9/11 Physics from Non-Experts

As I said, I'll be fair

Apollo20 said:
Gregory Urich:

When you claim that no one has published a detailed estimate of the mass of a Twin Tower, may I direct you to the very detailed calculations (18 pages worth!) under the heading "Mathematics of the WTC fires, Part Three", on the website www.takeourworldback.com.
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Thank you, I’ll look into it further, but at the surface it seems very credible. As I stated before, If I see an explanation that will fit Urich’s estimations with the amount of debris and the amount of materials used I would be fair and drop the point.
I hereby will, until I checked all the numbers (and for the moment I will assume they are right), I will accept the proposed estimate in the link and despite the fact that Urich’s estimation must be lower than the estimation proposed in the link, I will drop my point. If anyone can see where these calculations are wrong I would be interested to hear it...
 
Empty space

R.Mackey said:
Quite right. The WTC Towers, of course, were designed about 40 years ago, and modern design requirements are figured differently. I found NIST NCSTAR1-1 to be extremely dry reading but still useful in capturing the state-of-practice when the Towers were designed.

Agreed. This is why I keep pointing people to Chap 4 NIST NCSTAR1-2A, which indeed uses 20 psf as an as-built estimate of actual load for the partitions, along with other things added in. Gregory Urich also apparently considers the core to be nearly empty space, and that's hardly accurate...

Thanks for your comments.
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Please forgive my sloppiness earlier referring to 99 elevator shafts. There are 99 elevators with coaxially located shafts. I just wanted to check up on the empty space issue so I checked out the architectural drawings for the core floors 11-16 just as an example. Other floors especially higher in the building will likely have fewer elevator shafts.

Nonetheless floors 11-16 have no fewer than 50 elevator shafts with a cross section of approx 4300 sq ft. That's alot of empty space. Then there is the service shafts at 1800 sq ft. These contain pipes, ducts, wiring, etc. but are mostly space and there is no live load. There is also 1900 sq ft of hallway and stairs which have no permanent live load, only people moving about. The rest 3745 sq ft is restrooms, service rooms, closets, eating areas, etc. This means that 68% of the core is essentially empty with 52% not even having a floor.

Compare this to my calculation where on every floor I have used 20.5 psf combined live load and superimposed dead load, and also 5" concrete floor for the entire core area. If you add this up (which I will get around to) my guess is that the overestimated weight for 110 floors will dwarf the partitions, actual superimposed dead loads, mechanical floors and elevators combined.
 
If you chaps ever move onto metric for your calcs, let me know - I might be able to contribute something!

(Or ask one of the guys from Arup on my team!)
 
My heartfelt thanks to everyone on both sides of the argument who answered my questions. I understand the issues much better now.

Respectfully,
Myriad
 
Gregory Urich I still find your live load estimates laughable, and am beginning to wonder if you have ever, in your life, worked in an office, let alone had to move furniture around.

Also, are you aware that NIST provide an example of a typical floor layout in their report? One could simply look at this layout to get a better idea of the number of offices, internal walls, etc...

-Gumboot
 
GregoryUrich said:
This doesn't support my estimated actual live load. But it does indicate a safety factor of 2.8 if the building is fully loaded to design load. I don't claim a safety factor within the design load. I claim that buildings are rarely loaded over the entire floor area at the full design load.
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OK, so in other words, you're claiming that the ultimate strength of the floor structures is about 2.8 times the design load of 100 psf. I'm fine with that. As you point out, NIST NCSTAR1-1A discusses the actual destructive test criteria, and the report also includes deflection testing of every single type of floor truss.

But you're also claiming that the actual load in the structure -- which includes live loads and all superimposed dead loads -- comes to a total of 27 psf. I'm still waiting to see you support this. Your "common sense" office calculations just aren't rigorous enough, nor supported by actual evidence, just your assertions on how much an office weighs and your insistence that you've handled everything.

Why would the WTC designers plan for a structure that could handle 100 psf (actually, the reduced live loads alone, just for floors, vary from 55 to 82.5 psf on non-mechanical floors, as shown in NIST NCSTAR1-2A Figure 6-1 on page 136; superimposed dead loads add another 12.5 psf for area outside the core, same page), when in reality the structure would only face a quarter of that?

Are you suggesting the design engineers radically overestimated occupancy? Grossly misjudged standard office loads? Were forced to include yet another factor of four overstrength by the Man? Where did this error come from? It's too fantastic to believe.
 
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Apollo20 said:
I am not endorsing takeourworldback in any way!

Try:
www.takeourworldback.com/911/911fires3.htm before you assert there is no calculation of the mass of a twin tower...

I think I deserve an apology!
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It's possible that I made a mistake here. If that work isn't yours, then you do deserve an apology. I had assumed, however, that it is yours. Is it?

Very few things on that website are signed...
 
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gumboot said:
Gregory Urich I still find your live load estimates laughable, and am beginning to wonder if you have ever, in your life, worked in an office, let alone had to move furniture around.

Also, are you aware that NIST provide an example of a typical floor layout in their report? One could simply look at this layout to get a better idea of the number of offices, internal walls, etc...

