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Forum » WTC Collapse Simulator » Ryan Mackey Hardfire Modeling Challenge » WTCCS Model
WTCCS Model
femr2Date: Friday, 10-04-2009, 07:59:45 | Message # 1
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Hi.

The WTC Collapse Simulator is described in full on the home page of this site.

Please read the home page, the quick start guide, download the energetics spreadsheet, and we can begin...

 
femr2Date: Saturday, 11-04-2009, 22:14:35 | Message # 2
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I have finally received a useful reply to my Hardfire Modeling Challenge! The submitter has put together a simulation rather than a physical model, which is perfectly fine, and one that I believe is close enough to do some useful experiments. I've replied to its creator with a list of questions, most having to do with the assumptions used, but nonetheless this is good enough for some valid experimentation. The model is found here: http://femr2.ucoz.com/

Enjoy.

ETA: The model also includes a detailed spreadsheet with tower mass, by floor, in terms of steel and concrete. So, as you can see, even people in the Truth Movement are capable of finding these answers.


R. Mackey on JREF.

I have yet to hear from Ryan. Whoever contacted him about the simulation, it was not me. I suggest that he contacts me here.
I am quite happy to answer questions.

 
femr2Date: Saturday, 11-04-2009, 22:20:17 | Message # 3
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Tilt actually will have very little effect, unless your model is so high-fidelity that it accurately captures the connection failure modes in the steel. This model isn't that good, nor is any other I've heard of. This is an extraordinarily difficult detail.

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There are numerous assumptions built into the model that favor arrest -- which is fine, provided we declare and understand them. In any event, I've written the author back for clarification. So far he's preferred instead to complain that I work through e-mail rather than post specially on his forum (which I do because I desire a one-on-one discussion free of derails -- just look at what happened in this thread!). Hopefully I'll receive some responses and be able to better understand his setup.

R. Mackey on JREF.

Again, I have not been in contact with Ryan, and whoever he is conversing with, it is not me.
I am quite happy to discuss the simluation in full here.

There are currently a massive 2 members, so I do not think that a one-on-one discussion is a problem.
If discussion derailment from others is a concern I have no issue locking this forum such that only posts from myself and Ryan are allowed.

Hopefully Ryan will decide to approach me directly, here.

 
femr2Date: Sunday, 12-04-2009, 18:19:05 | Message # 4
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I have now been in contact with the person Ryan Mackey thought was me, and am sure that Ryan will now be informed of his unfortunate assumption.

The list of questions posed has been sent over, and I shall repost them here along with appropriate answers.

I am not aware of any conversation other than what I post here, so Ryan, when you do decide to comment here I would appreciate it if you refrain from any 'truther'/'debunker' discourse. I would, of course, be 'branded' with the 'truther' mantle given my very clear and firm viewpoint, but as I am sure you are aware, this simulation is clearly intended to be used as an educational and hypothesis testing medium, rather than any form of 'definitive' 'proof' of either side of the 'argument'.

My intention is to gradually extend the model over time, to incorporate more and more sophisticated mechanisms, including elastic collisions and more specific floor-by-floor parameters. (Such as specific mass loss through identification of ejecta). I will also be including the initial 'top-down' crush process evident for WTC 1 and a cap crush mechanism (although it is clear that in reality the cap itself did not survive the descent in any useful form)

Anyway...

 
femr2Date: Sunday, 12-04-2009, 19:20:31 | Message # 5
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1. Do you consider energy loss due to conservation of momentum and due to destruction of materials independently? It appears that you do. If so, this is wrong. The energy that "disappears" in the inelastic collisions goes primarily into deformation of materials, some ultimately ending up as heat but damaging steel and concrete on the way. Note that this energy is about 20-30% of the total potential energy -- it's huge. If this energy isn't enough to destroy materials, then you should of course take out an additional amount to reach that threshold, but if you consider momentum loss and damage as independent energy sinks, you are double-counting the energy loss due to collisions. That has the effect of artificially slowing your collapse an extra 0.2 g or so all the way down the stack.

C-O-M energy loss is now enabled for use in subsequent deformation of materials.

 
femr2Date: Sunday, 12-04-2009, 19:49:26 | Message # 6
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2. Do you require that all concrete is destroyed uniformly, and during the collapse? If so, this is also wrong. Even Dr. Steven Jones has remarked on this for years, noting that at best only a small fraction of the concrete was actually turned to dust, and even of the dust much of it is quite coarse. You could make a more rigorous mass estimate by looking at the dust study by Lioy et. al. and looking at the mass fraction relative to, say, dust from drywall, assume that most or all of that was turned to dust, and treat the rest of the concrete as uncrushed. Also, what concrete is ground up does not have to be ground up during the collapse. If it hits the ground with some leftover kinetic energy, this energy also contributes to grinding of materials. Furthermore, concrete exposed to fire (i.e. the fire floors, and practically everything in the Pile) will spall, and this will allow grinding with hardly any kinetic energy expenditure. In summary, the amount of concrete turned to dust during the collapse is small.

