Building Collapses in Rio

reply to post by IrishWristwatch

Of course I'm not suggesting the column loses capacity because it hits a floor. I was clearly talking about the FOS of the combined systems of column-column versus column-floor.

I do not understand your argument. A column will have the same capacity (and therefore FoS) to resist the impact of either another column end or an entire floor slab.

Originally posted by DrinkYourDrug
reply to post by IrishWristwatch

 

Of course I'm not suggesting the column loses capacity because it hits a floor. I was clearly talking about the FOS of the combined systems of column-column versus column-floor.

I do not understand your argument. A column will have the same capacity (and therefore FoS) to resist the impact of either another column end or an entire floor slab.

Yes. A column is a column, and what it impacts will not change its intrinsic properties. That's not what I'm saying.

The statement was made in the context of collapse progression, where the issue is an upper section plowing through and destroying the lower section. I responded to a comment about FOS, about how the building is designed to handle more than the static load. My remark is that the FOS for the building, or any part, is for the as-built geometry. That FOS does not apply if the columns are misaligned or miss each other altogether during the descent. If the latter, the two most likely things for a column end (top or bottom) to hit are floors and air.

It does not matter what the design FOS of an individual column is, if it's hitting air, no load is being borne. Floors are not much better. Floors do provide some resistance to punch-through, of course, it's over 4" of (lightweight) concrete plus pan. Compared to the pressure exerted by a column end? Hot knife through butter.

That's why buildings are built with columns stacked on top of each other and cross beams instead of floor diaphragms. The floors won't hold them; the columns are to support the floors, not vice versa.

So I am referring to the fact that design FOS is completely irrelevant in a messy collapse. The weakest link gives, and it has not one whit to do with whether it's upper or lower if columns are punching through floors. The capacity of a load bearing system is limited to the least of its members. It doesn't matter what the specs are on the column if the member it impinges on is a floor. Column wins.

No load bearing path with even an FOS of 1 (barely able to support static load) is likely to emerge from chaos. Yet it would have to be much greater than design FOS to absorb accrued momentum and arrest.

This is all VERY straightforward common sense.

reply to post by IrishWristwatch


I understand the context of of the statement and I disagree with the argument.

Floors are not much better. Floors do provide some resistance to punch-through, of course, it's over 4" of (lightweight) concrete plus pan. Compared to the pressure exerted by a column end? Hot knife through butter.

This is where I disagree. I maintain that the column being impacted by the floor will provide the same resistance as if it was being impacted by the end of another column (which is proportional to its axial capacity or FoS in original configuration).

Apparently the floor slabs would be stacking up thick and fast. A column end would not be getting very far punching through this stack and would be able to apply a great deal of its axial capacity to the falling floors.

reply to post by IrishWristwatch

TextFor those without an education which includes undergraduate mechanics: The governing principle here is least action, not path of least resistance. Physicists also understand, in this context, action does not mean force (as in the statement of Newton's 3rd law) but rather the integral of the Lagrangian or Hamiltonian over time.

You have never been an honest broker in this Irish.

This little snippet captures the argument:

If one is tracking the bead as a particle, calculation of the motion of the bead using Newtonian mechanics would require solving for the time-varying constraint force required to keep the bead in the groove. For the same problem using Lagrangian mechanics, one looks at the path of the groove and chooses a set of independent generalized coordinates that completely characterize the possible motion of the bead.

We are NOT trying to establish the motion of bead given the path, we are trying to establish the path given motion when the motion is not given completely. All that you are doing is interpreting the evidence from a framework that answers the question in the way you would like it to be answered.

It is nonsense and a completely invalid approach, and I think you know it full well, which is why it still apparently bothers you.

But I understand something about you now that I didn't before:

"When I got my bachelor's degree, I thought I knew everything. When I got my master's degree, I realized I knew nothing. When I got my Ph.D., I realized it doesn't matter."

This is very still very interesting on repeated viewing though:


The centre of gravity of that section would nearer the top, and the floors were not one piece donuts all the way round, but four independent slabs that could fail independently.



If a ROOSD occurred it should have occurred down one side of the building.

