What is Free Fall?

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Offline Aemilius

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Re: What is free fall?
« Reply #50 on: 04/12/2013 01:09:39 »
Unrelated, but, I was just thinking (after a couple of shots of whiskey)....  How remarkable is it for an eighth grade high school dropout to ever have the chance to enjoy any kind of meaningful exchange with a career Ph.D. research Physicist?
 
Even as a 55 year old (relative) newcomer to the internet.... Far out man!

Thanks in advance Mr. Calverd.
« Last Edit: 04/12/2013 10:04:02 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #51 on: 04/12/2013 06:50:15 »
Not nearly as remarkable as being asked a sensible question by someone who seems to care about the answer!

Keep drinking the good stuff.
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Offline evan_au

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Re: What is free fall?
« Reply #52 on: 04/12/2013 10:09:52 »
Reply #44 shows a graph of the building's velocity vs time.

The part outlined in red shows an approximately linear velocity vs time curve. This is representative of a building in free fall.

During this time period, the height of the building vs time would be parabolic, with the height proportional to the time squared.
« Last Edit: 05/12/2013 10:45:16 by evan_au »

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Offline Aemilius

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Re: What is free fall?
« Reply #53 on: 04/12/2013 15:05:39 »
Hello evan_au (nice to meet you)....

Sorry, I should've mentioned earlier that the graph is from the NIST report (I'm sure I can dig up a link if necessary).

Are you also a Physicist?
« Last Edit: 04/12/2013 15:07:20 by Aemilius »

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Offline Aemilius

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Re: What is free fall?
« Reply #54 on: 04/12/2013 15:37:09 »
Just noticed your "Profile". Telecommunications.... Electrical Engineer?
« Last Edit: 04/12/2013 20:31:43 by Aemilius »

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Offline Pmb

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Re: What is free fall?
« Reply #55 on: 04/12/2013 17:43:42 »
Is air/wind resistance a component of free fall?  It is negligible at low speeds, but can be significant as one reaches terminal velocity.
It's wrong to say that It is negligible at low speeds because what is "low speed" depends on the particular object. What is low speed for a cannon ball is not low speed for a feather.

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Offline Aemilius

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Re: What is free fall?
« Reply #56 on: 04/12/2013 19:02:54 »
Is air/wind resistance a component of free fall?  It is negligible at low speeds, but can be significant as one reaches terminal velocity.
It's wrong to say that It is negligible at low speeds because what is "low speed" depends on the particular object.

That's incorrect. It's not at all wrong to say that it's negligible in this case. As Mr.Calverd pointed out earlier (and I would have to agree), for an object like the 100 pound weight depicted in the animations falling through air a distance of 15 feet "....you would find it difficult to measure the difference between in vacuo and in air arrival times." This is, if I'm not mistaken or taking it out of context, the very definition of "negligible".


What is low speed for a cannon ball is not low speed for a feather.

That depends. In air or in vacuo? In air yes but in vacuo no. Again, as Mr. Calverd pointed out earlier (and again, I would have to agree), if I'm not mistaken or taking it out of context "....the mass of the objects is irrelevant (in vacuo). In free fall, all objects fall at the same rate."
« Last Edit: 04/12/2013 19:48:36 by Aemilius »

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Offline Aemilius

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Re: What is free fall?
« Reply #57 on: 04/12/2013 19:22:29 »
Very little pressure is required to blow out a building. If my "idealised shed" roof fell one third of the height of the building, the excess internal pressure would be over 700 lb per square foot. Windows - especially large ones - give way well below that level, and the rigidity of a modern bulding is partly conferred by the stressed skin window structure.

True, but really, when one reviews the video and graph concerning this particular building, it's glaringly apparent even to a layman that it went into free fall almost immediately, which would of course naturally rule out any bursting, or blowing out, due to a build up of air pressure in the lower part of the building some number of stories below (hidden from view).

In other words, it simply never had the chance to fall far enough before going into free fall for it to have plausibly developed the kind of pressure build up that could blow out 8 stories of glass, columns, etc. of the building.... Do you think we can we agree on that?         