No. There is no requirement for uniform destruction of all concrete.

The concrete crush energy sink can be switched off entirely, or either of two distinct selectable methods can be applied:

1) A user specifiable constant energy usage (in joules) for the crushing of concrete. Applied per-floor.
or
2) A user specifiable particle scale range (top to bottom) (in microns) applied on a floor-by-floor linear ramped scale, with a current limit of up to 500 micron (although of course this can be easily extended by the user. It's a linear energy usage scale)

The first method does not relate to concrete volume at all, only energy consumed per floor, and so is a fully specifiable constant allowing the user to use a value equivalent to the crushing of zero concrete thru to all concrete, and using their own external methods for determining the amount of energy required to do so.

Mechanically the second method does apply to the full concrete volume of each impacted floor, but again, the scale of the crush can be changed as described above. Many sources state an 'average' concrete crush scale well below the current maximum of 500 micron, which is why the limit was set as such. It could easily be extended to a maximum of say 1m if the user chose to.

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If it hits the ground with some leftover kinetic energy, this energy also contributes to grinding of materials.

In this situation the KE should be removed from the system via mass loss. Applying it's energy to post-descent material destruction is erronious.

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Furthermore, concrete exposed to fire (i.e. the fire floors, and practically everything in the Pile) will spall, and this will allow grinding with hardly any kinetic energy expenditure.

The model does not include energy sink for any concrete in the cap, only for the impacted floors below. Your point is redundant within the model.

I assume that by 'pile' you mean the descending mass I refer to as the 'cap' ? (As in pile-driving mass)

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In summary, the amount of concrete turned to dust during the collapse is small.

This is a much discussed point of contention within 9/11 debates, and does not belong here.

Further extension of the model will include the ability to specify mass loss on a per-floor basis for each possible material.
This extension will allow per-floor specification of the concrete volume crushed and per-floor max/min crush scale.
Additional energy sinks will also be included for the crushing of other materials, which are currently not included.
At this point, only the crush energy for concrete is included (if non zero). Crush energy required for dry wall, furnishings, fittings and a wealth of other required crush 'bodies' will also be included.

 
femr2Date: Sunday, 12-04-2009, 20:12:15 | Message # 7
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3. In your ejected mass fraction, are you considering the ejected mass of the new floor hit, or of the entire debris pile? If the latter, then you are going to overpredict mass ejection in the later stages of collapse. This is because the mass scales superlinearly with floors hit (as floors get heavier as you move down), and the rate at which floors are hit scales roughly quadratically with time. This means your mass ejection fraction scales somewhere around the three-and-a-half to fourth power of time. This isn't right. Admittedly, estimating this is quite difficult. I would suggest that you put in loss as a given fraction per unit time rather than unit distance as a rough estimate. The much more sophisticated way is to assume mass ejection scales with the resistance of lower floors (c.f. equal and opposite), since what is really happening is mass gets ejected because of momentum flow from the lower floors through the debris pile to loose elements near the edges. I haven't thought about it in much depth and this could use some more work.

Mass loss can be omitted entirely, or included as described below.

The ejected mass fraction is specified by two separate values:

1) A percentage of the 'Cap' mass, pre-impact.
and/or
2) A percentage of the impacted floor mass. (Storey)

Both of these values are clearly labelled.

Either or both can be set to a user specified percentage, including zero, or switched off entirely (Equating to zero for both, but allowing any percentages entered to be retained in the sheet).

These factors negate the verbose implications you describe.

The model will be extended in the future to include per-floor specifiable mass loss, per material, enabling much more precise control. I'm intending upon identifying specific ejecta from video footage as part of the exercise. Separation of external column structure mass will also be included within the model to account for the probable quantities of external columns which played no part in further descent, and did not form part of the 'pile driving' mass.

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since what is really happening is mass gets ejected because of momentum flow from the lower floors through the debris pile to loose elements near the edges

This natural behaviour inference is not directly conducive to this discussion, as I am sure you can understand given our differences of opinion on the cause of each descent. I intend on extending the model to include the energy required to laterally accelerate the ejected mass.

