The 9/11 Forum

It seems that WTC1 had collapsed primarily because of core failure, 102 minutes after the aircraft impact. Some members of this forum also believe that the south face inward bowing was a direct result of failing core structure.

Now, it makes me a little bit uncomfortable to see a very little discussion about the actual events that lead to the core failure.

Is it reasonable to believe the NIST impact damage estimate? Were the temperatures in the core high enough to cause substantial buckling? Did the connections suffer from any flaws?

If we assume core failure as a source of that inward bowing - how do we explain the core failure in the southeast region - with no impact damage and no substantial fires until the last 30 minutes? I think that these are questions worth asking.









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I am going to hazard a guess here. If one accepts the damage as shown in your post the perimeter of the core lost 12% of its load bearing capacity with the removal of those 3 columns. The hat truss was comprised of 8 trusses; 4 of them were atop the perimeter core columns and 4 of them were over the center of the core one of those trusses had 5 columns supporting it with 3 of them completely gone and one heavily damaged and the one to the south column 1004 supposedly undamaged.

My hunch is this transferred much of the loads carried by those 4 columns to to col 1004 and to the columns on the south facade which the hat truss ends were supported by. The north side facade columns supporting the hat truss was destroyed by the plane impact. I would think that the south facade was carry so much redistributed stress that there was little to no reserve strength left - the factor of safety was driven down to close to 1.

Now add an hour of heating to reduce the core's yield strength a bit further and the remaining columns eventually reach below FOS 1 and the first sign of buckling was the columns which were the most over stressed... the ones whose FOS was driven down to 1 sooner.. and that would be center of the south side.

Buckling is a sign of failure when the stress on the column exceeds its yield strength. This means that the loads were increasing (load redistribution) or the strength was driven down... columns were growing weaker from heating. There could have been other column destruction such as loss of strength when bracing failed... or even explosives removing more columns... causing load redistribution. But once the loads exceed the aggregate yield strength of the remaining columns they would all buckle. Buckling would lead to translation and mis alignment and the upper mass would simply then drop with little resistance.

Just a guess.

NIST says that the loads distributed to the south face were quite insignificant. What's more the maximum of IB occurred opposite to CC1007. NIST suggests that the east region didn't suffer any significant core damage. Also I am under the impression that the antenna motion suggests core failure in the eastern region as well.

And the final collapse happened at floor 98 which was almost unaffected by the plane and suffered only low temperatures in the core area. Is it possible that, for example, the core column connections had some kind of flaw that made the especially vulnerable to heat? NIST didn't see any big problems with the core (even though they had the ID and temperature model).

Core columns would loss carrying capacity from loss of bracing. It's possible that the bracing was more subject to heat stress failures because the sections were light and the exposure to heat greater. It's like trying to cook a hamburger on a BBQ cooking it on edge or laying flat.

All the floors above 78 between rows 500 and 600 had floor as the express shafts ended at the 78th sky lobbies. On the 900 and 1000 rows only the elevators that went to the top for WOW or the observation deck were continuous. So there was likely fire load in the floored areas inside the core and live and dead loads on those floors in the core.

Vertical transport of the liquid fuel would have to been down the few express shafts, some freight car shafts, the local shafts penetrating the impact region, stairways or from damaged slabs. We don't know the fuel load from the jet nor probably the precise load from the contents.

Perhaps floor 98 was seeing the most thermal stress as it was just above all the fires?

Natural question would be: how many floors of unsupported core column lenght would it take to endanger those columns? At what temperature? It still seems that the bracing would have to fail at temperatures around 400°C. Was the bracing fireproofed?

Perhaps floor 98 was seeing the most thermal stress as it was just above all the fires?

Unlikely, as there was no fireproofing damage.

So there was likely fire load in the floored areas inside the core and live and dead loads on those floors in the core.

The NIST sim seems to reflect this.

We don't know the fuel load from the jet

We do, less than 10 tons of aircraft combustibles plus a few tons of displaced office content - that's for floors 95 and 96. Nothing significant except ..maybe that the added fuel could have been piled up against CC1004....

We need to see the inventory of fuel loads. What temps are these burning at? There have been claims that the fireproofing inside the core was in poor condition, some of it succumbed to vibration from the elevators... and perhaps broke off from the vibrations of the impact. Hard to know the condition of the fireproofing. Not much of the steel in the debris seems to have any fireproofing attached to it... if I recall correctly.

If you observe the vids you can see that almost immediately after the plane hits smoke emerges from the top mech floors of tower 1 east side. My guess is that this was the result of voltage spikes and exploded transformers which could leak flammables that could have gone down into the lower floors via ducts/shafts. How would fires start 15 stories from the impact?

There were rise shafts and these might have acted like plenums and pathways for the fire and very hot gases to move upward.

Could voltage spice ignite propane tanks for cooking?

How do we know how much fuel made its way into the towers and how much remained on those floors?

Wouldn't something like an engine bounce / deflect around unless it made a direct hit to a column? Were all 4 engines recovered? If not what happened to them? What about the landing gear?

How reliable are the heat models of NIST? Based on what?

There have been claims that the fireproofing inside the core was in poor condition, some of it succumbed to vibration from the elevators

It was because of direct contact with the cables, if I recall correctly. I hoped that you knew the exact type of FP that was applied to the lateral supports - if any. I sense, that the performance of this elements could be a huge thing (as I do trust the NIST fire simulation). NIST didn't see any huge problem with the core - that's why I suspect surprisingly bad performance of those members.

and perhaps broke off from the vibrations of the impact

Like vibration induced cracks in the drywall? I don't think that SFRM could have been shaken off in any significant amount.

Could voltage spice ignite propane tanks for cooking?

Negligible heat source.

