Don't blame it on being an architect. I think your mindset has a lot more to do with it.No I didn't learn anything yet from this thread... I am just a dumb architect.
I see. Complicating matters is that the "J" in jaydee is my first initial, for my first name, Jeffrey. I sometimes inexplicably read posts referring to Jeffrey, as directed at me. Quickly remembering that the poster did not know my name.After reading over this thread I realized I somehow replied to you instead of Jeffrey Orling, for whom I intended the comments saying he shouldn't make proclamations while also acknowledging that he didn't understand the math and found it confusing. They weren't for you.
I do perform these types of calculations in my work and I am sure Dr. Nordenson is a fine structural engineer who is plenty capable. However, none of us are immune to occasional mistakes and it is clear that he made a mistake here by not considering the stiffness and deflection of the falling beam and girder assembly. There is a big difference in load applied to the impacted object and that experienced by the impacting object when the acceleration/deceleration takes place over a greater or lesser time frame.Tony...
I am not a physicist, nor a forensic engineer and I don't engineer the structure for the projects I do... My structure understand ended in statics in college... and my knowledge is basic and intuitive... more than the average Joe but that's it.
Nordenson seems to be making a case that a single falling girder and attached whatever falling down one story would set of a progressive collapse one floor section after the next. I can't prove or disprove and comment on the math. My suggestion was take it up with him not with people on this or other forums. Let HIM defend his work.
My gut feeling... considering his stature and body of work and his reputation... is he probably knows what he is doing... More so than Professor Hulsey or Oystein, or Ozzie... or you and certainly more than Gage, or Cole, or Greg Roberts or Neils Harrit... all of whom speak like experts in structural engineering and most of the internet people on these forums.
I do know that there ARE progressive floor collapses... Ronan Point was one... Why not 7wtc? Nordenson goes on to detail how the frame failure progresses... gutting the interior and then leaving the perimeter moment frame unsupported and unbraced.
I don't know a single architect that does these sorts of calcs... Maybe you do... Ask Gage ;-)
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A point load is infinitely stiff and has no deflection and thus will cause very high acceleration and deceleration of the impacted and impacting object. The only thing Dr. Nordenson's analysis has deflecting is the girder below and it is much stiffer at 10 to 12 inches from its seat than the long and flexible falling beam and girder assembly. What he has in his analysis is far from what would really happen here. The deflection of the impacting beam and girder assembly will make a big difference in the force experienced by the girder below and its seat and the analysis needs to be corrected to account for this.
Nordenson, who implicitly assumes an infinite stiffness for the falling girder (i.e. the falling girder absorbs no energy in the collision with the girder below other than the plastic deformation of the flange on the tip that is already accounted for), finds that the energy absorbed by the girder below (i.e. 100% of the net kinetic energy at the beginning of the impact) means a vertical force that is about an order of magnitude larger than the shear capacity of the affected connection. Therefore, he concludes, it wasn't possible for the connection to stay intact - the connection to c79 of the impacted girder would fail, that girder would fall on floor 11, fl11 would fail in the same way and fall on fl10 ... all the way to the ground.So what is the PRACTICAL difference? It seems to me you don't know that either - and are assuming it is significant.
Well it does and it doesn't. Recall that there were few studs installed and that they had broken. There is little to no composite action with the girder. Concrete floor mass would simply add to impact and dampen spring back.I am curious about the 3D geometry. Intuitively it seems that the collison would be not axially aligned and the failure could be related to the twisting of the lower assembly.
If the upper girder comes down it has sections of floor slab and the beams supporting it and framed into the 44-79 girder.
It would include falling mass of higher floors contributing to impact loading but in addition those lower floors would have some composite action from shear studs.(except possibly if the higher of the two floors experiencing fire was the initial failure. In that case one lower floor would have had less composite combination of floor and girder.Would the subsequent collapses below be identical to the first? What about the additional mass? Nordensen's cartoon diagram of the progression shows a sort of neat pile of slabs on grade.. or is this a pile of debris of 13 floors?
Well it does and it doesn't. Recall that there were few studs installed and that they had broken. There is little to no composite action with the girder. Concrete floor mass would simply add to impact and dampen spring back.
If the shear stud all broke... the concrete will still come down with the beams that supported them... and so will the superimposed dead loads. Why would the steel drop "ahead" of the concrete slab it supported broken shear studs notwithstanding. Seems like all the mass needs to be included... no?