I agree that a mechanism relating mass loss to collapse progression is a good idea. I shall have a think about the best way to do so, but first thought would be to relate to impact KE.

 
femr2Date: Sunday, 12-04-2009, 20:33:11 | Message # 8
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4. In computing energy to fail the steel, what mechanism are you assuming? Compressive failure, buckling failure, or fracture? It appears you are using compressive strain since you talk about plastic deformation, but I am not certain. In the actual collapses, plastic deformation is slight, as evinced by the relatively straight condition of nearly all structural elements after collapse. In all previous models -- Greening's and Bazant's -- they overpredict resistance, deliberately, rather than consider lower energy buckling or fracture modes.

The 'energy to fail the steel', termed 'Support Collapse Energy' in the model, can be omitted entirely. or included by two methods.

1) A user specifiable per-floor constant (in joules)
or
2) A per-floor calculated value based on several factors.

The user specifiable value is completely open, allowing any value supplied by the user to be used, for each floor.

The calculated value uses a base figure for the collapse of one floor (the failure floor), which is split between the external columns and the core columns, and then ramped on a scaled multiplier towards the tower base.

The base figure, and each split value can be set by the user.

The ramping formula can be found in the following spreadsheet column: TowerMass:AJ

The ramping formula accounts for the thickening of the steel towards the base, but admittedly it is an estimate (it underestimates the strength increase significantly). If I can get hold of more accurate steel mass and scale source data the model will be updated to use specific steel mass-to-collapse energy conversions.

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In the actual collapses, plastic deformation is slight, as evinced by the relatively straight condition of nearly all structural elements after collapse.

Again, this natural behaviour inference is out of place in this discussion. Modelling should be performed on expected behaviour, not forced implementation of mechanics which are limited by observed behaviours.

Future update of the model will include an implementation of inelastic collision, including much more refined steel deformation mechanics, and will include the appropriate time and energy requirements.

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In all previous models -- Greening's and Bazant's -- they overpredict resistance, deliberately, rather than consider lower energy buckling or fracture modes.

I suggest the inclusion of 'deliberately' is not necessary within this discussion.
 
femr2Date: Sunday, 12-04-2009, 20:48:58 | Message # 9
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5. What observed collapse time are you trying to match? The correct figure seems to be about 15-18 seconds, though you appear to be measuring all the way to the bottom of the basement, whereas we can't see into the basement and usually measure to grade. This will probably only add a few percent to your estimate, but you'll still overpredict collapse time.

I am not trying to match any collapse time. The intention is to model the effect of the various parameters upon collapse time.

This hopefully provides the user with an understanding of how such factors affect the time of collapse, and assists users in making their own conclusions about probable causes.

The measurement is to ground level ONLY (above grade).

Figures are included for mass below grade simply because they are available, and for completeness.

In addition, the post crush-down collapse of the cap is modelled without any energy sinks or resistance, which produces a slightly short full collapse time. This will be addressed in a future update and may also include below grade mechanics.

Official collapse times vary, though it is clear that Dr Shyam Sunder, Lead Investigator for NIST stated the times as 9 and 11 seconds. This clearly cannot be a timing for the descent of all debris though, as both are faster than pure free-fall. We shall of course disagree on the mode of descent, which complicates what is actually meant by the descent time, and I suggest that focus is placed on refining the model rather than specifying what value it is supposed to be 'generating'.

With all parameters set to mutually acceptable values, and the possible inclusion of additional factors and more complex mechanics, the resulting time will, I am sure, be a topic of further, separate, discussion in itself.

 
femr2Date: Sunday, 12-04-2009, 20:53:51 | Message # 10
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6. Your mass numbers for the structure look pretty good. Have you considered varying the live load applied to the structure? NIST and some researchers, such as Purdue, use a relatively low figure as you do here, but there are others such as Dr. Quintiere who suppose the live load was up to three times as high. There is some support for either argument, and it is also suggested that NIST's live load refers only to accessible flammable materials rather than actual mass. I accept your assumption here as reasonable, but it's a parameter that remains largely unknown and has the potential to affect your results, so you may want to make it controllable as well.

The live loads in the spreadsheet are set to the maximum rated loads for all floors, and are also specified separately for the inner core and outside core areas.