Once a couple of steel uprights have buckled, the stress on the remainder is no longer compressive but rotational, and they aren't good at sustaining a rotational load.

I hadn't thought of that.... but then, as I study the symmetry of the facades descent, as a whole, while considering the over 300 foot wide (largest visible) facade particularly, I'm unable to model a progessive structural failure and subsequent collapse anything like that shown in the video....
....or anything remotely corresponding to the NIST graph, as it would require a novel horizontal "Newtons cradle" type of transfer of vertical load forces....
....to effect a progressive failure of the thirty columns supporting the two visible surfaces, or facades, of the building simultaneously....
....and even if I could manage that, the mystery of free fall remains.
 
"Google" has been unrewarding.... Is there any known precedent setting mechanism you're aware of?   

It looks very much as though the video'd building was also minimally rigid.

Hah! To your Ph.D., Mr. Calverd, you may now feel free to add a Masters Degree in understatement!
« Last Edit: 25/02/2015 18:02:38 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #58 on: 04/12/2013 20:09:05 »
No, it's not glaringly apparent!

The NIST graph shows velocity, not height, versus time. As Evan pointed out, free(ish) fall produces a linear increase of velocity with time, and this is only apparent after the first 2 seconds of collapse, which is consistent with my aerostatic model of lower-floor blowout. 

If the building was entirely supported by the internal steels, it would be surprising that none of them is visible  after the collapse. But half of the static load was borne by the outer steels, as in a conventional brick building. No great surprise there, you can use a steel web, with concrete, brick or steel panel infills for sway rigidity - same problem: it can burst and collapse very quickly from internal pressure.
« Last Edit: 04/12/2013 20:13:42 by alancalverd »
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Offline Pmb

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Re: What is free fall?
« Reply #59 on: 04/12/2013 22:01:07 »
Quote from: Aemilius
That's incorrect.
Nope. In fact it's very correct.

Quote from: Aemilius
It's not at all wrong to say that it's negligible in this case.
You donít seem to have read my post very carefully. I said that you canít talk about what is low speed in all generality because what works in one case doesnít work in all cases.

Quote from: Aemilius
That depends. In air or in vacuo?
Please go back and read my post again. This time please read it very carefully and to what I was responding to. I was responding to the following statement

Quote from: CliffordK
Is air/wind resistance a component of free fall?  It is negligible at low speeds, but can be significant as one reaches terminal velocity.
Since it was this comment which I quoted it means that it was this comment I was referring to. This comment is about falling in an atmosphere where there is air resistance acting on it. He made this comment without referring to what the object was and what is low speed and can be ignored for one object does not hold for all objects. When there is no resistance then the body is truly in free-fall.

By the way, since Iíve been a physicist for over a quarter of a century you donít have to remind me that all objects fall at a rate in a vacuum which is independent of their mass. However in general relativity (GR) the rate at which something fall does depend on the velocity its moving with, i.e. the gravitational acceleration is velocity dependant. See http://home.comcast.net/~peter.m.brown/gr/grav_force.htm

If the bodyís spatial extension is large compared to the region over which tidal forces canít be ignored then tidal forces will affect the bodyís rate of fall. I.e. in a curved spacetime large objects donít move on geodesics.
« Last Edit: 04/12/2013 22:08:04 by Pmb »

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Offline alancalverd

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Re: What is free fall?
« Reply #60 on: 05/12/2013 00:13:28 »
Something of a circular argument developing here. "Low speed" is presumably any speed at which the acceleration of the falling object is for practical purposes indistinguishable from its value in vacuo, and will obviously be different for a sycamore piano or a sycamore seed. We dealt with form factors (though admittedly not autorotational lift) several pages ago, and it turns out that the questioner was specifically interested in concrete buildings falling down on the earth's surface. Even an old pedant like me thinks that we can ignore relativistic corrections when trying to work out why the building collapsed. 
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Offline Pmb

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Re: What is free fall?
« Reply #61 on: 05/12/2013 01:18:59 »
Quote from: alancalverd
"Low speed" is presumably any speed at which the acceleration of the falling object is for practical purposes indistinguishable from its value in vacuo, and will obviously be different for a sycamore piano or a sycamore seed.
I was trying to make the point that "low speed" was not an absolute but relevant to the particular scenario. The way he said it seemed to me that he was speaking in absolute terms. Let's leave it at that and not pit pick, shall we?

