9/11 Physics from Non-Experts

[rwguinn]

rwguinn:

I left M, the mass of a WTC Tower, as a variable since this has become the topic of this thread. You can substitute the high or low value of M and see what makes the most sense, 298 kg/m^3 or 151 kg/m^3.

Interestingly the WTC was claimed to involve lightweight construction and yet the John Hancock Center appears to be the most lightweight of the exampes I qoted, unless you support Gregory Urich's mass....

And by the way, on the point Mackey raised about whether or not the mass of the basement should be included in calculations of the mass of a tower, my guess would be "yes" since the lower six sub-floors that made up the basement appear to have largely collapsed and thus contributed to the potential energy release.

eas. The way this "debate" is going, the value of M has become somewhat like "The number of radii that can be drawn in a sphere is 2*n--2 because a radius is demonstrably 1/2 a diameter, and n being the smallest number that boggles the mind" (E.F. Russell, SF author)
Like I said--I am a structual analyst. I'll leave the design load and criteria to those who know it, and I have yet to be convinced that Gregory Urich is among them...

 

[R.Mackey]

I should also point out that the mass of the other buildings, i.e. Sears and John Hancock, may also involve some creative bookkeeping. Do those figures assume any occupants, and do they include the sublevels and foundation?

Very difficult to compare fairly until we know all the assumptions.

 

[beachnut]

rwguinn:

I left M, the mass of a WTC Tower, as a variable since this has become the topic of this thread. You can substitute the high or low value of M and see what makes the most sense, 298 kg/m^3 or 151 kg/m^3.

Interestingly the WTC was claimed to involve lightweight construction and yet the John Hancock Center appears to be the most lightweight of the exampes I qoted, unless you support Gregory Urich's mass....

And by the way, on the point Mackey raised about whether or not the mass of the basement should be included in calculations of the mass of a tower, my guess would be "yes" since the lower six sub-floors that made up the basement appear to have largely collapsed and thus contributed to the potential energy release.

I think all the weights could be suspect. How do you know John Hancock is really the weight the establishment tell you? Our new truth engineer needs to do a check on all of them.

Some one has all ready told the newest truth engineer, he can take away as much mass as you want and it only contributes to global collapse. Like a little boy quibbling over who has the fastest sled, the answer is still me. We can swap sleds, I will still beat you.

 

[Myriad]

I believe one of the things NIST actually did was calculate the fuel load per square foot in the towers. They didn't include things like paper inside cabinets, as they determined these would not catch on fire. However, given the temperature of the air on engulfed floors, I think it's possible the paper would have reached spontaneous combustion levels.

Yes, and that makes sense, for the short-term pre-collapse fire analysis (collision to collapse) that was NIST's concern. It's not so much that the paper wouldn't reach ignitiion temperature, as that as long as other fuels were available in the open, only an insignificant fraction of the available oxygen would get inside the cabinets to support combustion there. (In the longer term, which didn't get a chance to happen, under those conditions the paper would be pyrolized, cooking into carbon.)

But for the post-collapse fires, that additional cellulose has to be included in the available fuel load. No file cabinets survived intact. I imagine that most of the paper was more or less squeezed out of them, turning into something like paper pulp (except dry) in the process.

Respectfully,
Myriad

 

[Furcifer]

Some one has all ready told the newest truth engineer, he can take away as much mass as you want and it only contributes to global collapse.

Spot on beachnut. I have argued to exhaustion the poor representation of the distribution of structural steel as a linear function. However, this seems to have been supported by Mackey, Newton and Urich. If this is true the core plate thickness should be 21mm at the point of impact.(this is based on FEMA's report that the steel in the core ranged from 3/4" to 4", and not Mackey,Newton and Urich's 1/4" to 4", the result of which would lower the plate thickness at the point of impact) This is less than 1/3 of the plate thickness assumed by T. Wierzbicki of 67mm. If you plug this back in Greening's paper, you end up with an "impact energy to collapse one floor" of 3.8ee8 J There is a slight margin of error because I have retained the surface area reported by Greening for the exterior columns, and rounded. I should also note there is a slight miscalculation in the Greening paper (he shaved 2mm off the plate thickness from 67mm to 65mm) Again I should note there is a huge discrepancy from what has been assumed here in the exterior column plate thickness. T. Wierzbicki has stated that the plate thickness was 12.5mm-7.5mm, Newton, Mackey and Urich hold that the plate thickness was 101.6mm-6.35mm. The thing is, this does little to the plate thickness at the point of impact, both result in a thickness close to 9.5mm.

