The 9/11 Forum

OneWhiteEye wrote:

A drop height of 0.5m or about 20 inches is ridiculous.

No, it's not. A freefall drop of 12 feet is not conservative towards survival and is not representative of the initial descent. It bounds the solution to one extreme, my suggestion was to bound the problem at the other extreme, Bazant's criteria, as you note later.

You can't use that to determine whether or not a jolt should have been noticeable at the roof in the real situation where the upper section had to drop a full story before an impact would occur.

Not everything is about jolts!

We aren't trying to determine if Bazant's conjecture, that even a 0.5m drop would cause collapse, here.

Speak for yourself. It may have been Enik's purpose to examine for jolts (none), but a lot more comes out of it than that. The FEA indicates jolts is a non-issue, as I already know, so I'm concerned with issues that have some relevance. Bazant, who assumes single story failure, puts the threshold at 0.5m freefall drop. Simply doubling it for comparable destruction above and below is 1.0m. Thus the test points make a whole lot of sense, and are an attempt to advance understanding rather than allow it to keep wallowing in the mire of muddy physics.

A drop of about 9 feet or 3 meters would be realistic given how a 12 foot column would bifurcate when buckling and the simulation should have buckled columns with three hinge points.

No period of free fall in initial descent was observed, so apparently conditions were not as you assumed. Another reason to tone down the drop a little if something realistic is to be tested.

The real test would be to model all 236 perimeter columns 12 stories tall with their 52 inch deep spandrels every story with lateral support every 12 feet in the axis orthogonal to the spandrels and have a buckled column drop on its lower end which is the top of 97 stories of the same 236 columns with the same constraints but increasing wall thickness of both the columns and spandrels.

Of course, but this beats the crap out of anything I've seen done on this aspect of the collapse. One can learn from the results of simplified models before adding complexity beyond that which has any hope of being validated if done in isolation.

I can't help but sense that it sounds as though you and femr2 are hedging your bets as you aren't advocating doing the analysis based on observation. A 0.5 meter drop is not what happened and the north face of the upper section of WTC 1 was moving at about 19 ft./second after 9 feet of fall.

T_Szamboti wrote:I said 9

So ? I said I don't think 12ft is a great idea and that the distance should be calculated based on velocity derived from the actual drop data. If that's 9, fine. If not, then I don't agree, innit.

Additionally, let's see what a finite element analysis of the 12 story upper section with all 236 perimeter columns with lateral restraint every 12 feet shows first, before adding any tilt.

It would be wonderful if Enik extends the model, but I imagine it will have to be in phases, and I see no point whatsoever in deliberately omitting the tilt. We know it was there, so the only reasons I can think of NOT including it are 1) To quantify the effect of tilt, and 2) for you to ignore the run with tilt if was performed and state results from the zero-tilt run only. Sorry, but I suspect (2) as more the likely reasoning.

Although OWE would really like to see no velocity loss at the top, I think he will be disappointed.

Well perhaps we shall see. If the results are not to your liking, will you accept the results ?

I can't help but sense that it sounds as though you and femr2 are hedging your bets as you aren't advocating doing the analysis based on observation.

What ? I'm suggesting using a drop height correlated DIRECTLY with velocity determined from observation. Observation shows tilt, which you are suggesting is not *put in* until later. And observation clearly shows a non-rigid upper block, which your position on jolts is directly in contradiction with.

A 0.5 meter drop is not what happened and the north face of the upper section of WTC 1 was moving at about 19 ft./second after 9 feet of fall.

A 9 ft drop is not what happened either. Whilst we're there, the perimeter columns overlapped too, so there ain't no impact between 'em at all.

Not sure about that 19ft/s. Will have to check it out with the sauret trace data.

Achimspok,

Could you post two graphs using the same frame number synch...one with drop, and one with velocity for the NW corner ?

T_Szamboti wrote:I can't help but sense that it sounds as though you and femr2 are hedging your bets as you aren't advocating doing the analysis based on observation.

No, I advocate doing a full exploration of the nearby solution space for this and all subsequent models of increasing complexity and realism, to get a feel for sensitivity to parametric input. I believe femr2 prefers the same. If it were me, I'd be doing just that, but it's not me. I feel forward in asking Enik to do it again even once, let alone twice, I'm so grateful for what's there. It seems to have a fine mesh and Enik supplies the loads and sample locations, there's plenty to be learned from just that.

If going for a more realistic configuration significantly adds rigidity, and it is reasonable to expect it would, the balance of this currently very lobsided result will tip more towards your perspective, Tony, though how much I've no idea. It's a matter of degrees... speaking of which, I wonder what a half degree of tilt would do to tip the scales back towards my position?

A 0.5 meter drop is not what happened and the north face of the upper section of WTC 1 was moving at about 19 ft./second after 9 feet of fall.