Quote from: alancalverd
Even an old pedant like me thinks that we can ignore relativistic corrections when trying to work out why the building collapsed
I never mentioned a falling building. I was talking strictly about the statement In free fall, all objects fall at the same rate. and while true in Newtonian mechanics its not precisely true since GR has a correction for high speed motion. I'm not interested in whether you think I should make this point or not. It's a fact and we can't nor shouldn't say what people want to know.
« Last Edit: 05/12/2013 01:37:34 by Pmb »

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Offline Aemilius

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Re: What is free fall?
« Reply #62 on: 05/12/2013 01:31:34 »
Right.... sorry about all that Pmb. Thanks for the input (I need all the help I can get!).
« Last Edit: 05/12/2013 01:48:34 by Aemilius »

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Offline Pmb

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Re: What is free fall?
« Reply #63 on: 05/12/2013 01:39:58 »
Right.... sorry about all that Pmb. Thanks for the input (I need all the help I can get!).
You're most welcome, sir! I was and will assume that you want to know as much as you can about this even when I know there's things you might have the knowledge to inquire about such as the corrections GR has to high speed free-falling like that around a black hole.

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Offline Aemilius

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Re: What is free fall?
« Reply #64 on: 05/12/2013 01:49:09 »
Are you a Ph.D too Pmb? You've both given me something to think about. What do you think of alancalverds aerostatic model of lower-floor blowout? I've read a number of theories about how this building collapsed, but this is the first I've heard of it.

Just a note to add that it was a steel frame building, not concrete Mr. Calverd.... wasn't sure if I'd mentioned that.   
 
« Last Edit: 19/03/2015 13:41:12 by Aemilius »

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Offline Pmb

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Re: What is free fall?
« Reply #65 on: 05/12/2013 02:55:00 »
Quote from: Aemilius
Are you a Ph.D. too Pmb?
Not yet. I started working on it but had to stop due to an illness in the family. I plan on going back to graduate school next fall. I have the equivalence of a masters degree in physics.

Quote from: Aemilius
You've both given me something to think about. What do you think of alancalverds aerostatic model of lower-floor blowout?
I never saw it. He's in my ignore list for reasons I won't get into in open forum.
« Last Edit: 06/12/2013 02:22:21 by Pmb »

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Offline evan_au

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Re: What is free fall?
« Reply #66 on: 05/12/2013 10:48:11 »
Just noticed your "Profile". Telecommunications.... Electrical Engineer?
Yes, my formal education was in Electrical Engineering.

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Offline alancalverd

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Re: What is free fall?
« Reply #67 on: 05/12/2013 19:17:44 »
Just a note to add that it was a steel frame building, not concrete Mr. Calverd.... wasn't sure if I'd mentioned that.   
 

Steel frame, certainly, but apparently clad with concrete panels or something similar. Indeed a "concrete" building is essentially a combination of steel mesh to support tension loads and concrete in compression. A framework of rectangular steel lattice is not rigid. You need to brace it with triangulating beams (as in a bridge or a roof) or fill the spaces with panels made from something fairly incompressible like concrete - the usual procedure for large office blocks. Now if you blow out a few of those panels, the steels either side can buckle and the whole lot will collapse - as it did.
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Offline Aemilius

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Re: What is free fall?
« Reply #68 on: 05/12/2013 19:37:07 »
Hi Mr. Calverd....

The NIST graph shows velocity, not height, versus time. As Evan pointed out, free(ish) fall produces a linear increase of velocity with time, and this is only apparent after the first 2 seconds of collapse, which is consistent with my aerostatic model of lower-floor blowout.