 

[Newtons Bit]

I never said exactly what the plate thickness was. I said it did vary. If you're looking for what the shapes actually were, I believe the core wide-flanges were W14. And I just happen to have a copy of the 6th ed AISC Manual of Steel Construction (1960's) available which lists those dimensions.

Step into history boys!

Okay, I can't just add the image, however I can link to them!

http://bp1.blogger.com/_-e0bzNzFdXc/Rh7DWlUbSHI/AAAAAAAAAC8/2YMmNjMCP8g/s1600-h/w14p1.jpg

http://bp2.blogger.com/_-e0bzNzFdXc/Rh7DX1UbSII/AAAAAAAAADE/6PE_V_-71yQ/s1600-h/w14p2.jpg

There were lighter W14 shapes back in the day as well, and you can see the smallest one there has a web of 0.418 inches. They go alot smaller.

 

[Furcifer]

I never said exactly what the plate thickness was. I said it did vary.

No, no one did really, but we agreed on them varying from 4-1/4" as Urich has stated. Thanks for the Web limits.

 

[gumboot]

Yes, and that makes sense, for the short-term pre-collapse fire analysis (collision to collapse) that was NIST's concern. It's not so much that the paper wouldn't reach ignitiion temperature, as that as long as other fuels were available in the open, only an insignificant fraction of the available oxygen would get inside the cabinets to support combustion there. (In the longer term, which didn't get a chance to happen, under those conditions the paper would be pyrolized, cooking into carbon.)

But for the post-collapse fires, that additional cellulose has to be included in the available fuel load. No file cabinets survived intact. I imagine that most of the paper was more or less squeezed out of them, turning into something like paper pulp (except dry) in the process.

Respectfully,
Myriad

Ah right, I see what you're saying.

-Gumboot

 

[GregoryUrich]

Spot on beachnut. I have argued to exhaustion the poor representation of the distribution of structural steel as a linear function. However, this seems to have been supported by Mackey, Newton and Urich. If this is true the core plate thickness should be 21mm at the point of impact.(this is based on FEMA's report that the steel in the core ranged from 3/4" to 4", and not Mackey,Newton and Urich's 1/4" to 4", the result of which would lower the plate thickness at the point of impact) This is less than 1/3 of the plate thickness assumed by T. Wierzbicki of 67mm. If you plug this back in Greening's paper, you end up with an "impact energy to collapse one floor" of 3.8ee8 J There is a slight margin of error because I have retained the surface area reported by Greening for the exterior columns, and rounded. I should also note there is a slight miscalculation in the Greening paper (he shaved 2mm off the plate thickness from 67mm to 65mm) Again I should note there is a huge discrepancy from what has been assumed here in the exterior column plate thickness. T. Wierzbicki has stated that the plate thickness was 12.5mm-7.5mm, Newton, Mackey and Urich hold that the plate thickness was 101.6mm-6.35mm. The thing is, this does little to the plate thickness at the point of impact, both result in a thickness close to 9.5mm.

The 4" to 1/4" thickness is only for the exterior columns. This is NIST's value so argue with them not with me.

The core had built up "box" shapes at the base with three parallel plates each with a thickness of 7". Don't forget the core at impact levels (floor 94 for example) was wide flanges with a width that appears to be around 24" in the architectural drawings.

If we scale the cross sectional area 3 x 7" x 52" for the base to a wide flange shape with a width of 24" by my scaling factor adjusted for the 94th floor(16:2.2) we get 3.1" (79mm) thickness for the core plates on the 94th floor.

