasn't "extra energy", it was kinetic energy. You know, energy from motion instead of energy stored.
Of course, once you realize that, you realize it was gravity driven.

So you didn't understand what an avalanche (gravity driven collapse) is? The PE (snow) is up on the slope, initiation is to little friction to keep the PE in place, the snow starts to move = KE = avalanche (snow is pushed down the slope. No magic. No extra energy! Material of uniform density up top pushing similar material down below. It works for snow and soil, etc. Material of uniform density.

But it does not work for a 3D steel structure on top of a building mostly full of air. Not uniform density anywhere. And much too strain energy built into the structure below. Anybody believing that a gravity driven collapse of a steel building is possible is just ignorant. NIST and FEMA are two rotten examples.

You can demolish all the steel structure up top with fire, overloading, buckling, controlled demolition, etc. and no gravity collapse will be initiated. Only local collapses up top will take place. The solid pieces up top are too small to create a gravity driven collapse, and the strain energy of the structure below is concentrated in 1000's of very small cross joints that cannot be demolished by pieces dropping from above.

Quite obvious actually. Gravity driven collapse is only possible in a structure like snow or soil that has uniform density (and uniform strain energy to keep it together) everywhere.

That's why Seffen assumes uniform density in his WTC-model! Quite dishonest frankly speaking.

 

[Dave Rogers]

Maybe he needs that extra foot to count with because in the initial collapse of the North Tower a fall of 12 feet in 2 seconds isn't what was observed.

It is according to the website that you used for your reference to establish the drop of your antenna. You're arguing against your own source.

Dave

 

[Newtons Bit]

Of course, if you break the connection of the adjacent horizontal beams then and only then is your diagram accurate. You never said that until now and that isn't what you were portraying initially and the tower columns had horizontal beams in both axes. Your initial post was attempting to explain how the core column welds could break due to bending moments applied to the columns. I simply showed this wasn't true, due to the opposing horizontal beams providing a restraining force and counteracting moment alleviating the column of having to withstand the bending moment caused by the floor deflection.

No Mr. Szamboti, you still have no clue what you're talking about. Eccentric shear connections (which all shear connections except beams framing over the tops of columns are) will apply moments into the columns, regardless of whether or not they are moment frames or pinned gravity columns. The reason why this is important is due to moment magnification as the axial DCR approaches zero. That's this equation:

Can you point out to the class what that means? I seriously doubt it.

But in any event, you still don't know the difference between a FRAME and idealized connections.

Without yielding or breaking of the adjacent horizontal beam connections, the columns in your diagram would not have deflected due to bending moments. I don't believe I ever used the word zero, more like insignificant and maybe essentially zero, which is true.

Maybe some of you guys on JREF can maintain some credibility. Although I was amazed at some of the riciculous comments here on this, you did finally behave in a professional manner by admitting it.

I call shenanigans. Do not put words into my mouth. Do not even pretend you have any idea what structural engineering is. Yours words have proven this. You started off by saying the connection supports (seriously wtf?) would absorb all of the moment by referencing Timoshenko and Gere. They are talking about this:



Not about a moment frame! There's a huge difference.

You then went on to describe the moment connected beams would act like guys (again wtf) and absorb the moment. Again, you show us that you have no freaking clue what you're talking about.

And now you move on and try to claim victory in that I admit that the moments are small? Of course they're small, and idiot knows that, but they're not insignificant. A designer could, if he chose to, ignore the bending moments in the columns due to eccentric shear connections and have no problems with his structure. This is because these bending moments are only about 0.05 of the DCR. However when analyzing the FAILURE MODE of that column, which is the topic of this thread, these bending moments become extremely important due to moment magnification. Again, something you fail to understand. It's probably not even within your capacity to understand, you've thrown away any sort of critical thinking ability in search of your religion of 911 truth.

As far as credibility goes, you've shown that you never had any to begin with. Go back to school, take some intro to structures classes in the architecture school. Maybe they can dumb things far enough down for you to understand. But it's obvious that even our discussions here, that we reduce such that most laypeople can understand, is beyond you. All you are capable of doing is quote mining.

 

[Furcifer]

So you didn't understand what an avalanche (gravity driven collapse) is? The PE (snow) is up on the slope, initiation is to little friction to keep the PE in place, the snow starts to move = KE = avalanche (snow is pushed down the slope. No magic. No extra energy!

Congratulations, you just broke the laws of physics.

 

[Myriad]

So you didn't understand what an avalanche (gravity driven collapse) is? The PE (snow) is up on the slope, initiation is to little friction to keep the PE in place, the snow starts to move = KE = avalanche (snow is pushed down the slope. No magic. No extra energy! Material of uniform density up top pushing similar material down below. It works for snow and soil, etc. Material of uniform density.

...

