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9/11 Physics from Non-Experts

Interesting how this thread has evolved from physics from non-experts ... to physics (well, engineering) from experts. :D

And the experts all agree, apparently, on the overall collapse scenario, and are discussing what are, to this layperson, fine details.
:cool:
 
grmcdorman said:
Interesting how this thread has evolved from physics from non-experts ... to physics (well, engineering) from experts. :D

And the experts all agree, apparently, on the overall collapse scenario, and are discussing what are, to this layperson, fine details.
:cool:
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Da%n, guys--
we've been discovered!:D
 
3bodyproblem said:
Hey shagster, welcome. I was curious about your thoughts on the communition of Zone C to Zone B are for WTC 1 and 2 respectively. (not just the concrete, the whole upper mass). Second, what effect does the free standing core have on the amount of mass "shed", and hence the collapse duration, in your model? As I think you are aware, it is still my contention that the lateral forces on the exterior by Zone B preclude it (the exterior) from contributing to the available energy of Zones B and C. These two combined would certainly represent much more than 30% of the mass, while still allowing it to remain relatively close to the perimeter.
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I will try to comment on that later. I've been looking at the SAP2000 model lately and have been busy with other projects. I don't mean to ignore anyone's posts.

I'm getting total tower mass values in the range of 335E6 to 353E6 kg based on data in the SAP2000 model of the tower.

From what I've read in NCSTAR 1-2a, NIST used the self-weight feature in SAP2000 to automatically add up all the weight of the core and perimeter columns, spandrels, and the hat truss. They added in other loads such as non-column CDL, SDL, and LL by tacking them to columns as joint loads. By adding up these two classes of weights (self-weight and joint loads), the total tower weight can be determined from the SAP2000 model. Those weights can be extracted from the SAP2000 model without actually running an analysis.

I have to go through this more carefully to see if I'm misunderstanding anything.

I will try to run an analysis in SAP2000 and see what the loads were on all the columns at the base of the tower. This should also agree with a total mass of about 335 to 353E6 kg. I already ran one of the wind loading conditions sucessfully in SAP2000 and saw how the tower bends in the wind.

The NIST SAP2000 global model is detailed. There are tens of thousands of elements. The joint load table alone has about 269,000 rows of data.
 
A belated welcome to you, too, shagster.

What you have above looks correct. Question: Does this mass include the basement? The foundation? From representations of the model in NIST NCSTAR1-2 it appears that the SAP2000 model does include the structure in the basement, but does not include the foundation, treating it as a fixed boundary condition. But it's hard to say without looking at the model itself...
 
I've run the SAP2000 model. The model that shows up in SAP2000 doesn't show the foundation. It shows a fixed boundary condition at the very bottom of the tower. It includes the below-grade stories (sub-levels).
 
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I added up all of the joint loads in the data table for WTC2. These are the loads that are not self-weights that NIST added in as joint loads. They are labeled in the data table as CDL, SDL, and LL and appear in the F3 column. For WTC2, the total came up to 281E6 kg. The total tower weight has to be more than this since this value doesn't include the weight of the core and perimeter columns and the hat truss components, which NIST decided would be taken care of by the SAP2000 self-weight feature. This feature automatically adds up the weight of all these components. The self-weight plus the joint loads should give the total load from what I understand.
 
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Here is a summary of the gravity joint loads, self-weights, and their sums. Someone else familiar with SAP2000 should try this. I will try to run an analysis with no wind loading and measure the total of the loads on all the columns at the base of the towers. It should give the same results as what was derived from the data tables.

WTC1 tower mass:

WTC1 joint loads: 279.2E6 kg
WTC1 self-weight: 72.1E6 kg
WTC1 total: 351.3E6 kg

WTC2 tower mass:

WTC2 joint loads: 280.6E6 kg
WTC2 self-weight: 65.8E6 kg
WTC2 total: 280.6+65.8 = 346.4E6 kg
 
GregoryUrich said:
The correct mass and proper sequence of "crush up" first makes a huge difference in the timings. If Bazant is wrong about the amount of ejected debris this also makes a significant difference. I still think 20% is very low. I think we will find that the gravity driven collapse time will end up being around 20 seconds.
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Based on the height of the rubble pile within the tower footprint and assuming that the stories were squashed down to about 10 to 15% of their original height, a total mass shedding of 20% of the tower mass isn't very low. 20% to 30% is reasonable. When the shedding is higher than 30%, it becomes difficult to explain how the top of the pile could be near the bowtie level, unless the debris in the pit is considered to be loosely compacted.