This would indicate that my linear scaling is in favor of a higher PE and higher strength in the impact area. As I pointed out earlier, the scaling should probably be some logarithmic function.

 

[GregoryUrich]

You are acting as if the mass of a tower is a constant quantity; it is not. Mass may be represented as DL + f*LL, where f may be any percentage between 0 and 100% (theoretically). FEMA is basing its PE calculations on DL + 0.25*LL (Congratulations! You have independently validated FEMA's results!), which is typical because PE calculations are typically done in collapse cases, and D + 0.25 LL is the load case used in conjunction with extreme events (hurricane, earthquake, blast) which considers the low probability of an extreme event and can use a low percentage of live load accordingly (assuming that f follows some bell curve distribution centered around some percentage, the 25% may be assumed to fall on the left of the bell curve). (Generally, engineers have taken office loads as 25% sustained, 75% transient (for creep calculations, among others), this is no magic number however. We can argue all day long as to whether it should be 22%, 30%, 35%, etc...) The 500,000 ton figure, which as I have said several times now originated from a statement by Robert Fowler, one of the original engineers doing the "grunt work" tower calculations, is NOT a D + 0.25 LL, it is D + Lr*LL, where Lr is some live load reduction factor (see previous posts). Lr does not apply to all live loads, Lr is NOT less than 0.4. This is what I am talking about comparing apples and oranges; you are comparing two different load cases and arguing one is wrong because it is not something it was never intended to be. This is where you are in over your head as far as comprehension.

This is what I do not get about twoofers. Instead of running their numbers by people who could give them quick, easy answers (Ron Hamburger, Robert Fowler, Leslie Robertson), they talk to each other in an echo chamber of BS, or go onto an internet message board and argue with other people.

(PS I have zero confidence in your basement numbers, which do not affect the PE of the tower at all. Your floor loads, as I have said, for the D+0.25LL case, match up closely with NIST's and your PE matches up with FEMA. Your strawman, however, that 500,000 tons represents a D + 0.25*LL case, is wrong, and I would not be surprised to find that you are substantially underestimating the DL of the basement, as well as the LL.)

OKAY, now what EXACTLY is your criticism of Dr. Bazant's work?

I have never suggested that the oft quoted value is a the same type of estimate as mine. I have said it is not correct with respect to the actual weight of the building (at any point in time may I add). It would require an average load of 100% of design SDL and LL which we all know is very unlikely.

Remember that the 500,000 tons value is given as fact just about everywhere as "the building weighed 500,000 tons" with nothing about how it is calculated. These types of unqualified statements are missleading and furthermore cannot be correct.

Assuming the orange is mine. The assumed apples I picked were from architectural sites.

I will try to get in touch with the engineers. I never assumed they would be willing or have time to discuss this. I will also check with the architectural sites regarding their estimates of similar buildings (i.e. what type of calc).

I am a bit overloaded at work so I won't be able to respond to the Bazant question until the weekend. By the way I've seen a number of versions of Bazant's "Why Did the World Trade Center Collapse?—Simple Analysis". Do you have any "favorite" version so we can talk about the same article.

 

[GregoryUrich]

I should also point out that the mass of the other buildings, i.e. Sears and John Hancock, may also involve some creative bookkeeping. Do those figures assume any occupants, and do they include the sublevels and foundation?

Very difficult to compare fairly until we know all the assumptions.

Why is it that Sears and John Hancock would involve creative book keeping but not the WTC 500,000 ton estimate?

 

[R.Mackey]

I made no such implication. I only want everyone to be very clear about what mass they are including in their estimates. As we've seen, the live load and basement, in particular, can make an enormous difference. I looked for other comparable estimates as well, and I can't say with any certainty how those were computed, either.

The only way to be sure is to get a more detailed list of the basis-of-estimate, perhaps from Skilling et. al. or other engineers who worked on those projects.