Quite obvious actually. Gravity driven collapse is only possible in a structure like snow or soil that has uniform density (and uniform strain energy to keep it together) everywhere.

Um, dude, snow does not have anything remotely resembling uniform density. It is made of ice crystals which form a 3-D structure mostly full of air. The ice crystals have a density of 917 kilograms per cubic meter. Recently fallen snow (which often avalanches) has a density of about 100 kilograms per cubic meter, so it's about 89% air. Even wind-compacted snow (which also often avalanches) is about 75% air.

If you were looking for a naturally-occurring substance on this planet that is remarkable for having a highly non-uniform density, it would be hard to find a better example than snow. Except perhaps for soil, which of course you chose as your other example of a uniform density material! (Pumice stone is another, but it's far less common than snow or soil.)

So, you were saying...?

Respectfully,
Myriad

 

[Furcifer]

But it does not work for a 3D steel structure on top of a building mostly full of air. Not uniform density anywhere.

As opposed to your 2-D avalanche analogy? Spending too much time on your computer has distorted your reality I'm afraid, real avalanches are 3-D.

Most avalanches are a direct result of non-uniform density which leads to stratification of the snow layers. The input of energy by sound or by heat via the sun reduces the static coefficient of friction inbetweeen the layers. (There's also the possibility that the addition of potential energy via mass increase exceeds the force due to static friction)


edit: the input of energy by heat will reduce the static coeffcient of friction, the input of sound will subtract from the force due to friction.

 

[Max Photon]

WTC failure modes: In summary, a summary.

Dave Rogers
Myriad
Newton's Bit
Norseman
R.Mackey
Rwguinn
3bodyproblem
et al.,

Is this a good summary?

1. Jets dislodged SFRM;
2. Fires heated poorly-protected floor trusses;
3. Heated floor trusses expanded and sagged;
4. Sagging floor trusses exerted catenary forces on perimeter columns;
5. Catenary forces caused perimeter columns to bow inward;
6. The eccentricities of inward bowing perimeter columns exceeded the moment capacity of the column splice bolts, causing the bolts to fail;
7. Inward bowing columns did not buckle along individual 3-storey columns, but rather separated and pivoted at the failed splices.

Max

 

[R.Mackey]

It's not bad, but you might also want to add:

0. Jets destroyed perimeter and core columns

3.5 Heated core columns experience creep and thermal shortening
3.75 Action of hat trusses redistribute load away from the cores to the perimeter columns

4.5 Contracting floors after local burnout accentuate pull-in forces, particularly along long-span trusses

And also that there were cases of bowing and buckling in less than three-story lengths, just that this is the dominant failure mode in the global collapse. Bowing at initiation, for instance, was spread through many "waffle sections," and since these were staggered, the bowing was sometimes greatest near connections, and sometimes in the middle.

 

[Max Photon]

WTC failure modes - A summary

Thanks Ryan!

Here's the new, improved summary:

1. Jets destroyed perimeter and core columns;
2. Jets dislodged SFRM;
3. Fires heated poorly-protected floor trusses;
4. Heated floor trusses expanded and sagged;
5. Heated core columns experience creep and thermal shortening;
6. Action of hat trusses redistribute load away from the cores to the perimeter columns
7. Sagging floor trusses exerted catenary forces on perimeter columns;
8. Contracting floors after local burnout accentuate pull-in forces, particularly along long-span trusses;
9. Catenary forces caused perimeter columns to bow inward;
10. The eccentricities of inward bowing perimeter columns exceeded the moment capacity of the column splice bolts, causing the bolts to fail;
11. The dominant failure mode for inward bowing columns was to separate and pivoted at the failed splices;
12. However, there were cases of bowing and buckling in less than three-story lengths. (Bowing at initiation, for instance, was spread through many "waffle sections," and since these were staggered, the bowing was sometimes greatest near connections, and sometimes in the middle.)
13. Once the upper block gets moving, the dynamic load exceed the carrying capacity of the structure, and total collapse ensues.

ETA: If anyone wants to reword this and repost it, please feel free.

 

[Myriad]

Is this a good summary?

1. Jets dislodged SFRM;
2. Fires heated poorly-protected floor trusses;
3. Heated floor trusses expanded and sagged;
4. Sagging floor trusses exerted catenary forces on perimeter columns;
5. Catenary forces caused perimeter columns to bow inward;
6. The eccentricities of inward bowing perimeter columns exceeded the moment capacity of the column splice bolts, causing the bolts to fail;
7. Inward bowing columns did not buckle along individual 3-storey columns, but rather separated and pivoted at the failed splices.

It's not bad, but you might also want to add:

0. Jets destroyed perimeter and core columns

3.5 Heated core columns experience creep and thermal shortening
3.75 Action of hat trusses redistribute load away from the cores to the perimeter columns

4.5 Contracting floors after local burnout accentuate pull-in forces, particularly along long-span trusses

Hi Max,

You might also want to add:

- Besides creep and thermal shortening, the fire heated and caused weakening of the core columns.