As for the original topic of the thread, the building in Rio did not fall straight down, but on top of two other buildings. Hardly an analogue for WTC7.

reply to post by DrinkYourDrug

Apparently the floor slabs would be stacking up thick and fast. A column end would not be getting very far punching through this stack and would be able to apply a great deal of its axial capacity to the falling floors.

But the exterior columns would be pulled unward by the trusses being bent downward from the debris pile.
The vertical beams in the core would also buckle sideways by the debris deflecting the horizontal beams.

Originally posted by Darkwing01
You have never been an honest broker in this Irish.

So you say. I care not what you say.

But I understand something about you now that I didn't before:

"When I got my bachelor's degree, I thought I knew everything. When I got my master's degree, I realized I knew nothing. When I got my Ph.D., I realized it doesn't matter."

What? You thought I had a higher degree? Or no degree? Which is better according to you? Don't answer. I don't care what you think.

Originally posted by DrinkYourDrug
reply to post by IrishWristwatch

 

I understand the context of of the statement and I disagree with the argument.

To disagree with my argument is the same as claiming:

- the floor slabs have the same psf rating as the columns (they don't)
- the upper block would be supported if it were set on a floor assembly (it wouldn't)

If you feel both of those are statements are true, then you do understand the context.

Floors are not much better. Floors do provide some resistance to punch-through, of course, it's over 4" of (lightweight) concrete plus pan. Compared to the pressure exerted by a column end? Hot knife through butter.

This is where I disagree. I maintain that the column being impacted by the floor will provide the same resistance as if it was being impacted by the end of another column (which is proportional to its axial capacity or FoS in original configuration).

There are two separate things to address in your statement. The first concerns the part "the column being impacted by the floor will provide the same resistance as if it was being impacted by the end of another column" and is why I believe you still don't get the context.

You are talking about a column having the same properties no matter what it impacts, I already agreed with that but that's NOT what I'm talking about. I'm talking about the stability of a load bearing path through columns end-to-end versus column end abutted to floor. You say you understand that context, but statements like this say otherwise. Once again, to be as clear as possible, I'm saying that a floor is not rated to hold the same load per unit area as a column. I'm going to be as gentle as possible about this - it's a fact. It's not subject to negotiation. If you do understand the context, then you're wrong.

Let's take the perimeter columns, which are weaker than the core columns. From 911research.wtc7.net...:

The structural steel used in the exterior 14-inch by 14-inch columns that were spaced at 3 feet 4 inches on center around the entire periphery of each of the WTC towers was fabricated from various grades of high-strength steel with minimum specified yield stress between 36 kips per square inch (ksi) and 100 ksi (PATH-NYNJ 1976).

The unit kips refers to kilopounds, ksi to kilopounds per square inch (units of pressure). Let's take the smallest value of yield stress given for the columns, 36 ksi. That equates to 36000*144 = 5,184,000 pounds per square foot (psf). All perimeter columns had the same outside dimensions, which I will round up to 15"x15" = 1.6 sq ft. The upper, weakest perimeter columns were 1/4" thick and I will round down the outside dimension to 14" to calculate the cross-sectional area as 13.9 sq in ~ 0.1 sq ft . Thus, a single column can bear a load of 518,400 pounds and presents an effective end area of 1.6 sq ft.

Now the floors. I've seen figures for the floor between 80-150 psf, I'll use 200 to be safe. The maximum live load before theoretical yield on an area equal to a column end is then 200 x 1.6 = 320 pounds. By comparison, a column can exert a force of 518,400 pounds, an overload ratio for the floor of over 1600x. To be extremely generous to this already lopsided estimate, call it 1000x.

You can disagree all you like, but you're wrong. Hot knife through butter.

I think you don't get the context. I was talking about the ability of the floors to resist collapse when they are being impacted by columns, in response to the absurd notion that the typical design FOS of the building applied during a messy collapse. To illustrate that point I referred to a single column encountering a single floor slab. I am not talking about the isolated capacity of the column in any case.

Continued...