-Gumboot
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Let me see: Mercedes North America Inc., Computer Science Coporation Inc., Eriksson AB, Astra-Zeneca AB, Ikea IT AB, Sigma AB, Cerdo Bankpartner AB, etc., all in all about 20 years of office space occupancy. Oh yeh, I forgot about my own company that we moved and set up ourselves, USBs, server room and all. What do you think that stuff weighs? Have you ever been in an office?

NIST gives 25,000 workspaces per tower.
 
Thank you all for undermining my question, which has yet to be answered. We can debate till we are blue in the face how much paper weighs. Good job at overlooking the obvious.
 
Mackey:

Thankyou!

I accept your apology.... and NO, it's not my work! But it looks to be a very detailed calculation. For example it gives an estimate of the mass of decking for one floor. From this you can estimate how much ZINC was on each floor from the decking alone. Then you need to add in the contribution from the duct work. At least 3 tonnes of zinc per floor looks reasonable. But I digress...
 
3bodyproblem said:
In going back to your summation notation, which you have defended rather vehemently, i would like to point out that if indeed there were 12 grades of steel ranging 4-1/4 inch thickness, the effect of smoothing this over the entire length of the building greatly reduces the overall mass. Correct?
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Sorry 'bout that.
Here it is, so maybe he'll answer it sometiime this millineum
 
Somehow i don't think so...but if you can't see what he's doing then whatever. I've defended papers before, i know the game...Right Greg?
 
Correction again

R.Mackey said:
OK, so in other words, you're claiming that the ultimate strength of the floor structures is about 2.8 times the design load of 100 psf. I'm fine with that. As you point out, NIST NCSTAR1-1A discusses the actual destructive test criteria, and the report also includes deflection testing of every single type of floor truss.

But you're also claiming that the actual load in the structure -- which includes live loads and all superimposed dead loads -- comes to a total of 27 psf. I'm still waiting to see you support this. Your "common sense" office calculations just aren't rigorous enough, nor supported by actual evidence, just your assertions on how much an office weighs and your insistence that you've handled everything.

Why would the WTC designers plan for a structure that could handle 100 psf (actually, the reduced live loads alone, just for floors, vary from 55 to 82.5 psf on non-mechanical floors, as shown in NIST NCSTAR1-2A Figure 6-1 on page 136; superimposed dead loads add another 12.5 psf for area outside the core, same page), when in reality the structure would only face a quarter of that?

Are you suggesting the design engineers radically overestimated occupancy? Grossly misjudged standard office loads? Were forced to include yet another factor of four overstrength by the Man? Where did this error come from? It's too fantastic to believe.
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I'm claiming that live load (25 psf) plus superimposed dead load (8 psf) is:

33 psf outside the core, which I correct you on a second time.

Why design for 100 psf? This takes care of areas where heavier things will sit. Even if the average is 25 psf, the floor still has to take three people standing together at the coffee machine or a 300 lb filing cabinet where ever you want to put it.

I know it's not rigorous. I'm not saying I handled everything. I have 100 tons extra for anything else. You could even park 80 cars in there. What else can you think of?
 
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Apollo20 said:
Mackey:

Thank you!

I accept your apology.... and NO, it's not my work! But it looks to be a very detailed calculation. For example it gives an estimate of the mass of decking for one floor. From this you can estimate how much ZINC was on each floor from the decking alone. Then you need to add in the contribution from the duct work. At least 3 tonnes of zinc per floor looks reasonable. But I digress...
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Yes, in that case, you do indeed deserve my profound apology. I was careless.

Their steel calculations are quite thorough. It appears, however, that they conclude the 500,000 short ton estimate for the total tower weight is accurate assuming the Towers were loaded to full live load. Thus even if we assume a service live load of only 25% design live load (which feels pessimistic), we lose only 1000 short tons per floor, suggesting the 500,000 short ton estimate is high by perhaps 110,000 short tons. This would give us an answer in the 390,000 short ton ballpark.

I've said before, I don't know whether the true weight was closer to 400,000 or 500,000 tons, but I'm still having a lot of trouble swallowing 250,000 tons.
 
3bodyproblem said:
Somehow i don't think so...but if you can't see what he's doing then whatever. I've defended papers before, i know the game...Right Greg?
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Rigorous defense, or the old "Give 'em some minor error to quibble over so they won't notice the glaring inconsistencies" defense?
Or the "if you cant baffle 'em with brains, bewilder them with bull ----" defense?
Have used all of them at one time or another, depending on circumstances--and audience:D
 
GregoryUrich said:
I'm claiming that live load (25 psf) plus superimposed dead load (8 psf) is:

33 psf outside the core, which I correct you on a second time.
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Your correction is incorrect. I already explained the source of that number, which is an average between the floor outside the core and the floor inside the core. It is, simply, the total of all your supposed non-structural loads divided by the total floor area. This is necessary until you come up with a more accurate model of the core.
 
No

3bodyproblem said:
In going back to your summation notation, which you have defended rather vehemently, i would like to point out that if indeed there were 12 grades of steel ranging 4-1/4 inch thickness, the effect of smoothing this over the entire length of the building greatly reduces the overall mass. Correct?
Click to expand...

This only effects the PE not the total mass. I still have 100,000 tons of steel.

One problem related to this is that the floor joists shouldn't be smoothed because they are the same on every floor. This introduces a maximum error for PE of around +5% and -5% at the top and bottom of the building respectively. I will correct for this in my next version.
 
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