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9/11 Physics from Non-Experts

GregoryUrich said:
I was suggesting to 3Body that 95% debris ejection as proposed by Blanchard seems like too much and wouldn't fit his scenario.

Extreme force was overkill actually. The only force acting on the core at that point was gravity.

I said it looks like...

So sue me.
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If you add "induced" after "was gravity", I"ll buy that. Collision forces were gravity induced (falling in a 1 g field produces velocity), but are the result of dv/dt, and are additive to the existing 1 g-loading on the remaining structure.

:DI do happen to know there is a lawyer on these forums:D

You have to be very careful with "Looks Like". It's a term we engineers always use with calculations and hypothesizing trends-- but seldom, if ever, as a serious hypothesis without a large amount od numbers and analysis to back us up.
Appearances are often deceptive, and actuality will trump "looks like" every time
 
GregoryUrich said:
I'm not sure I agree. The combined mass of most of the floors (would include the live loads) and the upper section would be around 50% of the total mass. Another problem is that if all that mass precedes the core collapse, that there would be very little mass to drive the core collapse. The core by itself would be one of the strongest buildings ever built with the exception of resistance to lateral forces, of which we have none.

I don't say I know the collapse sequence but your explanation seems improbable.

I've watched videos with the "spire" from wtc1 a number of times. It appears to me that it is some small portion of the core (e.g. 5-9 columns). I have seen a fairly clear photo of it.

Some interesting photos of wtc2 show what appears to be the entire bottom half of the core standing (I'm not sure how long it stood). What I can't figure out is that, if the intact part of the core was standing for a period of time, why it fell later with no extreme forces acting on it.
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I'm not sure if the core would have been able to stand freely without the truss connections to the exterior columns. Everything I have read suggests that the WTC was highly dependent on this for its rigidity. For WTC2 it appears from all the video that a 75 story tall core section remained standing after the collapse wave, but did fall before the collapse reached the ground. A certain portion of the core would be damaged and taken away by the collapse wave. From my posts at PhysOrg i have the feeling that the core of WTC2 may have been totally intact at thee 41-42nd floor. It then tapered gradually to the 75th floor. But as we have discussed before, the mass of steel in the lower portion was significantly higher closer to the ground.
 
rwguinn said:
Ejection of mass--even 50% mass ejection (which I consider very high) onlyreduces the available energy by 50%. There was much more than 200% energy budget during the collapse.
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This does seem high, however there is no reason to assume any of the exterior steel fell anywhere but outside the footprint. The horizontal forces exerted by the mass contained within the "funnel" of the outer exterior walls being peeled away would not allow this. All the push from the debris would be radially outward from the core. And I believe much of the core steel did not contribute to the collapse wave, as it remained standing after it had passed. Thus the ejected mass may only be as low as 50%, but the core steel that does not contribute to the collapse may be as high as 30%. So far it seems that only 4% of the PE in the collapse was needed to break the exterior truss seats from the columns. This gives me tons of room to play with ejected mass, pulverization and heat.
I'm just trying to put a formula and equation to how i see the collapse mechanism.
 
rwguinn said:
Define "Extreme force"
I suggest you look up "Creep load" , buckling, and column stability if you're having trouble with the core collapsing post-event.
Ejection of mass--even 50% mass ejection (which I consider very high) onlyreduces the available energy by 50%. There was much more than 200% energy budget during the collapse.

And I want to know how it is you can see through all the dust to determine
Define dramatically--and show us how you determined this.
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I've spent the past couple of weeks knee-deep in post-buckling behavior of columns and stability. I seriously doubt he could get into it without losing his mind. Engineering textbooks are like the necronomicon kids, you can lose your SOUL to them! Okay, it's not that bad. But it could be and you'd never know!
 
rwguinn said:
If you add "induced" after "was gravity", I"ll buy that. Collision forces were gravity induced (falling in a 1 g field produces velocity), but are the result of dv/dt, and are additive to the existing 1 g-loading on the remaining structure.

:DI do happen to know there is a lawyer on these forums:D

You have to be very careful with "Looks Like". It's a term we engineers always use with calculations and hypothesizing trends-- but seldom, if ever, as a serious hypothesis without a large amount od numbers and analysis to back us up.
Appearances are often deceptive, and actuality will trump "looks like" every time
Click to expand...

