What exactly has NIST proven and HOW did they prove it?

I think the thread title is self-explanatory.


Constantly seeing the word "proof" thrown around by sad individuals convinced that they have something that other people they call "truthers" are supposed to refute, I've started this thread as a reality-check.


If anyone can show what NIST proved in their reports, and how, let them post it here, firstly if they can even find any proofs, and secondly to show something about NIST's work that will become self-apparent if the thread even gets that far.


Good luck, and may I suggest starting with their main hypothesis: the WTC tower trusses got hot, sagged, and by this mechanism were able to pull enough perimeter columns inward to cause the entire building to collapse. They never actually proved this at all, but why spoil all the fun so early?


Like I said, if anyone thinks they can do this, good luck.

Hi Bsbray...
NIST used finite element modeling to model the events preceding the collapse. What they show is that the combination of column damage and fire led to the collapse. We know that NIST stopped their investigation at the collapse initiation.

I reached out for some help by a structural engineer with your question(s) and may have received some answers. I would like to know your thoughts on this as he offers calculations and drawings:

Why did the columns bow in? There’s been quite a bit of speculation and misinformed opinion about the mechanics of the structure that caused that, so I hope to do a little bit of enlightenment. The first thing that needs to be looked at is the problem. Figure 1 shows a typical building section through a building such as the WTC towers.



Everything here is fairly straight forward. The gravity load path can be seen very easily. The floor trusses deliver the vertical floor loads to the columns which deliver them to the foundations. But what happens when a core column is severed as in Figure 2?

Figure 2
Things start to get a little bit more complicated. The first thing that should dawn on most people is that the floor is no longer being supported by the middle column which is going to cause some problems. The middle column will drop unless there is a force that can resist it, see Figure 3.


Figure 3


As the column drops, the top chord of the floor truss develops tensile forces (it is quite literally stretched). This tensile force has two parts, a vertical portion that pulls the column up and a horizontal force that pulls the rest of the structure in. This can be seen in Figure 4.


Figure 4


The other things to note here is that the left column is still under its full axial loading (P2, which will be important in the analysis). The left column is already shown pulled in to some degree, however it is not to scale. Further modifying the problem, we know that the fires caused the trusses to sag to some degree. If the fire is hot enough, the truss will become a tension only member. This means that the top chord of the truss will act something like a rope and pull inwards at its connections. This can be seen in Figure 5. There is another condition as well (which I have no illustrated) that will cause the heated floor to expand outwards without losing its bending capacity and thus not sagging. It is a condition that likely proceeded that of the sagging floor trusses.


NIST (1-6D) estimates that the total pull-in force at the exterior columns is roughly 6 kips (6,000lbs) at each column. The truss to column connection consisted of (2) 5/8” diameter bolts. Even non-structural grade bolts of this size will have a shear capacity of over 5kips each, so it is reasonable to assume that the top chord of the truss will not pull off of the columns at the connections due to a 6kip load.

Is this 6 kips enough to pull the column in several feet as seen in the photos of the tower?


Continued:


I am editing the pictures now.... sorry


[edit on 5-7-2008 by ThroatYogurt]

Math is needed here. First some assumptions need to be made. For the purpose of this analysis, let the exterior column be HSS14x14x5/16 tubes at 25% of the maximum axial load prior to any damage. The column has the following properties:

Similar to HSS 14x14x5/16
A = 15.7in^2
I = 739 in^4
S = 92.3 in ^3

Pn = 557k (from AISC LRFD 3rd, table 4-6 with an unbraced length, KL = 12’-4”)
Pu = ¼* 557k = 139k
Mn = 92.3in^3*46ksi = 4645 kip*in (Mn = maximum bending capacity)

The column itself will bend inwardly until it snaps if the column itself ever becomes inelastic. This can be defined by the ratio (I’ve simplified this a bit): Mu/Mn + Pu/Pn < 1. The other limit state is P-delta. When the exterior column is pulled inwards, it deflects. This deflection(Δ) generates a moment, specifically P2*Δ. This moment, creates more deflection, which further magnifies the moment, creating more moment, and so forth. P-delta has two outcomes: the moment reaches equilibrium at some point, or becomes unstable and continues to grow. This phenomenon can be easily shown with a simple experiment. Take a straw and try to compress it between your fingers. It has a surprising amount of strength. Now push the middle in slightly. This is p-delta.