If we take your assumptions, e.g. leave out the sublevels and assume a minimal service load, then you should naturally expect a lower value for the WTC Towers and other structures as well. I still have serious concerns over your superimposed dead load estimate, and your assumptions (or anyone's) are open for some debate, but it would go a long way towards explaining the discrepancies.

 

[Augustine]

I have never suggested that the oft quoted value is a the same type of estimate as mine.

Then why are you comparing your answer to it?

I have said it is not correct with respect to the actual weight of the building (at any point in time may I add). It would require an average load of 100% of design SDL and LL which we all know is very unlikely.

That is because it represents D + Lr*LL. That should be a value far on the right side of the bell curve. That is never intended to be "actual weight of the building" - it is the design load on the foundation which we can be 99% sure will not be exceeded. It is quite accurately described as the design gravity loads at the base of the tower.

Remember that the 500,000 tons value is given as fact just about everywhere as "the building weighed 500,000 tons" with nothing about how it is calculated. These types of unqualified statements are misleading and furthermore cannot be correct.

Where? Wikipedia? PBS? Perhaps this should be a lesson to twoofers to stop doing their research at websites; there is a world of technical publications, journals, presentations, books, conferences, etc. that offer much more information.

Also, I have been misquoted at times, or had a story published in a newspaper that is not quite technically accurate (usually because the reporter is not an engineer). No editor in his right mind is going to stop a story to point out this is dead load, this is live load, this is live load reduction factor, etc etc. Robert Fowler was presenting his comments to an audience of MIT engineers, and he presented the gravity load magnitudes along with the wind base shear magnitudes. An audience of engineers does not need to be spoonfed that these numbers are not everyday values, but instead represent design maxima. The reporter put weight in parentheses after gravity loads.

I will try to get in touch with the engineers. I never assumed they would be willing or have time to discuss this. I will also check with the architectural sites regarding their estimates of similar buildings (i.e. what type of calc).

I do not know whether the engineers will be willing to discuss this with you. That depends on you. If you are truly interested in “truth”, in learning and in educating yourself, I am confident they will be happy to answer your questions. If however, like many twoofers, you are only interested in arguing or stubbornly refusing to listen to people more experienced and knowledgeable than yourself, then I would not expect them to humor you for long. There is nothing more mind boggling than some arrogant twoofer out of his depth arguing with an expert in his field.

I hope you note how few engineers there are in the “twoof” movement. Quick questions about the “twoof” movement:
How many structural engineers are there?
How many structural engineers who design office buildings?
How many structural engineers who have designed buildings over four stories?
How many structural engineers who have designed buildings over twenty stories?
How many structural engineers who have designed skyscrapers?
Why do you think this is?

I personally believe the lack of engineers (of most stripes) within the twoofy movement is attributable to the engineers code of ethics (which most engineers understand and practice). It states (among other things) that engineers will confine their practice to areas of competence, will issue public statements only in an objective or truthful manner, and will not attempt to injure, maliciously or falsely, directly or indirectly, the professional reputation of other engineers.

Critique whichever Bazant article you want. I have the correct versions, and if your critique refers to something not in the article I will let you know. (BTW, I would read your Bazant article a lot closer. His mass value is only 50% higher than yours, not 3 times, and he openly admits he was freeballing it - because he wasn't writing a paper picking nits over the "actual weight", he was demonstrating that the order of magnitude of the dynamic overload was so great that collapse was inevitable REGARDLESS of whether his weight was off by a factor of 2 or even 4...)

 

[Apollo20]

Augustine:

"Collapse was inevitable...."

Would that be with or without x-amount of steel at greater than 400 deg C?

And on the topic of live loads, three classic papers of relevance to the WTC live loads question would be:

Jong-Cherng Peir et al., "Spatial and Temporal Variability of Live Loads"

J. of the Structural Division... May 1973,

R. K. McGuire et al., "Live Load Effects in Office Buildings"

J. of the Structural Division... July 1974,

and,

B. Ellingwood, "Analysis of Live Loads in Office Buildings",

J. of the Structural Division... Aug 1977.

The numbers quoted in these papers are quite revealing...