An equivalent free fall drop to achieve that velocity (~5.6 ft) would be fine with me. Then can I ask Enik for 1.0m? It would be a bitch if it snapped through like butter even at 0.5m, wouldn't it?

femr2 wrote:

T_Szamboti wrote:I said 9

So ? I said I don't think 12ft is a great idea and that the distance should be calculated based on velocity derived from the actual drop data. If that's 9, fine. If not, then I don't agree, innit.

Additionally, let's see what a finite element analysis of the 12 story upper section with all 236 perimeter columns with lateral restraint every 12 feet shows first, before adding any tilt.

It would be wonderful if Enik extends the model, but I imagine it will have to be in phases, and I see no point whatsoever in deliberately omitting the tilt. We know it was there, so the only reasons I can think of NOT including it are 1) To quantify the effect of tilt, and 2) for you to ignore the run with tilt if was performed and state results from the zero-tilt run only. Sorry, but I suspect (2) as more the likely reasoning.

Although OWE would really like to see no velocity loss at the top, I think he will be disappointed.

Well perhaps we shall see. If the results are not to your liking, will you accept the results ?

I can't help but sense that it sounds as though you and femr2 are hedging your bets as you aren't advocating doing the analysis based on observation.

What ? I'm suggesting using a drop height correlated DIRECTLY with velocity determined from observation. Observation shows tilt, which you are suggesting is not *put in* until later. And observation clearly shows a non-rigid upper block, which your position on jolts is directly in contradiction with.

A 0.5 meter drop is not what happened and the north face of the upper section of WTC 1 was moving at about 19 ft./second after 9 feet of fall.

A 9 ft drop is not what happened either. Whilst we're there, the perimeter columns overlapped too, so there ain't no impact between 'em at all.

Not sure about that 19ft/s. Will have to check it out with the sauret trace data.

Achimspok,

Could you post two graphs using the same frame number synch...one with drop, and one with velocity for the NW corner ?

The exercise is to see how much attenuation of velocity loss there would be in a real collision of the perimeter columns 12 strories below the roof. Let's see what it is. I say it won't be all that much based on hand calculations and OWE seems to think it will be a lot based on his intuition.

T_Szamboti wrote:The exercise is to see how much attenuation of velocity loss there would be in a real collision of the perimeter columns 12 strories below the roof. Let's see what it is. I say it won't be all that much based on hand calculations and OWE seems to think it will be a lot based on his intuition.

Well, so far, what's shown in FEA is theoretically unobservable in video measurement 2.3 stories from the collision point...

Jolt not visible in FEA at video measurement resolution 2.3 stories above impact


... I don't need anything more than intuition of basic physics to come to the conclusion there won't be any less attenuation another 10 stories up. Make it stronger, more rigid, it will increase the jolt transmitted to the roofline accordingly. I wouldn't bet that it would get out of the noise for video measurement, though, even per-frame data points with subpixel resolution. Maybe, but it sure won't be a great big whack. Then the block can be tilted a half a degree to see what happens - my intuition says it'll drop like a stone in 2/3rds gravity if colliding after a drop of 5.6 feet. I could be wrong!

T_Szamboti wrote:The exercise is to see how much attenuation of velocity loss there would be in a real collision of the perimeter columns 12 strories below the roof.

Real collision ? Can you show me perimeter column impacts in a video ? Looks to me like perimeter sheets overlap, and the corresponding collisions are between perimeter columns and floor assemblies.

Let's see what it is.

No harm in that at all, but let's not kid ourselves that's a scenario that actually happened eh.

T_Szamboti wrote:I say it won't be all that much based on hand calculations and OWE seems to think it will be a lot based on his intuition.

Mighty presumptuous statement. You've ignored what I've done; there's a difference. I only do hand calculations once, in code, then run them hundreds and thousands of times with various parameters to represent the uncertainties in input. It is precisely because of those thousands of iterations from one extreme to the other that I have such an excellent intuition about the problem, not the kind obtained from doing a single lengthy and painstaking calculation based on a host of assumptions, and then some bad physics to boot.

This is an overlay of a case within the nominal band of a rigid top crush down calculation, my calculation, against Enik's point 4:



Looks like my calculations correspond to the FEA results much better than yours. This is from weaker than midband realistic energy dissipation of an axial impact scenario.

Don't say there are no calculations when there are.


Edit: This also exemplifies why a rigid top is a sufficient approximation, if the calculations are done correctly.

Looking back now I think I understand...

Tony: " The elastic compression of the lower stories would be about 20 inches if the entire length of the 97 stories were considered."

Using a 97 story spring is, well... odd? I assumed you were calculating failure energy to be the integral of the load displacement function for a single story, which has a hump of similar length but which is mostly plastic phase. Instead, it appears you're letting 97 stories simultaneously climb the elastic portion of that hump based on impulse at one end. No can do. This is not a problem in statics. When's the last time you saw cars in a head-on collision crush like an accordion, with the back side equally crumpled as the front? There's a reason for that.

Have a look at the Vehicle Crashes thread for a couple of traces showing how impulse applied to one end of a deformable body results in a gradient of stress, therefore compaction.



Front end's already gone before the crash dummy's head starts to snap forward.