The collapse duration is relatively insensitive to mass shedding. I've written a Greening type of model that accounts for a constant amount of mass shedding per story impact during crush-down. A total shedding of 30% of the tower mass adds about one second to the collapse duration compared with no shedding, so the increase in collapse duration isn't huge. 30% doesn't put the predicted collapse duration beyond the observed duration.
 
In his zest to try to obtain a 20-second collapse duration and insinuate an explosives conspiracy, Hoffman has stated that a mass shedding per impact of 'only 6%' of the upper block mass is needed. Consider how much shedding that is for the above-grade stories. For WTC1, 6% of the 14 stories of the upper block is 0.84 of a story mass. For 110 impacts, the total shedding would be 110*0.84 or 92 stories. That leaves only 18 stories for the pit. That's not realistic considering the pile height in the pit was up to the bowtie level and the way the materials were squashed in the pit to the point of looking like a meteorite.

Hoffman's figure of 'only 6%' is even more problematic for WTC2. 6% of 29 stories in the upper block is 1.74 stories. That means 1.74 stories would be shed each impact. For 110 impacts, the total shed mass would be 1.74*110 = 191 stories, which is more stories than the tower had. All of the mass of an impacted story would need to be shed plus an additional 0.74 stories. That 0.74 story worth of mass would have to come from the next intact story of the lower section that hasn't yet been impacted or it would have to be shed from the upper block somehow.
 
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I'm getting about the same values in my model as Hoffman's java model for the increase in collapse duration as a result of shedding. However, I don't agree with his claim about 6% of the upper block mass shed per impact since it doesn't agree with the height of the rubble pile.

A more realistic total shedding of 30% of the tower mass amounts to 30% of a story mass shed per impact. In terms of upper block mass for WTC1, 30% of a story mass is 0.3/14 = 0.02 or 2% of the upper block mass shed per impact. For WTC2 it would be 1% of the upper block mass shed per impact.
 
shagster said:
I'm getting about the same values in my model as Hoffman's java model for the increase in collapse duration as a result of shedding. However, I don't agree with his claim about 6% of the upper block mass shed per impact since it doesn't agree with the height of the rubble pile.

A more realistic total shedding of 30% of the tower mass amounts to 30% of a story mass shed per impact. In terms of upper block mass for WTC1, 30% of a story mass is 0.3/14 = 0.02 or 2% of the upper block mass shed per impact. For WTC2 it would be 1% of the upper block mass shed per impact.
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In the videos, at least for wtc1 it appears that ramps up during the first part of the collapse and that there is a huge increase in sheading around the mechanical floors (79-81). After that is is really hard to see what is happening.

Are you using "crush up" first? (I.e. the top part collapses from the bottom up onto the bottom part and then the bottom part collapses top down.)
 
GregoryUrich said:
In the videos, at least for wtc1 it appears that ramps up during the first part of the collapse and that there is a huge increase in sheading around the mechanical floors (79-81). After that is is really hard to see what is happening.

Are you using "crush up" first? (I.e. the top part collapses from the bottom up onto the bottom part and then the bottom part collapses top down.)
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I'm using crush-down of the lower section followed by crush-up of the upper block.

The overall mass shedding percentage needs to agree with the height of the rubble pile within the tower footprint. There's no other way around that. Claims of very high shedding don't agree with the observed height of the rubble pile.
 
Sole crush-up at the start of collapse wouldn't be able to continue on its own for more than a few impacts without crush-down. A pile of debris would build up at the stationary front at the damaged aircraft impact region. The floors between the core and perimeter could support only a couple stories worth of static mass. They needed to be reinforced to hold the batteries for the UPS system at Fuji and a safe. They wouldn't be able to support a growing pile of mass at a stationary front and would collapse after a couple impacts, which would start crush-down.
 
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Take a look at Gordon Ross's presentation regarding crush down vs crush up. It is obvious that crush up starts before crush down and that crush up finishes early in the collapse.

Regarding mass sheading, are you referring to the height of the rubble pile within the foot print?
 
If you draw the lines in the wrong places, then it can give the appearance that there was more crush-up at the start of collapse than what actually occurred. For example, drawing a line about five stories below the point where the collapse actually initiated and claiming that's where the collapse started.

Crush-up of more than a couple stories without crush-down can't occur for a structure like the towers had. The top-most floor of the lower section will fail when a couple stories worth of mass accumulates there. Drawing lines in the wrong places doesn't change that.

Videos show that some sections of perimeter wall lingered for up to 20 stories after the collapse front had passed. That can give the impression that crush-down wasn't occurring when it was.