We know that the floor diaphragm along the wall were greatly damaged due to fire and the impact of the aircraft. Let us assume that there are two floor diaphragms that are damaged to the point that they no longer provide bracing against buckling. There is thus a pull-in force of 6kips at two places along the length of the column.


CALCULATION 1: DIAPHRAGM DAMAGE, NO FIRE EFFECTS
Unbraced length = 37’-0”
Pu = 139k
Pn = 465k (from AISC LRFD 3rd, table 4-6 with an unbraced length, KL = 37’-0”)
Mn = 4645 kip*in

Mu = P*a
Mu = 6kip*1/3*37ft
Mu = 74kip*ft or 888 kip*in


Deflection = P*a *(3L^2 – 4*a^2)/(24*E*I) (Formula from AISC LRFD 3rd)
a = 1/3*L
= P * 1/3L *(3L^2 – 4/9*L^2)/(24*E*I)
= 23*P*L^3/(1296*E*I)
= 23*6k*(37ft*12in/ft)^3/(1296*29000ksi*739in)
= 0.435in

Additional moment due to P-delta
Mu+ = 0.435in*139k = 61.02 kip*in


Additional Deflection = Mu+*L^2 / (4*EI)
= 61.02kip*in*(37 * 12ft\in)^2 / (4*29000ksi*739in^3)
= 0.140in

Additional moment due to P-delta2
Mu++ = (0.435+0.140)in*139k = 79.93 kip*in


Additional Deflection = Mu++*L^2 / (4*EI)
= 79.93kip*in*(37 * 12ft\in)^2 / (4*29000ksi*739in^3)
= 0.184in

As seen, the first p-delta iteration results in an increased deflection of 0.140in. The second results in a deflection of only 0.184in. We can thus conclude that p-delta will eventually converge and that no further iterations are necessary. The 6kip pull-in force with no effect of fire will not result in the column becoming unstable.


In a 600C fire, the Modulus of Elasticity will have reduced to approximately 0.3 of its original value, and the yield strength to 0.5 of its original value. The effect of the Modulus of Elasticity being so greatly lowered is of far greater important than the yield strength, however.

CALCULATION 2: DIAPHRAGM DAMAGE, 500C FIRE
Unbraced length = 37’-0”
E = 0.3*29000ksi = 8700ksi
Pu = 139k
Pn = 465k*0.5 = 233k
Mn = 4645 kip*in *0.5 = 2323kip*in

Mu = P*a
Mu = 6kip*1/3*37ft
Mu = 74kip*ft or 888 kip*in

Deflection = P*a *(3L^2 – 4*a^2)/(24*E*I) (Formula from AISC LRFD 3rd)
a = 1/3*L
= P * 1/3L *(3L^2 – 4/9*L^2)/(24*E*I)
= 23*P*L^3/(1296*E*I)
= 23*6k*(37ft*12in/ft)^3/(1296*8700ksi*739in)
= 1.45in

Additional moment due to P-delta
Mu+ = 1.45in*139k = 201.6 kip*in


Additional Deflection = Mu+*L^2 / (4*EI)
= 201.6kip*in*(37 * 12ft\in)^2 / (4*8700ksi*739in^3)
= 1.55in


Additional moment due to P-delta2
Mu++ = (1.45+1.55)in*139k = 417 kip*in

Additional Deflection = Mu++*L^2 / (4*EI)
= 417kip*in*(37 * 12ft\in)^2 / (4*8700ksi*739in^3)
= 3.20in

This results in the column becoming unstable due to p-delta. This can easily be seen in that the deflection due to P-delta2 is double that of P-delta1. It can therefore be concluded that it was necessary for both fire and damage to result in the collapse of the towers.

[edit on 5-7-2008 by ThroatYogurt]

Originally posted by ThroatYogurt
Hi Bsbray...
NIST used finite element modeling to model the events preceding the collapse.

First, why does the computer modeling take precedence when NIST also conducted laboratory experiments?

Secondly, can you provide the parameters for that modeling so that it can be reproduced and verified, ie peer reviewed? Are these not the same tests where NIST states in their report that they increased certain parameters in their computer model to unrealistic levels before reaching the state they were looking for?

reply to post by ThroatYogurt


Thanks for going out of your way to ask someone else to "help" me, but I don't have any misunderstanding about the effects of buckling on the ability of a column to transfer a load to the next column.