OneWhiteEye wrote:Looking back now I think I understand...

Tony: " The elastic compression of the lower stories would be about 20 inches if the entire length of the 97 stories were considered."

Using a 97 story spring is, well... odd? I assumed you were calculating failure energy to be the integral of the load displacement function for a single story, which has a hump of similar length but which is mostly plastic phase. Instead, it appears you're letting 97 stories simultaneously climb the elastic portion of that hump based on impulse at one end. No can do. This is not a problem in statics. When's the last time you saw cars in a head-on collision crush like an accordion, with the back side equally crumpled as the front? There's a reason for that.

Have a look at the Vehicle Crashes thread for a couple of traces showing how impulse applied to one end of a deformable body results in a gradient of stress, therefore compaction.

Front end's already gone before the crash dummy's head starts to snap forward.

We don't use the 20 inches of elastic compression for the lower 97 stories in the Missing Jolt paper. It is only calculated to find a ratio, as we realize that the deformation would go plastic in the first story of the collision well before the 20 inch elastic deflection would be realized. Did you read page 24 of the Missing Jolt paper http://www.journalof911studies.com/volu ... gJolt7.pdf?

I ran the model thinking it was the limiting conservative case, drop of 12 feet (1 floor). I never anticipated doing a buckling type of analysis because I have yet to be able to reproduce the buckling using a deformed floor truss. I can redo the entire wall and repost my results for the roofline but I need to open the mesh up a bit. I am also trying to reduce the mass of the upper wall to get a "bounce" for comparison. Right now the server is full and I have to wait my turn.

If someone can direct me in the best way to post the numerical data (either excel, txt, csv, etc.), I will do that so that you can use the actual data and not have to rely on interpreting the graphs.

CSV is fine (for me), you can either attach it to a post or enclose it in the CODE markup

leftsquarebracket CODE rightsquarebracket
...data....
leftsquarebracket slash CODE rightsquarebracket

I'll give it a shot. The data is arranged by time (sec), velocity (in/sec), time, velocity, ... for the graphs given earlier. A quick graph of the data should allow you to correspond it to the appropriate graph. I used an excel equation of delta v/delta t to get the corresponding acceleration (-384 in/s^2 = 1g).

Code: Select all

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,,,,,,1.33501,-106.312,1.33501,-187.043,1.33501,-473.866,1.33501,-482.471
,,,,,,1.35001,-100.505,1.35001,-198.598,1.35001,-481.924,1.35001,-487.318
,,,,,,1.365,-99.2375,1.365,-213.871,1.365,-489.87,1.365,-492.715
,,,,,,1.38002,-96.1476,1.38002,-227.717,1.38002,-496.251,1.38002,-497.109
,,,,,,1.39501,-91.0335,1.39501,-242.461,1.39501,-502.71,1.39501,-502.777
,,,,,,1.41001,-87.7424,1.41001,-259.136,1.41001,-507.208,1.41001,-507.02
,,,,,,1.425,-89.6405,1.425,-281.991,1.425,-511.857,1.425,-511.381
,,,,,,1.44,-89.093,1.44,-304.735,1.44,-515.813,1.44,-515.288
,,,,,,1.455,-94.341,1.455,-326.3,1.455,-521.854,1.455,-519.745
,,,,,,1.47001,-99.9461,1.47001,-351.856,1.47001,-525.565,1.47001,-526.562
,,,,,,1.48501,-120.606,1.48501,-375.128,1.48501,-529.693,1.48501,-532.345
,,,,,,1.5,-147.756,1.5,-396.7,1.5,-535.619,1.5,-537.389

Thanks. All such work is greatly appreciated.

I did another analysis with the 12 story external wall section.

The first analysis was to drop it from a height of 1 story (12 feet) on to a 3 story external wall section, which was pinned at the base.
Note the points for the velocity data.

Column setup for first analysis

Below are the corresponding velocity/acceleration graphs for Steel to Steel contact.
Velocity & acceleration at Point 1
Velocity & acceleration at Point 1a
Velocity & acceleration at Point 2
Velocity & acceleration at Point 3
Velocity & acceleration at Point 4
Velocity & acceleration at Point 5

The second analysis was to drop a lighter structure from a height of 1 story (12 feet) on to a 3 story external wall section. In this
case we will have a "bounce" and acceleration data generated up the structure.
Note the points for the velocity data.

Column setup for first analysis

Below are the corresponding velocity/acceleration graphs for Lighter Structure to Steel contact.
Velocity & acceleration at Point 1
Velocity & acceleration at Point 2
Velocity & acceleration at Point 3
Velocity & acceleration at Point 4

Below is an image from one of my models and comparing it with my simple 12 story model. The actual model is heavier.
Structure comparison

Video of Steel to Steel drop.
Video of Lighter Steel to Steel drop (bounce).

Could you post two graphs using the same frame number synch...one with drop, and one with velocity for the NW corner ?

Sorry, I'm a little late here. Here it is:

large:http://img19.imageshack.us/img19/6762/dropvsvelocitynwcorner.png