The second part of the statement, "proportional to its axial capacity or FoS in original configuration", is interesting in its own right. Again, it does not matter what the capacity of the column is in terms of resisting collapse if the element it impacts is much weaker than it is. The point I wanted to illustrate is the loading "impedance mismatch"; the capacity of the column-floor system is given by the floor, which is the weakest link. But there are more interesting considerations.

I noticed you used the words "proportional" and "axial", and I applaud you for your precision. In doing so, you seem to be aware of two things: some proportionality is involved in out-of-spec conditions, and out-of-spec geometry is also involved. I'm only guessing, but I'd bet if a single column impacted an intact floor at a severe enough off-axial angle, it would bend instead of punching through. But a perimeter tree, which consists of multiple columns and spandrels, would not do so or would require an even more severe angle.

More importantly, the issue of proportionality comes in. What if the impact is not axial, regardless of what the column impacts? Then the FOS rating of the column does not apply. More than a few degrees off axial, and the failure mode shifts from hinge buckling under axial compression to bending under applied moment. The force required to fail the column (i.e. irreversible deformation and further capacity loss until full compaction) is reduced by one or even two orders of magnitudes. There goes your FOS, and that also serves to reinforce my point.

Which is (to beat a dead horse): the FOS of the as-built structure can be X, but the effective capacity during collapse is some value MUCH less than X. That is, not even enough to support the static load. This is in response to the ABSURD (and yes, stupid) claims that the design FOS should apply in a messy collapse. No more, no less.

Apparently the floor slabs would be stacking up thick and fast. A column end would not be getting very far punching through this stack and would be able to apply a great deal of its axial capacity to the falling floors.

If you're going to change the context to columns abutting a stack of floors, it's a different story. My remarks apply to my context. It is fair to shift the context if doing so presents a more realistic situation within the larger context of the collapse progression. Fair enough; the context I described would apply mostly if not exclusively in the initial impacts between upper and lower, and is therefore quite a limited context.

Does not mean your context is more probable or even applicable.

You presuppose the context must necessarily be pancaking. Why? Why can't it be loose (uncompacted) rubble? Why can't it be an extent of perimeter wall shearing (up or down) the floor pans off at their perimeter connections? But, okay, I'll go with pancaking.

You presuppose there is a nice stack of floor slabs one atop the other when it's far more realistic to expect pancaked floors to be fragmented, therefore semi-permeable under concentrated impulse. Okay, I'll go with that.

There are two situations to consider. The first is a column(s) from above impacting a stack of floors below, the second is a stack of floors impacting a column(s) below. In the first case, the stack may either be stationary or moving. If moving, the upper section must overtake at a higher velocity for there to even be an impact. Upon impact, the column either fails and crush up begins on top of the pancake, or the column survives and comes to rest atop the moving pancake, adding the full mass and transient impulse of the upper section to the moving pancake. If the pancake is stationary, then the column can also either fail or survive. If it fails, crush up has started, if it survives, collapse is arrested - after some arbitrary amount of pancaking! If you like, but that begs the question of what stopped the pancake, deferring it to the second situation.

The second situation, a stack of floors impacting a column below, can also result in failure or survival. If failure, game essentially over, it keeps going. If survival, then can a column end punch through a stack of (e.g) 20 floors? No, I don't think so. One? Yes. Two, very likely. Three, still probable. How then would floor slabs remain intact to accrue a 20-floor pancake? But, let's suppose 5 stacked, intact floors were effectively impenetrable and manged to accumulate, THEN hit column ends below.

Arrest is possible, assuming the momentum of the integral pancake mass doesn't overload the columns. But, by definition, the pancaking mass is detached from the rest of the structure. Only the pancaking mass is arrested. If interior pancaking collapse is all there is, then there is arrest. But both upper sections go into motion at the start of the collapses, therefore pancaking is at most part of the picture, if any.

All of this minutiae obscures the central point, as is typical for these discussions.

Design FOS is meaningless in a messy collapse. They WERE messy collapses, regardless of cause. Appeals to FOS or capacity greater than static load are rubbish. The effective capacity under the conditions of severe misalignment and eccentricity DIRECTLY OBSERVED is reasonably expected to be a small fraction of the static load. That is, the towers would not be stable in a static loading with that degree of misalignment, let alone able to resist dynamic overloading.