My questions are about the NIST investigation and where they actually prove what they assert with their hypothesis.


Now, similar calculations from the actual NIST report, with the structural documentation, and modeling along those lines (especially how much force a sagging truss will exert laterally) would be more along the lines of what this thread is asking for. Better yet, an actual lab test showing the mechanism at work in the real world.

[edit on 5-7-2008 by bsbray11]

Thats the problem that I have also. While 100% of what you have posted could be true, it only 'proves' there was enough energy initiate the collapse.

This is indeed a very good start.

However, the buildings fell strait down. This defies everything I had ever heard about SE.

As a taxpayer, I simply cannot be satisfied that it took 7 years to come up with a report that explains how the collapse was initiated. I expect a full criminal investigation. One has yet to take place.

reply to post by jprophet420


JP,

Would you have expected it to topple like a tree? [i'm not being sarcastic)

Thanks

Originally posted by ThroatYogurt
reply to post by jprophet420

 

JP,

Would you have expected it to topple like a tree? [i'm not being sarcastic)

Thanks

In the papers I have read and from the professionals I have talked to it would not fall into itself unless the damage was perfectly symmetrical. It would not necessarily topple over like a tree but it would fall decidedly to one side as opposed to strait down. Also, from everything I have read, the tower that was leaning 23 degrees or so (sorry cant remember which one ATM, its blatant in the videos) would never have fallen into itself without help.

reply to post by jprophet420


WTC 1 & 2 Did not fall straight down. Please try not to quote me, but I believe the majority of the debris on both towers landed outside their own foot prints.

Bsbray may have the info. If I find it i will post it.

reply to post by bsbray11

Unfortunately, the experiment that NIST performed on a floor section of joists did not include columns, nor did it include any removed fire protection. This is an attempt to compare apples to oranges.

This is basic statics, basic engineering, basic mechanics of structures. If they can't refute the math, then perhaps they should realize that they're too far over their head to be rejecting or accepting the information presented.

-Structural Engineer

reply to post by ThroatYogurt

Indeed. And no information has been presented that explains the events in full, hence the problem.

Ray Dougherty - structural engineer

www.alum.dartmouth.org...

After about 40 minutes, as I saw (I have telescopes, binoculars, etc.) the top segment of the building listing about 3 degrees, I left my apartment and went out to walk in the street. Buidlings collapse if they list more than 3 degrees. As I walked down Bleecker Street, people gasped as the building collapsed. Like Lord Jim, my imagination surpasses any reality. I should have stayed and watched. I did for the second tower.

...We watched the second building, and I noticed it was more than 3 degrees, but as the telescope revealed, that was because the beams were buckling on both sides. A building like the WTC does not 'break off in the middle' and fall like a tree.

Mark Loizeaux - demolition expert

wtc7lies.googlepages.com...

Loizeaux said he had an enhanced video of the collapses, and he talked about them in a way that indicated he had watched the video more than once. "First of all, you've got the obvious damage to the exterior frame from the airplane—if you count the number of external columns missing from the sides the planes hit, there are about two-thirds of the total. And the buildings are still standing, which is amazing—even with all those columns missing, the gravity loads have found alternate pathways. O.K., but you've got fires—jet-fuel fires, which the building is not designed for, and you've also got lots of paper in there. Now, paper cooks. A paper fire is like a coal-mine fire: it keeps burning as long as oxygen gets to it. And you're high in the building, up in the wind, plenty of oxygen. So you've got a hot fire. And you've got these floor trusses, made of fairly thin metal, and fire protection has been knocked off most of them by the impact. And you have all this open space—clear span from perimeter to core—with no columns or partition walls, so the airplane is going to skid right through that space to the core, which doesn't have any reinforced concrete in it, just sheetrock covering steel, and the fire is going to spread everywhere immediately, and no fire-protection systems are working—the sprinkler heads shorn off by the airplanes, the water pipes in the core are likely cut. So what's going to happen? Floor A is going to fall onto floor B, which falls onto floor C; the unsupported columns will buckle; and the weight of everything above the crash site falls onto what remains below—bringing loads of two thousand pounds per square foot, plus the force of the impact, onto floors designed to bear one hundred pounds per square foot. It has to fall." [Note that many of Loizeaux's comments about the conditions of the towers were accurate, but that the assumed cause of collapse – floors pancaking – was not. Determining the actual cause of collapse would take much research.]