(MAXIMUM Live Load as per ASCE 7-02 as applied to original design specification for WTC. NISTNCSTAR1-2A P102)

I feel that without further definite data, that the usage of the maximum design rated load is more than reasonable, however, as the data is housed in a spreadsheet, the loads can easily be changed by the user on a per-floor basis. (Sheet Tower Mass, Columns P thru V)

 
femr2Date: Sunday, 12-04-2009, 20:58:29 | Message # 11
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Having said all that, if you use the correct parameters, you should be reasonably close to the actual collapse -- and it appears that you are. In your slower cases, it appears that your collapse time is dominated by destruction of concrete. If you correct this factor, you should be pretty close. The other issues above will also lead you to overestimate the collapse time. Your results therefore make sense to me.

Again, the intention is to provide a model enabling experimentation with a range of values.

Users are encouraged to modify the data, though of course I'd like to know about any useful sourced and confirmed data so I can incorporate it into the downloadable source model.

I hope my answers to your questions result in rewording of your statement here, or more preferably retraction. It does nothing to progress the model, and expresses a position-based bias which is wholly discouraged within this discussion.

 
femr2Date: Sunday, 12-04-2009, 21:01:02 | Message # 12
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Looked at another way -- again, assuming I read your spreadsheet correctly -- your results predict that (a) if it was essential to crumble all of the concrete before the collapse could proceed, and (b) the concrete was all crumbled to particles of (fill in the blank) size, then the collapse would be slowed significantly, enough to be a poor fit to what we saw. The model therefore demonstrates that the concrete was not crumbled in such a fashion. I believe this result is correct.

Again, I suggest a rewording or retraction of this statement once you have digested my answers to your questions.

Asking questions, then applying assumed and position biased behavioural predictions before receiving answers, is not a method of discussion I wish to pursue here.

Only through much further discussion could a set of parameters (and final mechanics) be agreed.

Setting of parameters and implementation of mechanical behaviours deliberately to match observed behaviour, rather than natural extension and refinement through identification of relevant factors, is very specifically 'dry labbing', and is something I shall be trying to avoid most fervently here.

 
femr2Date: Sunday, 12-04-2009, 21:13:33 | Message # 13
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In summary, it would help if you explained your model in narrative rather than burying calculations in the spreadsheet. Nonetheless, it looks like you've done a good job on it, nothing a little documentation can't fix. You are one of the very few who has done so, and thus far yours is clearly the best entry in response to my "Hardfire Modeling Challenge." With a little more work we should be able to use this model to make some pretty good predictions and learn something, and that is the hallmark of a good model.

I fully agree that more documentation is required, though I would not agree to implications of 'burying' calculations. The calculations are quite necessarily inter-dependant but have been made as clear as possible by sheet separation and the inclusion of the parameters sheet.

I fully intend on documenting each and every calculation used, though until then it is of course simply a matter using the dependancy tools in excel for those curious about such matters.

I thank you for your questions, and acceptance of the model in your challenge, and hope we can progress and clarify it with mutually satisfactory results.

As I hope my answers clarify your questions, I see no immediate need to extend the model mechanics, although I fully agree that more documentation is required.

Explanation relating to how users may change sources values they disagree with would seem to cover most of your concerns, although of course, we may disagree on the values individuals may choose to actually use.

As far as I can see, focussed discussion can only result in improvement to the current model, so that's more than fine by me.

We will obviously have totally different opinions on the cause of the actual events, but I assume you will accept that biased discussion and inference should play no part in discussion of the model here.

I will now gather a list of assumptions made, beginning with those which work in favour of continued collapse, as they are more easily identified. I'm sure you will highlight those which work the other way round if I miss any.

 
femr2Date: Sunday, 12-04-2009, 22:00:27 | Message # 14
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Factors which work in favour of continued collaspe:

1) The 'Cap' mass is modelled as a single, non-deformable, solid, and the entire mass is applied to subsequent collision calculations.
2) All impacts are currently inelastic, ignoring the significant levels of energy absorbed by the structure in a more accurate elastic collision impact simulation. This will be addressed in a future update.
3) All descent calculations, including the initial drop, are calculated using standard vacuum free-fall equations. The initial drop and every subsequent drop after the impact of each floor, is performed as a free-fall descent. In reality this obviously was not the case, as the descent involved continual deformation of the building structure. Inclusion of inelastic collisions and steel structure deformation mechanics will help address this assumption.
4) The time implications of the deformation of the structure are not included.
5) The energy expenditure for the crushing of all materials other than concrete and steel collapse energy is not included. This includes dry wall, fixtures and fitting such as tables, computers, filing cabinets, and specifically the energy required to collapse each floor truss structure. (An almost endless list 'could' be compiled)
6) The effect of Cap tilting is not included, as this would require a finite element analysis rather than what is intended to be a physics/math model.
7) The initial drop assumes all core and external columns for the failure floor have 'vanished'. This works VERY much in favour of continued descent (Paper)
8) If energy expenditure during any impact exceeds available energy, the simulation is artificially allowed to continue, with the initial cap velocity being zero. The collapse failure floor is recorded and displayed on the Parameters sheet, and deficiencies are included on the videos by the value being highlighted in red. If this situation occurred in reality, collapse would stop. This behaviour is included to enable users to see the effect on energy deficiencies upon collapse time.
9) The maximum rated loads are used on all floors. No inclusion of energy expended in deformation of any of the live or construction dead load materials is included.