Your initial post appeared to still have girder attached to beams/floor.the upper girder comes down it has sections of floor slab and the beams supporting it and framed into the 44-79 girder.
The reason the concrete in the northeast corner would not come down with the beams and girder if all of their shear studs were broken is the high strength welded wire grid in the slabs that would have been supported by the still stable beams to the west and the south.If the shear stud all broke... the concrete will still come down with the beams that supported them... and so will the superimposed dead loads. Why would the steel drop "ahead" of the concrete slab it supported broken shear studs notwithstanding. Seems like all the mass needs to be included... no?
If the next floor down doesn't collapse then nothing happens.Your initial post appeared to still have girder attached to beams/floor.
Yes the girder, will come down ahead of floor. There will be cantilever resistance to the floor pan dropping, but with no composite connection to the girder means that the girder falls on its own. Floor mass impact will not be far behind the girder and IMHO would dampen spring back of the girder while adding to the forces on the lower floor.
TSz says that the initial impact of the girder will not fail the lower girder connection to col79. If true one notes that this is that particular impact, in isolation. Question is then, what happens next and what happens in addition.
Yes, the Oakland highway overpass about ten years ago. I showed a slide of it on this thread. It did not collapse the lower deck.I know its a highly specific thing but, does anyone know of any videos of a heavy girder rotating to impact in similar fashion as this set up? Industrial accidents for instance?
If the next floor down doesn't collapse then nothing happens.
We are talking about whether girder A2001 falling off its seat at the 13th floor could have caused a propagation of the floors under it. I don't see how you can say that is what happened. There is also no evidence that column 79 buckled at or near the 13th floor.YES!... but of course that is precisely what happened...not only did the girder and the floor around it collapse... but col 79 dropped as well.
Or so it seems.
"Next" as in directly following, wiithin milliseconds perhaps, of initial girder impact. The flooring and beams are detached from the girder but will be following the girder down. It represents more, albeit following in time, force. If initial girder contact damages the lower girder more damage may occur when the extra flooring and office contents mass also falls.If the next floor down doesn't collapse then nothing happens.
Yet that remains the only sequence for which there is ANY evidence. Outward visuals imply failure of col 79. They imply this failure occurred low down in the structure. They imply a horizontal progression of core failureWe are talking about whether girder A2001 falling off its seat at the 13th floor could have caused a propagation of the floors under it. I don't see how you can say that is what happened. There is also no evidence that column 79 buckled at or near the 13th floor.
I am saying the jury is still out as to whether a girder falling off its seat at column 79 at the 13th floor could have propagated a collapse. It does not look like it can develop enough force due to deflection cutting down the deceleration/acceleration and nobody has shown it could happen yet. Nordenson's analysis has been shown to be erroneous for not considering the beam and girder stiffness and deflection. My preliminary calculations show that when his analysis is corrected there is a good chance the math will show it won't."Next" as in directly following, wiithin milliseconds perhaps, of initial girder impact. The flooring and beams are detached from the girder but will be following the girder down. It represents more, albeit following in time, force. If initial girder contact damages the lower girder more damage may occur when the extra flooring and office contents mass also falls.
Your calcs, disputed by Ozeco, show less than required impact to fail the lower floor/girder.
Rather than conclude demolition, it would behoove you to investigate other plausible mechanisms that could contribute to collapse progression.
Yes Tony, we all know what you do and don't believe.I am saying the jury is still out as to whether a girder falling off its seat at column 79 at the 13th floor could have propagated a collapse. There is a good chance the math will show it won't. Nobody has shown it could happen yet, as Nordenson's analysis has been shown to be erroneous for not considering the beam and girder stiffness and deflection.
I have good reason to believe what I do.Yes Tony, we all know what you do and don't believe.
Agreed to A, It is looking like you are right on B and C is the underlying issue of dampening of an impact load which has never been in doubt as a matter of principle ... the doubts have been about how it applies.I do perform these types of calculations in my work and I am sure Dr. Nordenson is a fine structural engineer who is plenty capable. However, none of us are immune to occasional mistakesA and it is clear that he made a mistake here by not considering the stiffness and deflection of the falling beam and girder assembly.B
There is a big difference in load applied to the impacted object and that experienced by the impacting object when the acceleration/deceleration takes place over a greater or lesser time frameC..