For those operating under the delusion (for that's what it is, if you choose to cling to it after being shown otherwise) that this is false, I offer this article:

THE FIFTY-NINE-STORY CRISIS

It seems I have to pull this out again and again to stress how much misalignment can reduce capacity. In this article, Leslie Robertson (yes, of WTC fame) orders a massive, expensive retrofit of the building due to a design error. Quote:

A great deal of work remained. Robertson was insisting on a complete reevaluation of the Citicorp tower: not just the sensitivity of the chevrons to quartering winds but the strength of other skeletal members, the adequacy of braces that kept the supporting columns in plumb, and the rigidity of the building's corrugated metal-and-concrete floors, which Robertson feared might be compromised by trenches carrying electrical connections.

His insistence was proper--settling for less would have compromised Robertson's own position. It amounted to a post-construction autopsy by teams of forensic engineers. For LeMessurier, the reevaluation was harrowing in the extreme; every new doubt about his design for Citicorp Center reflected on him.

In one instance, Robertson's fears were unwarranted: tests showed that the tower floors were entirely sound--the trenches were not a source of weakness. In another, Robertson, assuming the worst about construction tolerances, decided that the columns might be slightly, even though undetectably, out of plumb, and therefore he ordered the installation of supplemental bracing above the fourteenth floor.

Bolding mine. Slightly, undetectably out of plumb. According to what I see around here - who cares? Robertson was a fool for ordering expensive and inconvenient reinforcement work! You could take columns and just stand them on the floor slabs, it wouldn't matter!

I recommend that article for any with a genuine interest in the subject at hand, as opposed to only being interested in hearing their lips flap. It's dated 1995, but I suppose it could have been planted post 9/11 and backdated to throw the public off and promulgate the official propaganda that things like good assembly practices, alignment and plumb have anything to do with capacity.

Geeeeeeeez.

This isn't the first time I've said these things here. Like water off a duck's back.

I'm under no delusion that I'll convince anyone of anything. I look around and what do I see? Hundreds of members... many thousands of posts... going on for years... and the discussion has advanced no further than this? Hopeless, utterly hopeless.

Go ahead, believe whatever you want, you will anyway. Inward bowing is explained by warpage of the aluminum cladding, floor slabs have the same rating as columns, there were no core remnants, momentum counts for nothing, a haphazard collection of parts has the same capacity as they would properly assembled according to drawing, blah blah blah yackety schmack. Whatever.

Originally posted by samkent

But the exterior columns would be pulled unward by the trusses being bent downward from the debris pile.
The vertical beams in the core would also buckle sideways by the debris deflecting the horizontal beams.

Do you think the 5/8 bolts holding the trusses up were stronger than the columns?

Why didn't the bolts just break?

If the bolts were stronger than the columns then why did the floors collapse at all? I thought it was the weak connections that kept the floors from resisting the collapse?

BTW it was not only 5/8 bolts, there were also two 1" bolts...

Originally posted by IrishWristwatch
momentum counts for nothing

That is the most relevant thing you said and needs addressing because it shows your confusion of the argument.

No one has EVER said momentum counts for nothing.

What we are saying is momentum is not everything like you all think it is, or want it to be.

You are just confused as to the effect of momentum in a collision. You can't ignore the fact that in ALL collisions the forces on both objects is the same, regardless of momentum. An increased momentum means the forces on BOTH objects increases, not just the one being impacted.

So an object with a small mass can not destroy a larger mass and not be destroyed itself, no matter what it's momentum is. The same reason 15 floors can not crush 95 floors without being destroyed itself long before the 95 floors are. Just because the floors are moving down it doesn't mean they will not feel the same force as the floors they impact. So if the impacted floors are being destroyed then the falling ones will also.

This is not rocket science. Newtonian physics is the first thing you learn in engineering.

For every action, there is an equal and opposite reaction.

The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

www.physicsclassroom.com...

Two 5/8" bolts (in slotted holes!) at the perimeter, 1" bolts at the damper.