Propane itself is a negligible sort but it can begin fires in other combustibles on other floors. In the vids you can see fires popped up above the impact zone.

I don't know how anyone knows the cause of the fireproofing knocked off the core columns. Would this claim apply to ALL shafts - express, local and freight?

I thought the fireproofing was the same for all structural steel - columns and bracing... and trusses.

NIST didn't see any huge problem with the core but they DID see that there were core columns which buckled. We all see that. But how did those core column buckle? NIST has us believe I suppose that the facade was "buckled" by the trusses pulling them inward... hardly likely. and the facade loads were redistributed by the hat truss the the core causing it to buckle?????

I have a bridge for sale.

I have just one irrelevant question. Woulf it be enough to take out the 1000 row in order to cause the collapse (without any additional damage - like the plane strike)? If so, than all what it would take to send the towers down would be 8 6kg charges (assuming the use of HMX). We're talking about two people and maybe 10 minutes of preparation.

What about the Willis Tower?

Based on the cross sectional areas of the columns at the 83rd floor the percentage of each column's share of the total support can be deduced. The 24 perimeter columns were 67.5% of the core's axial strength and the 23 central columns accounted for 32.5% of the core's total strength.

4.62% 2.81% 3.40% 2.48% 2.21% 3.35% 2.81% 4.62%
1.57% 1.75% 1.63% 1.57% 1.57% 1.63% 1.75% 1.57%
1.81% 1.81% 1.12% 0.47% 0.39% 1.07% 1.93% 1.66%
1.57% 2.16% 1.20% 0.94% 0.93% 1.65% 1.67%
1.57% 1.85% 1.64% 1.11% 1.04% 1.65% 1.65% 1.57%
4.74% 2.92% 3.86% 3.48% 2.06% 3.53% 2.92% 4.74%

Row 800 had only 7 columns

If all of one outside row of the core was take out...say 501-508 about 26% of the core's total strength would be gone in one fell swoop. All the floors connected to that side of the core would collapse - 3 of them since each column carried three floors.

The axial loads supported by row 500 would then be redistributed to the remaining columns. Assuming that the 500 loads were equally redistributed to all remaining core columns (I doubt it would be equally distributed) each core column would see 26/41 or an increase share of the total load of .56%.

Here's the new percentages in row 600 after 500 is gone:

2.13% 2.31% 2.19% 2.13% 2.13% 2.19% 2.31% 2.13%

Here is were the FOS comes into the picture. The redistributed loads to row 600 have increased by about 1/3. If the FOS was 1.65 it has not been reduced to about 1.33

Look at row 700 with the .56% of the additional load:

2.36% 2.37% 1.68% 1.03% 0.95% 1.63% 2.49% 2.22%

It appears that columns 704 and 705 are now seeing more than 2x their design load. they would fail with an initial FOS of 2 let alone an initial FOS of 1.65.

And the above two rows are for equal load redistribution. If the load redistribution was more on the columns directly under the hat truss, then 8 columns would not carry redistributed loads...602, 603, 606, 607, 902, 903, 906, 907. The hat truss had 8 trusses, 4 in each axis... 2 over the perimeter and two of the center columns. If those 8 columns saw no redistributed loads then the redistribution would fall to 33 columns... 26% of the lost capacity again equally redistributed to the remaining 33 columns or about an additional .8% of the total original core axial load. In this scenario if the original FOS was 1.65 the following columns would fail 601, 604, 605, 608,801 805, 901, 904, 905, 908. When this happens there would be another rapid redistribution to the remaining columns which would all buckle except maybe 1001 and 1008.

The same scenario would apply if it was row 1000 taken out and the tower's structure above would lose the entire core, the facade columns would then buckle as well or the floors at and above the core destruction would collapse and the facade would lose bracing.

FOS was the key to how long the core could stand and how few columns could support the entire axial load.

It should be noted that the typical FOS for a steel frame is 1.42 and it appears that the FOS for the twins was a low as 1.65 but no more than 2.0 depending on the total mass of the towers. And of course the FOS was not uniform for each column and not the same at each level. There was some variation.

so collapsing the Willis Tower or some other steel frame constructions would be a relatively easy thing to do. With only 20l of explosives... big enough man can carry 10l under clothes with no problem... rather scary

Let's have a look at the Willis Tower's plans.

This site explains the system used.

http://khan.princeton.edu/khanSears.html

There was essentially a 15' x 15" column grid and the braced tubes were 5 bays on a side (75') arranged in a pattern of 3 tubes x 3 tubes.

This system is much different from the twin and looks to me much less vulnerable to collapse. No long span truss supported OOS floor. It wouldn't experience a ROOSD.

NIST FOIA: Fireproofing Condition of the Floor Bar Joists, 27th Floor, WTC1



And:

There appears to be a remarkable correlation between the floors upgraded for fireproofing in the WTC towers, in the years preceding 9/11/01, and the floors of impact, fire and failure.

http://911blogger.com/news/2008-01-06/a ... elated-wtc

The Sears tower had a foot print of 225' x 225 feet. It has a column each 15' feet or 225 columns at the base. Each column supported 225' of floor load, plus the load from the column above... except the one at the facade and they supported 112 or 56 for the corner columns. The average percentage of load for a column was under .5% of the total axial load

This was a totally different approach to the load distribution than the twin towers where the 24 perimeter core columns supported 43% of the floors load and the 236 facade columns carried 57% of the floor loads. The average facade column carried about .25% of the total axial loads and the 24 perimeter of the core columns carried an average load of 2%.

It's obvious that the twins were putting a lot of the eggs in one basket... and hence the vulnerability... not to mention of OOS system to a global ROOSD collapse.

Do you ever wonder, as an architect, whether or not the towers were designed and built to be taken down? I've heard some people say that.