All that basic physics agreed. A good summaryThe energy in a drop is the area under the acceleration/deceleration vs. time curve and it does not change for a given drop height of an item with a certain mass. It is the potential energy (mgh) turned into kinetic energy. While we can't change the energy involved, we can absorb it over more time, and the acceleration/deceleration peak, which is what affects the force involved, is lowered. Imagine a tall slender curve on an X-Y graph for something that is hard being dropped onto a hard surface and then imagine a lower but longer curve for the same item dropped from the same height onto a softer surface. The softer impact surface deflected and lowered the acceleration/deceleration peak even though the energy did not change. It is the same reason we put foam in packages. They allow for energy absorption over more time by deflecting. This lowers the deceleration of the item and thus the force it experiences due to its own mass times that deceleration.
Energy in a drop is absorbed as force x distance. For a given energy, if the distance it is absorbed over is small, because there is no deflection, then force will be high. The deflection can be in either the impacting object or the impacted object or both. It is the amount of deflection that ultimately determines what the forces involved are.
A point load is infinitely stiff and has no deflection and thus will cause very high acceleration and deceleration of the impacted and impacting object...
The two flexibilities are in parallel - not series. Excuse the "lay persons language" of the next bits of explanation.The only thing Dr. Nordenson's analysis has deflecting is the girder below and it is much stiffer at 10 to 12 inches from its seat than the long and flexible falling beam and girder assembly. What he has in his analysis is far from what would really happen here. The deflection of the impacting beam and girder assembly will make a big difference in the force experienced by the girder below and its seat
Why? For who? It was a paper prepared for legal purposes for a past and concluded action. Why would his clients need corrections at this late stage? Who else has standing to ask for corrections?and the analysis needs to be corrected to account for this.
IMNSHO - no difference whatsoever.So what is the PRACTICAL difference? It seems to me you don't know that either - and are assuming it is significant.
We are seeing progress on understanding the single issue in focus. The overall status is IMO unchanged - and it makes no "PRACTICAL difference"... How does an error in Nordenson's report have the slightest effect on the status of the NIST report?
Interesting logic - I'll leave it for someone to identify the Formal Fallacy. Bill said "A". Fred said "B". Fred is wrong THEREFORE Bill is wrong?
The "NIST" report on WTC 7 might as well be called the "missed" report because it does not hold up to scrutiny and seems to be an attempt to promote an impossible explanation for what appears to be political reasons. Things like the omission of the girder stiffeners, ignoring the lateral trapping of the girder by the column 79 side plates, and falsely claiming there were no shear studs on the girder all show the report is bogus.IMNSHO - no difference whatsoever.
1) The IMPLIED goal here is "Prove CD";
2) The path to that - acknowledged by Tony at post #5 - is "Denigrate and discredit NIST";
3) ...
N) Tony's specific single issue focussed goal here is prove Nordenson was wrong -- on one point.
I think I summarised the overall "logic" at my Post #6.
We are seeing progress on understanding the single issue in focus. The overall status is IMO unchanged - and it makes no "PRACTICAL difference"
The lateral spring rates (lateral stiffness) of the two girders are in series, not parallel, as they act in the same line of action. You could say their axial spring rates are in parallel but that isn't involved here.Agreed to A, It is looking like you are right on B and C is the underlying issue of dampening of an impact load which has never been in doubt as a matter of principle ... the doubts have been about how it applies.
My commendation on this following concise explanation of the underlying principle of applied physics for Jeffrey (Sander) Orling
All that basic physics agreed. A good summary
The two flexibilities are in parallel - not series. Excuse the "lay persons language" of the next bits of explanation.
Nordenson has accounted for the energy that is dissipated - used up - lost - consumed in the earlier failures and the deformation of the flange at the point of impact. BUT the NET energy remaining at the time we are considering - the start of "dampening" as the impact occurs - is still "live" - still available hence the "NET". It is becoming temporarily stored in bending strain energy - in two beams - until EITHER the connection fails or it doesn't fail and some complicated oscillation starts.
Let's take the next step down the path. Because the two beams are in parallel as far as energy flow goes (physically in series BUT that is not how the energy "sees" it.) the net energy will flow two ways. And it will split those two ways in inverse proportion to the stiffnesses of the two beams. So - if the falling beam is (say) 1/9th the stiffness of the lower beam the energy will flow 90% to the falling beam and 10% to the lower impacted beam.
So Nordenson only has 10% of the NET energy he thought he had.