All the same, I agree with ANOK's contention that floor connections cannot be assumed weak one moment and strong the next, according to convenience. The 5/8" bolts were supplemented by welds, but neither the bolts, the welds, nor the angle brackets were likely to be able to sustain the significant rotation required by caternary sag of the floor assemblies, so as to produce tension, so as to produce pull-in of perimeter panels. I realize NIST said it, and I'm nobody, but give it some thought.

I was down in Ecuador a few years ago. We went to a resort town on the coast and they were building several high rise condominiums.

The first condo I saw being built was like something right out of the US. Modern equipment, reinforced concrete construction.

The two other condos that I saw being constructed were third world construction. The concrete was being mixed right next to where it was being poured. The whole process was manual with concrete being moved in white plastic pails. Two floors below, the forms were being stripped off and you could see the incomplete penetration of the concrete into the forms. This was being plastered over with more a cement and sand mixture.

If an earthquake should hit, I'm pretty sure which structure I'd rather be in or around.

Originally posted by IrishWristwatch

Design FOS is meaningless in a messy collapse.

Again your opening statement shows you are confused.

FoS is NEVER meaningless, it is the structural capacity of the building. A 'messy' collapse doesn't mean physics is ignored.

Factor of safety for structural applications is the ratio of the allowable working unit stress, allowable stress or working stress. The term was originated for determining allowable stress. The ultimate strength of a given material divided by an arbitrary factor of safety, dependant on material and the use to which it is to be put, gives the allowable stress.

Unless you know the FoS of a component you can not claim x amount of force would cause failure. You have to know the FoS to do that calculation. That is why we laugh when people ask for calculations to prove gravity could not cause the collapses.

You only think it's not important because it's not part of the OS.

Originally posted by IrishWristwatch
Two 5/8" bolts (in slotted holes!) at the perimeter, 1" bolts at the damper.

I don't think slotted holes make any difference.

All the same, I agree with ANOK's contention that floor connections cannot be assumed weak one moment and strong the next, according to convenience. The 5/8" bolts were supplemented by welds, but neither the bolts, the welds, nor the angle brackets were likely to be able to sustain the significant rotation required by caternary sag of the floor assemblies, so as to produce tension, so as to produce pull-in of perimeter panels. I realize NIST said it, and I'm nobody, but give it some thought.

So you're agreeing with me? Sagging trusses could not pull in columns?

Originally posted by ANOK
So an object with a small mass can not destroy a larger mass and not be destroyed itself, no matter what it's momentum is. The same reason 15 floors can not crush 95 floors without being destroyed itself long before the 95 floors are. Just because the floors are moving down it doesn't mean they will not feel the same force as the floors they impact. So if the impacted floors are being destroyed then the falling ones will also.

Small rock vs large window? Besides that, who cares if the top is destroyed or not. Nobody cares, only truthers who think it is significant. A destroyed top section still has the mass do to more destruction. Thats all that matters. But in truther world "destroyed" means "poof, magically disappeared".

ps, I see that you choose to not post anymore in the thread where I confronted you with your miserable understanding of physics. I think denial is indeed your best approach to cope with it.

Originally posted by -PLB-
Small rock vs large window?

What? We're talking about steel, not glass. Why do always take it out of context and think you have a point?

If it was steel falling on glass you would have a point.

Besides that, who cares if the top is destroyed or not. Nobody cares, only truthers who think it is significant. A destroyed top section still has the mass do to more destruction. Thats all that matters. But in truther world "destroyed" means "poof, magically disappeared".

LOL you really don't have a clue what the argument is. The top did not have the mass to do any destruction.
This is why in vids you can see the tops acting independent of the bottom. The tops were being destroyed before the bottoms started. Whatever energy collapsed the towers started in the top sections first.

ps, I see that you choose to not post anymore in the thread where I confronted you with your miserable understanding of physics. I think denial is indeed your best approach to cope with it.

What are you talking about? What thread? Confront me now genius.

What was stronger PLB the bolts or the columns?

Originally posted by ANOK
What? We're talking about steel, not glass. Why do always take it out of context and think you have a point?