I'm sure there are more, which I will add via edits to this entry as I document them.

 
femr2Date: Friday, 24-04-2009, 11:15:33 | Message # 15
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I've now been in contact with Ryan via JREF, who has made it clear that he will not be commenting here.

Our conversation became, how do you say, stalled, on a point of contention, namely the energy loss arising from a conservation of momentum calculation.

It took some additional detail from another user to clarify the source of the contention as I'll explain below.

The positive outcome of the discussion has been the discovery of a fundamental contradiction in the usage of a perfectly rigid Cap (pile-driving mass) used in conjunction with a perfectly inelastic collision. (Which is the fundamental basis of many WTC crush-down models)

In a virtual world 'my' view (that the energy loss was accounted for in the energy exchange in decelerating the impacting mass, and accelerating the impacted mass) made perfect sense, making my viewpoint 'rigid', it was however, wrong. Damn and drat, but hey, it's hard to let go of a personal 'understanding' when it appears to make so much sense, all the numbers add up, and be so much simpler than the alternative...

As time (t) forms no part of the conservation of momentum equation, my usage of it assumed an instantaneous application of the physical behaviours implied, resulting in instant velocity change and instant energy exchange within the 'virtual' model.

Whilst in a virtual model the instantaneous application of these behaviours does appear to explain all energy 'loss' from the system, this is not actually the case, as the implied necessity to include time in some form cannot be excluded, and results in the conservation of momentum calculation energy loss being necessarily consumed by deformation of the impacting bodies.

This situation is compounded further by the use of perfectly rigid bodies, which in a virtual world appears to make perfect sense but again allows incorrect interpretation of conservation of momentum calculations to also seem to make perfect sense.

Again, however, in reality, perfectly rigid bodies cannot exist, so in 'application' of conservation of momentum to a perfectly rigid body, a paradox arises as the body itself cannot deform, and so a perfectly inelastic (and also elastic) collision cannot be applied to a perfectly rigid body.

Given the widespread usage of perfectly rigid bodies in many physics texts, I think it's easy for this misconception to result in such a misinterpretation, and the implications of the time domain being necessary in a conservation of momentum calculation, which itself does not reference time in any way, also compounds interpretation and explanation of the issue.

So, in summary, the energy loss through the conservation of momentum calculation must be made available for deformation of materials within the model being discussed.

This only really forms a half-way-house solution to the problem, as perfectly rigid bodies are still used, and this also must be addressed in some way. (Implementation of non-rigid bodies and elastic collisions involving the time domain could be interesting to try and express in spreadsheet format, but I'll give it a go)

The model has now been updated to allow usage of the c-o-m energy loss in subsequent deformation calculations, and will be available for download shortly. (Numerous other additional features have also been included, such as the ability to specify mass loss on a per-floor basis and usage of specific floor heights rather than an averaged height in floor drop calculations.)

I noticed a few others observing the conversation, who began by agreeing with my 'stance', have now also changed position, which, given that I have now found out that the c-o-m energy loss usage issue has been a point of contention between many others, I think allows me to emerge from a bit of a humble-pie situation with at least a good basis for the apparently common misconception (virtual use of rigid bodies and perfectly inelastic collisions) and the knowledge that others have also benefited from the 'exchange'.

The model is now also more accurate, which is all that really matters. Hey ho...onwards.

 
femr2Date: Saturday, 25-04-2009, 21:44:31 | Message # 16
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An updated calculation model spreadsheet is now available for download:

http://femr2.ucoz.com/load/1-1-0-4

 
guarneriDate: Sunday, 20-02-2011, 05:18:35 | Message # 17
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Hi femr2,

I am new to this forum and wonder where to find the new spreadsheet (I get a "page not found 404 error"). Also I am very much interested how the story from here proceeds re has proceeded.

I am very impressed about your work especially the neutrality you have put into it.

regards

 
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