- Besides creating forces from the sagging floor trusses, the fires heated and caused weakening of the perimeter columns.

- Regarding your items 6 and 7, it's not clear that the bolts failed first in the perimeter columns on the collapse initiation floors. (That was certainly the predominant exterior column failure mode in the collapse as a whole, though.) Inward bowing columns did buckle, inward bowing IS buckling. At collapse initiation they might have buckled more, or buckled UNTIL their bolts failed, or some might have failed one way and some another.

- Ultimately, it was the weight of the upper floors, the load on the columns, that buckled columns and/or failed their connections at collapse initiation. All the other factors mentioned either redistributed greater than normal loads to particular columns, or compromised their ability to support their loads loads, or both. This is something that people seem to frequently forget. It makes no sense to focus only on eccentric forces from sagging floor trusses, heat weakening, creep, etc. on the columns without keeping in mind that there was also the weight of a building pressing down on them.

By the way, if you're planning to suggest a theory involving sabotage by means of additional heat-weakening of column connections via some exothermic chemical reaction (such as, perhaps, thermite?), please first address the serious flaw in that theory that I've demonstrated at the end of this thread.

Respectfully,
Myriad

ETA: Cross-posted. The reference to list items 6 and 7 refers to your original list in post 593.

 

[Norseman]

Most avalanches are a direct result of non-uniform density which leads to stratification of the snow layers. The input of energy by sound or by heat via the sun reduces the static coefficient of friction inbetweeen the layers. (There's also the possibility that the addition of potential energy via mass increase exceeds the force due to static friction)


edit: the input of energy by heat will reduce the static coeffcient of friction, the input of sound will subtract from the force due to friction.

I have to kindly point out that ordinary sound, like shouting or gunfire, cannot release an avalanche, this is a myth. The pressure generated on the snow surface will be far, far to low to be able to trigger an avalanche. But the shock wave from an explosive charge in the snow, on the surface or close above the snow surface can trigger an avalanche. Therefore explosives are used in avalanche control work in connection with ski resorts and roads passing through areas with high risk of avalanches. But it is still possible for a skier to trigger an avalanche on a slope where explosives failed to do so.

Snow has the same properties as most engineering materials, like steel for instance, but since it is normally encountered close to its melting point it is a highly unreliable and unpredictable material. When fresh snow crystals lands on the ground they start to physically connect together through a sintering process that creates ice bonds between the snow crystals, this increases the strength within the snow layers and between the snow layers. So both physical bonds and friction combine to keep the snow layers in place on a slope. But these bonds are vulnerable to sudden changes in loads or rapid temperature changes. The physics behind snow and avalanches are quiet complex and not a fully understood area of science. But now I am rapidly moving off topic here.

Within the avalanche safety community there is an saying that goes like this:
"The avalanche does not know that you are an expert!"

Modified to this, it is just as relevant in relation to the WTC collapses:
"The collapse does no know that you are an expert!"

But I am quiet certain that the wisdom contained in these words will be lost on certain participants in this thread.

Norseman

 

[Furcifer]

I have to kindly point out that ordinary sound, like shouting or gunfire, cannot release an avalanche, this is a myth.

No, but I have seen it done in the interior of British Columbia with compressed air cannons, I beleive they are called concussion cannons?


I agree, the physics behind avalanches is quite complex and still largely understood. Mudslides should also be included as well. However, there are indicators used to determine avalanche prone areas when sampling cores. A layer of lower density between two higher ones in particular.

 

[Norseman]

No, but I have seen it done in the interior of British Columbia with compressed air cannons, I beleive they are called concussion cannons?


I agree, the physics behind avalanches is quite complex and still largely understood. Mudslides should also be included as well. However, there are indicators used to determine avalanche prone areas when sampling cores. A layer of lower density between two higher ones in particular.

The compressed air canon is used to launch an explosive charge on to a suspect slope:
http://avalanchemitigationservices.com/photos.htm
http://avalanchemitigationservices.com/images/evolution.pdf

Also for snow avalanches there are many indicators that can be used to determine the likelihood of an avalanche or not, including tests like digging a snowpit or doing a Rutschblock test. But there is no method that gives us an 100% reliable answer unfortunately. It is a bit like earthquake prediction.

Norseman.

 

[R.Mackey]

Just to echo Norseman's comments, Mythbusters attempted to create avalanches using gunfire, specifically fully automatic 9mm Para (HK SP9's I think). They went through about $300 of ammo, and failed.

It takes a pressure wave of a given magnitude and area to trigger an avalanche, obviously. Pressure waves are basically sound, but that sound typically has to be quite large. Artillery can do it easily. Gunfire or yelling might be able to do it in an extremely unstable situation, but this would be extraordinary.