Regarding shedding, I'm referring to the height of the rubble pile within the tower footprint at the end of collapse. I define shedding as mass that went outside the tower footprint during collapse.

There may have been some additional 'shedding' when the collapse front finally hit the ground and if some of the mass of the pile that was within the tower footprint got rammed outside the footprint near the end of the collapse at or near the ground level. That would reduce the rubble pile height and give the impression that more shedding had occurred story-by-story throughout the collapse than what actually occurred.
 
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Shagster: Just out of curiousity, based on what you have defined as "shedding" do you agree or disagree with the assertion that virtually all of the exterior mass was shed during the collapse? I am under the assumption that the lateral forces on the exterior would exclude it from contributing to the collapse front, but this does not appear to be supported but the most recent papers I have read.
 
From the aerial photos there were many exterior panels outside the footprint. It appeared that most of the panels went outside the footprint but I don't have any way of quantifying that.

I wouldn't expect the shedding of the exterior panels alone to have a large effect on collapse rate, especially in the upper half of the tower where the column wall thickness was thin.

The exterior columns were on the order of 0.25 inch thick in the aircraft impact regions and higher. I calculate the mass of the exterior columns and spandrels as 8.8E4 kg per story near the 96th story. From NIST 1-6d, the mass of an office story was about 1.6E6 kg. The average mass per story in the part of the tower above the aircraft impact region was higher than this due to the extra mass of the mechanical floors and hat truss (in the range of 2.1 to 2.8E6 kg per story depending on how many stories are averaged for the region above the aircraft impact region, anywhere from 14 to 29 stories).

Ratio of mass of exterior panels in a story to the total mass of an office story near the 96th story:

8.8E4 kg / 1.6E6 kg = 0.055 = 5.5 %

The exterior panels were a small fraction of a story mass in the upper region of the towers.

A Greening type model modified to include the effects of mass shedding shows that about 30% of a story mass needs to be shed to increase the duration of the collapse by about a second. Shedding all the perimeter columns in the aircraft impact regions would be about 5.5% of a story mass, which in itself would increase the collapse duration by much less than a second.

The columns in lower regions were thicker and were a higher percentage of a story mass. Then again, the collapse front had picked up much KE by the time it reached the lower regions of the towers. So the effect of a larger percentage of exterior panel mass being shed later in the collapse would be expected to not have as large an effect on slowing the collapse front as when the front was moving much more slowly and with less accumulated mass.
 
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shagster said:
From the aerial photos there were many exterior panels outside the footprint. It appeared that most of the panels went outside the footprint but I don't have any way of quantifying that.

I wouldn't expect the shedding of the exterior panels alone to have a large effect on collapse rate, especially in the upper half of the tower where the column wall thickness was thin.

The exterior columns were on the order of 0.25 inch thick in the aircraft impact regions and higher. I calculate the mass of the exterior columns and spandrels as 8.8E4 kg per story near the 96th story. From NIST 1-6d, the mass of an office story was about 1.6E6 kg. The average mass per story in the part of the tower above the aircraft impact region was higher than this due to the extra mass of the mechanical floors and hat truss (in the range of 2.1 to 2.8E6 kg per story depending on how many stories are averaged for the region above the aircraft impact region, anywhere from 14 to 29 stories).

Ratio of mass of exterior panels in a story to the total mass of an office story near the 96th story:

8.8E4 kg / 1.6E6 kg = 0.055 = 5.5 %

The exterior panels were a small fraction of a story mass in the upper region of the towers.

A Greening type model modified to include the effects of mass shedding shows that about 30% of a story mass needs to be shed to increase the duration of the collapse by about a second. Shedding all the perimeter columns in the aircraft impact regions would be about 5.5% of a story mass, which in itself would increase the collapse duration by much less than a second.

The columns in lower regions were thicker and were a higher percentage of a story mass. Then again, the collapse front had picked up much KE by the time it reached the lower regions of the towers, so the effect of a larger percentage of exterior panel mass being shed later in the collapse would be expected to not have as large an effect on slowing the collapse front as when the front was moving much more slowly and with less accumulated mass.
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For me the duration of the collapse appears insignificant in the overall model. There would appear to be at least a 3 second variation in the overall collapse time when you consider both the video evidence and the seismic. As you have noted the mass distribution does little to put this outside the realm of possibility, a crushing blow to the CT controlled demolition theory once again (pun intended). I just feel it represents a serious distraction from validity of the new Bazant et. al. paper, rainsing concerns with the calculation of mass in the crushing front. When I combine this with the video evidence of the free standing core I see an opportunity for the CT rebuttal and subsequent dismissal of the global collapse theory as per Bazant. Just an observation.
 
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