And I picked the 10% - 90% split for simplicity of explanation - in the Nordenson figures is is about 7 or 8:1 so Nordenson saw about 6-7-8 times oversupply of energy. (The exact number doesn't matter - yet - till we get the principle on the table.)
The actual outcome depends on the actual stiffness in play for that falling beam. If is even less stiff than my guess then your intuition is correct - there will not be sufficient energy flowing to the lower beam and via it to fail the girder<>column connection.
If the falling beam is stiffer than my guess then there will be enough energy flowing to the impacted beam to fail the connector.
And in that second scenario of "failure" Nordenson's outcome conclusion would be correct BUT he would be in the position of being "right for the wrong reasons".
And the odds are your way Tony that the beam is much less stiff (???) I don't accept your "hundreds" but even 10 times makes your claim on that aspect of the Nordenson paper valid.
However:
Why? For who? It was a paper prepared for legal purposes for a past and concluded action. Why would his clients need corrections at this late stage? Who else has standing to ask for corrections?
The "NIST" report on WTC 7 might as well be called the "missed" report because it does not hold up to scrutiny and seems to be an attempt to promote an impossible explanation for what appears to be political reasons. Things like the omission of the girder stiffeners, ignoring the lateral trapping of the girder by the column 79 side plates, and falsely claiming there were no shear studs on the girder all show the report is bogus.
The correction of the Nordenson analysis will add another nail in the coffin of that ignominious tome and fraud known as the NIST WTC 7 report as it never even bothered to show the falling girder would cause a collapse propagation of the floors below. It just assumed and pronounced it.
My personality doesn't lend itself to playing along with a fraud. There needs to be a new investigation so nobody has to.
The lateral spring rates (lateral stiffness) of the two girders are in series, not parallel, as they act in the same line of action. You could say their axial spring rates are in parallel but that isn't involved here.
The force applied to the girder below in the Nordenson analysis is probably about 20 times too high and the actual force is not enough to shear the seat of the girder below and cause collapse propagation.
I developed the math in the PDF I uploaded earlier:Figures?? Calculations to support this?? You know - actual evidence?
I could say that Tony but I wont.The lateral spring rates (lateral stiffness) of the two girders are in series, not parallel, as they act in the same line of action. You could say their axial spring rates are in parallel but that isn't involved here.
Could well be true as I explained in broad concept in Post #146. It depends very much on the actual flexibility/stiffness of the dropping beam which forms a parallel path for the diversion of the still live - not "dissipated" energy. Try tracking energy flow....The force applied to the girder below in the Nordenson analysis is probably about 20 times too high and the actual force is not enough to shear the seat of the girder below and cause collapse propagation.
Perhaps you could, in layman's language, say that strain energy build-up is "parallel", but when talking about springs, which the beams are, the technically and physically correct language demands that we talk of "springs in series" as opposed to "in parallel":...
Energy - anything going two separate ways is parallel by my use of language. And by my language "serial" means one after the other.
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Could well be true as I explained in broad concept in Post #146. It depends very much on the actual flexibility/stiffness of the dropping beam which forms a parallel path for the diversion of the still live - not "dissipated" energy. Try tracking energy flow.
Here - a free "hint" - if you put the springs in series the more flexibility - less stiffness - the upper one has the MORE energy passes through to the lower beam "spring". Exactly the reverse of what your claim needs.
Perhaps you could, in layman's language, say that strain energy build-up is "parallel", but when talking about springs, which the beams are, the technically and physically correct language demands that we talk of "springs in series" as opposed to "in parallel":
https://en.wikipedia.org/wiki/Series_and_parallel_springs
Again, check the math in my PDF, it shows that the effective stiffness is that of springs in series.
You might be surprised at how low the temp would be on the top of the concrete slab. The WWM is still below 300C in the ARUP analysis.Are these values for the lower 12th floor assembly assuming a stone cold... pristine strength condition? If so I find that preposterous. The fires that were impacting the 13th floor surely including fires below on the 12th floor. If so wouldn't that change the outcome of a "collision" of the 13th floor falling on the 12th?
I believe that JO was referring to the temp of the lower girder and associated seat on col 79.You might be surprised at how low the temp would be on the top of the concrete slab. The WWM is still below 300C in the ARUP analysis.
Quick addition - I should also have mentioned that the welded mesh will be crucial, and should be accounted for in the calculation.