If it was steel falling on glass you would have a point.

It just demonstrates your sloppiness in communicating what you mean to say. Probably because you don't really know what you are talking about.

LOL you really don't have a clue what the argument is. The top did not have the mass to do any destruction.

LOL, it had. Even if you do not believe it destroyed the whole tower, any sane person would still believe it would do some destruction. But you are probably just using the wrong words again to communicate what you mean to say. And the world is left to guess whats going on in that head of yours.

This is why in vids you can see the tops acting independent of the bottom. The tops were being destroyed before the bottoms started. Whatever energy collapsed the towers started in the top sections first.

Whats that phrase again. "That which can be asserted without evidence can be refuted without evidence".

What are you talking about? What thread? Confront me now genius.

The denial is strong in you. But to give you a hint, it had to do with sagging trusses.

Originally posted by ANOK

Originally posted by IrishWristwatch
momentum counts for nothing

That is the most relevant thing you said and needs addressing because it shows your confusion of the argument.

No one has EVER said momentum counts for nothing.

For you to seize on that one thing out of all I've said to you is very telling in itself.

For all the discussion about collapse progression, NO ONE here is talking about the necessary condition of arrest being net momentum of zero, no one is including momentum in the dynamical considerations (except psikeyhackr, who only emphasized the resistive aspect, not the overload aspect) - in short, no one is counting momentum, which is to say momentum counts for nothing.

You, for your part, explicitly waved momentum away with this:

If there is an equal reaction between objects colliding, then what is it that defines the destruction? Momentum?
No, because no matter the momentum the forces on both objects will be equal. So what is left? MASS!

To any native English speaker that is virtually synonymous with saying momentum doesn't count. You claim, falsely, that MV doesn't matter, only M. This is claiming the dynamic situation is the same as the static, and it is not - because of momentum! QED.

It may not be surprising that you don't follow what I say, but it's remarkable you don't even understand yourself.

What we are saying is momentum is not everything like you all think it is, or want it to be.

Oh dear. Please do enlighten me.

You are just confused as to the effect of momentum in a collision.

No, I'm not, but you are. Read on.

You can't ignore the fact that in ALL collisions the forces on both objects is the same, regardless of momentum.

I haven't ignored that, in fact I acknowledged it (and refuted this statement of yours one page back). But do you know from what the force derives? Force is the time rate of change of MOMENTUM. Did you know that (even though I already said it upstream)? If not, you have no business lecturing me on the role of momentum in collision; if so, then you ought to know why you can't wave momentum away. It is the reason for the force above static load, which is called dynamic loading! It is proportional to mass AND velocity, not just mass. So you are wrong in a very fundamental way. You want to reduce it to mass alone when it can't be, and that's because you have no skills in physics. None. You do not know the difference between a problem in statics and one in dynamics. That's baby steps.

An increased momentum means the forces on BOTH objects increases, not just the one being impacted.

Duh. And what if that force exceeds the yields of one or both objects? Huh? And how would you know if it did based on mass alone, without factoring in the velocity? You can't.

You lecture, but you don't know. This is precisely the reason for my recently expressed exasperation. And my statement, including "momentum counts for nothing." You denied it, then turned right around and tried to handwave it away again!

So an object with a small mass can not destroy a larger mass and not be destroyed itself, no matter what it's momentum is.

Assuming they are made of the same material, and that there is no geometric advantage (e.g., strong axis orientation), and that the densities are the same (no void space or average void characteristics the same), yes. But that totally misses the point. With sufficient momentum, a smaller mass can destroy both itself AND the larger mass. Since you're not counting momentum, and think it doesn't count, you don't understand this.

The same reason 15 floors can not crush 95 floors without being destroyed itself long before the 95 floors are.

That is Heiwa nonsense almost word for word. Are you a devotee? Look, this is wrong on so many levels I don't even know where to begin, but I have covered all of these points in your presence before and I'm not insane, so I don't expect a different outcome.

edit on 30-1-2012 by IrishWristwatch because: (no reason given)

edit on 30-1-2012 by IrishWristwatch because: (no reason given)