 

[GregoryUrich]

I made no such implication. I only want everyone to be very clear about what mass they are including in their estimates. As we've seen, the live load and basement, in particular, can make an enormous difference. I looked for other comparable estimates as well, and I can't say with any certainty how those were computed, either.

The only way to be sure is to get a more detailed list of the basis-of-estimate, perhaps from Skilling et. al. or other engineers who worked on those projects.

If we take your assumptions, e.g. leave out the sublevels and assume a minimal service load, then you should naturally expect a lower value for the WTC Towers and other structures as well. I still have serious concerns over your superimposed dead load estimate, and your assumptions (or anyone's) are open for some debate, but it would go a long way towards explaining the discrepancies.

In my calculation, sub-levels are included in the mass with an average SDL+LL = 133 psf. Minimal service load is zero.

 

[Gravy]

I never said exactly what the plate thickness was. I said it did vary. If you're looking for what the shapes actually were, I believe the core wide-flanges were W14. And I just happen to have a copy of the 6th ed AISC Manual of Steel Construction (1960's) available which lists those dimensions.

Step into history boys!

Okay, I can't just add the image, however I can link to them!

http://bp1.blogger.com/_-e0bzNzFdXc/Rh7DWlUbSHI/AAAAAAAAAC8/2YMmNjMCP8g/s1600-h/w14p1.jpg

http://bp2.blogger.com/_-e0bzNzFdXc/Rh7DX1UbSII/AAAAAAAAADE/6PE_V_-71yQ/s1600-h/w14p2.jpg

There were lighter W14 shapes back in the day as well, and you can see the smallest one there has a web of 0.418 inches. They go alot smaller.

Sorry if this has been mentioned before, but NIST does give core column dimensions for floors where the columns transitioned from box to WF (web thickness not included though). That may help Gregory somewhat.

​

 

[GregoryUrich]

Augustine wrote:

Where? Wikipedia? PBS? Perhaps this should be a lesson to twoofers to stop doing their research at websites; there is a world of technical publications, journals, presentations, books, conferences, etc. that offer much more information.

Also, I have been misquoted at times, or had a story published in a newspaper that is not quite technically accurate (usually because the reporter is not an engineer). No editor in his right mind is going to stop a story to point out this is dead load, this is live load, this is live load reduction factor, etc etc. Robert Fowler was presenting his comments to an audience of MIT engineers, and he presented the gravity load magnitudes along with the wind base shear magnitudes. An audience of engineers does not need to be spoonfed that these numbers are not everyday values, but instead represent design maxima. The reporter put weight in parentheses after gravity loads.

Quoting Ashley:

"He calculated that the approximate maximum wind shear force that a single face needed to withstand to be somewhere around 11,000,000 pounds. The gravity loads (weight) produced by the towers at their bases were on the order of 500,000 tons, Fowler said."

Nothing here about design, you are inferring that from the previous sentence. In my article I have compared my calculated mass to that reported in the several common sources as the actual weight. Are we in agreement that the actual weight was not 500,000 tons?

You can stop hassling me about being your idea of a twoofer. If I am, your berating me won't change anything. Since, I'm not it's just very annoying.

 

[GregoryUrich]

Augustine:

"Collapse was inevitable...."

Would that be with or without x-amount of steel at greater than 400 deg C?

And on the topic of live loads, three classic papers of relevance to the WTC live loads question would be:

Jong-Cherng Peir et al., "Spatial and Temporal Variability of Live Loads"

J. of the Structural Division... May 1973,

R. K. McGuire et al., "Live Load Effects in Office Buildings"

J. of the Structural Division... July 1974,

and,

B. Ellingwood, "Analysis of Live Loads in Office Buildings",

J. of the Structural Division... Aug 1977.

The numbers quoted in these papers are quite revealing...

Very interesting. Do you know where I can find them?

 

[Apollo20]

GregoryUrich:

I doubt that any of these papers are on the internet. I found them in the Science and Engineering Library at my local university.

 

[T.A.M.]

find them using Google Scholar, perhaps??

TAM