South Tower column bowing




South Tower collapse initiation

i think NIST proved that they are incompetent and can't address the problem they set out to address.
in fact, they produced a huge report that is nothing less than elusive and nebulous. a lot of form with little substance.

what the two dimensional drawings above fail to recognise or consider, is that the floor joists and columns were a web, with cross members running perpendicular to the joists and columns shown. the columns could not drop down without the roofline sinking, which it did not. the columns could not drop because they were supported from both above and three sides on the outside of the core (two on the corners), and four in the center.

if the columns can't drop, then the floor joists don't drop. if the floor joists sag, it is because they are deforming and becoming more taffy-like, not getting any stronger or heavier.

nice try.

reply to post by ThroatYogurt


Nice calculations.

I have a question though: Why is he/she using LRFD (Load Resistance Factor Design)?

The towers would have been designed using ASD (Allowable Stress Design) as LRFD wasn't introduced until the 80's I believe.

While similar, the ASD method actually results in more of a safety factor as it is more conservative of an approach.

Here's a good explanation:

With ASD Steel, you have

Load Factors = 1.0 (or, in other words, no load factors)
Allowable stress reduction factor = 0.6 (yes it varies but let's use 0.6 for our conversation). This is applied to the yield stress - let's use 36 ksi as you used above.

This would mean that, under the actual loads (1.0 x load) you would design your member to not be stressed higher than 0.6 x 36 = 22ksi. Your safety factor then is 36/22 = 1.64.

With LRFD Steel, you have (per the IBC)

Load Factors for Dead = 1.2
Load Factors for Live = 1.6
Strength reduction factor = 0.9 for flexure

For a case where DL = LL your Load factor averages to 1.4.
So your safety factor is 1.4/0.9 = 1.56. In this case I've just used the single load combination of DL + LL (there are others of course) and looked at flexure only (shear has a different SRF).

www.eng-tips.com...

Originally posted by bsbray11
Good luck, and may I suggest starting with their main hypothesis: the WTC tower trusses got hot, sagged, and by this mechanism were able to pull enough perimeter columns inward to cause the entire building to collapse.

My questions on the floor trusses sagging theory is that they were part of a composite floor design. Meaning the trusses were attached to concrete. Concrete is stiff and doesn't want to bend like the trusses would. So, if the trusses were attached to the concrete (as in normal practice), wouldn't the concrete resist this deflection? Or would the loss of strength of the trusses add to the sagging because they couldn't hold the concrete anymore?

Anyway, what I'm getting at is we always here of the steel trusses but not so much that they would have been attached to the composite concrete deck.

wtc.nist.gov...

Page 48 of this report shows a good picture of how the trusses conncected to the concrete.

reply to post by Griff


WHAT ????

The trusses are attached to the concrete?

The trusses were attached to the ext and core columns.

The 4" concrete was poured over the corrugated steel decking.

Wow.....

Originally posted by ThroatYogurt
NIST used finite element modeling to model the events preceding the collapse.

Its just too bad thier model stated that neither the plane impacts or fire caused the collapse.

reply to post by Seymour Butz



the floor joists went right through the decking, so there were rows of loops of the joist steel poking up above the floor. it is on page forty three of the PDF that griff linked to.

the floor assemblies were integral concrete/steel structures.


[edit on 5-7-2008 by billybob]

Originally posted by Seymour Butz
WHAT ????

The trusses are attached to the concrete?

The trusses were attached to the ext and core columns.

The 4" concrete was poured over the corrugated steel decking.

Wow.....

Wow indeed. It would do you well to research a little before putting your foot into your mouth.



wtc.nist.gov...

Notice the truss knuckles and reinforcement bar extend 3-inches up into the 4-inch concrete. That's considered an attachment.

Originally posted by Griff

Wow indeed. It would do you well to research a little before putting your foot into your mouth.

LMAO.

This from the civil engineer that I had to correct after he put forth the idea that the core columns had 6" of "layered" insulation. And didn't have any idea that the ext columns, while they maintained the same exterior dimensions, were made from steel that got "thinner" as they got farther up, and was again corrected by me. And even though stated repeatedly that NIST never referenced the "swiss cheese" columns, whining about how important it was to his line of work, and then had to be shown, again by me, that NIST did address it.

Tsk, tsk Mr Griff. He who casts the first stone.......

On topic - Are you seriously suggesting that attaching a 4" slab would lend much stiffness to the floors? Seriously?