We were talking about the point in time where the collapse had ceased and the core remained standing (if briefly). At that point there are no more collisions, only gravity.
 
3bodyproblem said:
This does seem high, however there is no reason to assume any of the exterior steel fell anywhere but outside the footprint. The horizontal forces exerted by the mass contained within the "funnel" of the outer exterior walls being peeled away would not allow this. All the push from the debris would be radially outward from the core. And I believe much of the core steel did not contribute to the collapse wave, as it remained standing after it had passed. Thus the ejected mass may only be as low as 50%, but the core steel that does not contribute to the collapse may be as high as 30%. So far it seems that only 4% of the PE in the collapse was needed to break the exterior truss seats from the columns. This gives me tons of room to play with ejected mass, pulverization and heat.
I'm just trying to put a formula and equation to how i see the collapse mechanism.
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Don't forget conservation of momentum.
 
GregoryUrich said:
We were talking about the point in time where the collapse had ceased and the core remained standing (if briefly). At that point there are no more collisions, only gravity.
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Sorry-lost track ofthe point.You are correct at that point in time.
However, refer to Newtons Bit's posts on buckling for answers here.
And watch for the health of your soul. Textbooks can be eeevil!

ETA:
You cannot assume that the core was vertical and symetrically loaded. If any of the remaining vertical columns were missing, and the structure tied together by horizontals (It is difficult to determine from the few stills I have sen) the horizontals would be cantalevered off of the verticals, and the bending moments induced would be enormous. Welds and riveted joints would be failing very quickly and catastrophically
 
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rwguinn; said:
ETA:
You cannot assume that the core was vertical and symetrically loaded. If any of the remaining vertical columns were missing, and the structure tied together by horizontals (It is difficult to determine from the few stills I have sen) the horizontals would be cantalevered off of the verticals, and the bending moments induced would be enormous. Welds and riveted joints would be failing very quickly and catastrophically
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I have assumed a substantial deformation to the core based on the upper mass giving it a substantial working over as it passed and the torsional effects of the trusses as they swung away from the exterior. These trauma events and gravity alone should have been more than sufficient to collapse the standing core.
 
3bodyproblem said:
I have assumed a substantial deformation to the core based on the upper mass giving it a substantial working over as it passed and the torsional effects of the trusses as they swung away from the exterior.
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I woud hope so!



3bodyproblem said:
These trauma events and gravity alone should have been more than sufficient to collapse the standing core.
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I have no doubts about that--but the twoofers will make comments about assumptions--we will have to justify them, even to each other.
No justification or logic will reach the twoofers, of course, but we expect that.
 
You can't neglect the wind load here either. Though there wasn't a gale, the wind load on the standing structure would probably be somewhere around 5psf. Which is small, but for something so tall and missing any real lateral stability, is very significant.

There would also likely be earthquake effects from the other collapses. However I can't put a number to that.
 
There's also the question of what the "standing core" would have been standing on after the rest of the structure collapsed. Since if I understand correctly the subterranean floors were traditional space frame structures, it's hard to imagine the basement levels being crushed by debris and filled with debris and yet leaving the vertical core columns intact between street level and foundation floor. The lateral pressure at the bottom of the pile as it formed would be enormous.

Respectfully,
Myriad
 
I'm having trouble figuring out how much energy was needed to sever all the floor trusses from the exterior. The upper limit appears to be the gusset plates. I have found this referenced to be 90kips.(911 research Appendix B) The shear force on the bolts to be 53.9 kips. This is for each standard truss. I believe every other one was doubled? Can I safely assume a uniform distribution of impact mass over the entire floor and just add up the the force needed and average them?
 
Myriad; said:
There's also the question of what the "standing core" would have been standing on after the rest of the structure collapsed. Since if I understand correctly the subterranean floors were traditional space frame structures, it's hard to imagine the basement levels being crushed by debris and filled with debris and yet leaving the vertical core columns intact between street level and foundation floor. The lateral pressure at the bottom of the pile as it formed would be enormous.