I understand.... It's actually a pretty cool theory. Like I said I thought I'd seen them all. So in your theory, it would be in the first seconds 1 and 2 (approx.) that the building descended to one degree or another and a rapid pressure build up occurred as a result. Then in the next seconds 3 and 4 (approx.) the actual period of free fall following the blow out event occurs....



Am I close?
« Last Edit: 22/12/2013 00:20:46 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #69 on: 05/12/2013 20:23:40 »
It certainly consists with the video and graph, and also explains why the graph turns over at the top. At some point the descending piston will be travelling faster than the air beneath can escape, so it slows down a bit until there is enough pressure to cause another blowout - and of course it's now falling through rubble as well as air. 
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Offline Aemilius

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Re: What is free fall?
« Reply #70 on: 05/12/2013 22:25:49 »
Hey evan_au....

Electrical engineering, interesting. So, about the aerostatic model of lower-floor blowout (replies 58 and 68).... What do you think?
« Last Edit: 05/12/2013 22:29:01 by Aemilius »

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Offline Aemilius

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Re: What is free fall?
« Reply #71 on: 07/12/2013 03:25:19 »
So I still can't quite get the aerostatic blowout model to work.

The collapse of the "Penthouse", on the left, is the first sign (below) of any catastrophic progressive structural failure....


The rest of the rooftop structure to the right of the now collapsed "Penthouse" atop the building, however, remains stationary for about 4 seconds, and then it (the remaining rooftop structure) suddenly begins to descend largely intact, meaning the core columns must all have given way almost simultaneously (the dreaded horizontal "Newtons cradle" effect). As the remaining rooftop structure begins to descend, even before it reaches the roofline, the roofline also begins to descend, also largely intact, and they're seen afterwards essentially descending (below) together into the free fall period .....


So.... How could the brief 30 to 35 foot drop of the remaining rooftop structure on the right have developed the kind of pressure that would blow out 8 stories of glass, cladding and steel columns many stories below?

The initial collapse of the "Penthouse" above column 79 (on the left) couldn't have built up pressure to the point of being able to blow out 8 stories about the girth of the lower part of the building (required for the observed period of free fall) because the facade continues to stand afterwards for about 4 seconds.

If the symmetry of descent and free fall for 8 stories, or over 100 feet, of the building is to be explained by a symmetrical blowout about the girth of the building, it obviously can't have occurred prior to the blowout. In other words.... While the initial symmetrical descent can explain an eventual blowout, an eventual blowout cannot explain the initial symmetrical descent, which leaves unanswered the original question as to how any initial symmetrical descent could have started to begin with (the dreaded horizontal "Newtons cradle" effect).

Otherwise, if there were to be a blowout event, I would expect increasing pressure to find and then cause to burst only the weakest part of the facade as just part of a larger overall conventional progressive structural failure....


The debris field surrounding the post collapse zone doesn't reflect what one would expect to see from a blowout event of that magnitude (spanning 8 stories about the girth of the building) either....


If pressure had built up sufficiently to blow out all the glass, cladding and perimeter columns at once over a span of 8 stories (over 100 feet) we should expect to find a more substantial debris field, certainly greater than the reported 70 feet in any direction....



....and the way the walls all seem to have fallen inward rather than having been blown outward (notice the undamaged dust covered windows of a neighboring building)....


....and finally, as you noted earlier, "Very few buildings (apart from nuclear power stations and the like) are designed to withstand outward force." So, this obviously wouldn't be the expected post blowout/collapse appearance of a debris pile resulting from that scenario. I couldn't go with the aerostatic blow out model/theory of collapse at this point.