Respectfully,
Myriad
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I have been told that aerial photographs show a 4-7 story section of the core remained intact and upright (most likely being held up by the debris pile?) after the collapse. I have not seen these photos so I can't really comment. I am trying to track them down.
(I found the LIDAR digital enhanced photos, perhaps this is where the analysis comes from? http://www.globalsecurity.org/eye/html/wtc_noaa_wtc3.htm)
 
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3bodyproblem said:
I'm having trouble figuring out how much energy was needed to sever all the floor trusses from the exterior. The upper limit appears to be the gusset plates. I have found this referenced to be 90kips.(911 research Appendix B) The shear force on the bolts to be 53.9 kips. This is for each standard truss. I believe every other one was doubled? Can I safely assume a uniform distribution of impact mass over the entire floor and just add up the the force needed and average them?
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Are we still talking "free-standing core" here?
Because if you are talking about "during collapse" issues, that assumption would be conservative--failures in the truss/exterior interface were like localized, resulting in "zipper" type failure, albeit at a (very) rapid rate--but catastrophic failures are hard to judge without instrumentation.
 
rwguinn; said:
Are we still talking "free-standing core" here?
Because if you are talking about "during collapse" issues, that assumption would be conservative--failures in the truss/exterior interface were like localized, resulting in "zipper" type failure, albeit at a (very) rapid rate--but catastrophic failures are hard to judge without instrumentation.
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No, not for the free standing core. I'm thinking the free standing core represents two things: an energy sink, absorbing KE from the collapse wave and a depreciation of mass from the collapse wave KE (as assumed by Greening etc.)

I agree with what you are saying, almost instantaneously the failure would "zipper" from exterior truss connection to the next. I believe this can be seen in close up video footage on the North and East walls of WTC2 within seconds of collapse initiation. Some people have erroneously pointed to this as "explosives" propelling debris at 90 degree angles to the face of the wall. I am most certain that this is indeed the zipper effect. As the floor trusses pulled the exterior columns inwards to the point of failure and suddenly released, the exterior columns would "snap" back. This force would be the same if not greater than the effects of explosives.
As you have pointed out, without instrumentation how can I arrive at a reasonable determination of the energy absorbed from the available KE in shearing/breaking the exterior truss connections? I have looked through NIST for this, which i am sure they must have determined, but you need to be a well trained structural engineer to pull this information out properly.
 
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3bodyproblem said:
I have been told that aerial photographs show a 4-7 story section of the core remained intact and upright (most likely being held up by the debris pile?) after the collapse. I have not seen these photos so I can't really comment. I am trying to track them down.
(I found the LIDAR digital enhanced photos, perhaps this is where the analysis comes from? http://www.globalsecurity.org/eye/html/wtc_noaa_wtc3.htm)
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Interesting. I could be wrong, of course, and I don't have the expertise to evaluate those LIDAR images. But "intact" is sometimes a relative term in such cases, and I wonder if the 4-7 story height you read about is above street level (which would bring it close to surmounting most of the debris pile) or above foundation level. It's not surprising that the lower stories of the core remained upright within the debris pile. I was thinking, rather, of lateral compression and/or displacement. If the base stories of the core structure were compressed laterally to even a relatively small degree, its ability to stably support any freestanding core above it would become doubtful.

Respectfully,
Myriad
 
3bodyproblem said:
No, not for the free standing core. I'm thinking the free standing core represents two things: an energy sink, absorbing KE from the collapse wave and a depreciation of mass from the collapse wave KE (as assumed by Greening etc.)

I agree with what you are saying, almost instantaneously the failure would "zipper" from exterior truss connection to the next. I believe this can be seen in close up video footage on the North and East walls of WTC2 within seconds of collapse initiation. Some people have erroneously pointed to this as "explosives" propelling debris at 90 degree angles to the face of the wall. I am most certain that this is indeed the zipper effect. As the floor trusses pulled the exterior columns inwards to the point of failure and suddenly released, the exterior columns would "snap" back. This force would be the same if not greater than the effects of explosives.
As you have pointed out, without instrumentation how can I arrive at a reasonable determination of the energy absorbed from the available KE in shearing/breaking the exterior truss connections? I have looked through NIST for this, which i am sure they must have determined, but you need to be a well trained structural engineer to pull this information out properly.
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Don't just look at the connection, look at the truss itself just before the connection and at the welds connecting the web members (if you have that data).
 
Kent1; said:
Here's a good one also.
http://aboutfacts.net/LargePictures/Else21/Ground Zero.jpg
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Wow, what a picture. That appears to be a large portion of the corner section of the core, standing at least 7 stories tall. It's hard to discern what the rest of the core's condition was but it appears to be about 3-4 stories high. I also note that there doesn't appear to be a single truss attached to the exterior or core.
 
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