Thanks again Mr. Calverd, it's been very interesting..... I'll let you guys have the last word. Looking forward to chatting with you now and again about other things.
« Last Edit: 03/12/2015 01:17:19 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #72 on: 07/12/2013 12:42:04 »
If you look carefully at the part just below the penthouse you can see the initial blowout happening about 5 floors below, and you can see the pressure wave propagating downwards and to the right. Now you have not only the roof but some 5 to 8 concrete floors descending as a piston so you don't need a lot of initial blowout to weaken the entire structure. As the collapse progresses, there will be an inward rush of wind behind the descending piston, so a fair quantity of outer wall debris will end up inside or close to the footprint of the original building.

The pressure wave doesn't have to propel the walls very far - a few feet will be enough to relieve the pressure but destroy the integrity of the structure.
« Last Edit: 07/12/2013 12:46:38 by alancalverd »
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Offline Aemilius

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Re: What is free fall?
« Reply #73 on: 08/12/2013 00:18:01 »

Is this what you're referring to Mr. Calverd?

« Last Edit: 03/12/2015 01:22:22 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #74 on: 09/12/2013 18:01:46 »
Indeed.
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Offline Aemilius

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Re: What is free fall?
« Reply #75 on: 10/12/2013 19:05:59 »
Hi Mr. Calverd....

So, let me see if I really understand your aerostatic blowout model....
 

The building has been damaged (the 12 columns toward the rear) and there are several fires continuing to burn on various floors, but it's still standing....


In the first visible stage of the collapse, column 79 (circled) weakens and buckles due to heating from one of the fires. The other 5 columns supporting the West Penthouse immediately follow, buckling/failing in rapid succession, leading to the collapse of the West Penthouse into the building....


For about the next 4 seconds, though nothing much appears to be happening from the outside, on the inside the West Penthouse is actually continuing to crash its way downward like a giant piston through floor after floor gaining both mass and momentum as it goes, and this leads to a sudden rapid aerostatic pressure build up in the lower part of the building (out of view)....
 
 

In the next visible stage of the collapse, as the West Penthouse continues to descend, the aerostatic pressure becomes great enough that a catastrophic bursting, spanning about 8 stories, in the lower part of the building (out of view) begins, starting with the core columns, and it literally blows them away, leading to the beginning of the collapse of the East Penthouse into the building....
   

As the bursting process continues, all the connections between the outer structure and the collapsing core columns are quickly severed by the sudden removal of 8 stories of core support. As a result, all at once within a second of the beginning of the descent of the East Penthouse, 8 stories of perimeter columns, cladding, glass and other materials making up the walls, unable any longer to withstand the increasing pressure and no longer restrained by any connections to the core suddenly expands and blows out, and it's this completion of a symmetrical blowout that breaks up/removes any remaining intact supporting perimeter columns, along with trusses, girders and other structural components, including interior/exterior walls, cladding, glass and other materials of the lower part of the building (out of view)....
 

Finally, in the last visible stage of the collapse, all the various structural components that might have resisted the descent of the upper part of the building have been thoroughly dismantled/largely removed by the completed blowout, allowing the upper part of the building to quite naturally go into free fall for about 8 stories with no apparent resistance from either the core columns or perimeter columns as they had already been largely dismantled/removed by the blowout.... Is that close?
« Last Edit: 05/02/2014 13:10:05 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #76 on: 10/12/2013 20:07:10 »
You can't analytically separate the columns of a completed building from each other, the cladding, or the floors. The whole thing is designed to be just adequately rigid. If you blow out the cladding it will buckle some of the exterior columns to which it is attached, at which point the outer edge of the floor will fall, thus twisting the inner columns (because the concrete floors are floated on steel crossmembers bonded to the columns) . Steel columns are rigid in compression or tension, but not designed to withstand much torsion, and once buckled, they will collapse under their design compression load.

You can see the bursting on the video in your previous post. As the penthouse begins to fall, the texture of the face of the building changes as the windows bulge. The "texture wave" actually begins  fairly near the top and moves sideways (left to right) possibly quicker than downwards because corridors allow the air to move horizontally but the floors impede vertical propagation of the pressure wave.  If the building had a large atrium or substantial service shafts, this would allow quicker vertical propagation.

Remember that it isn't necessary to "blow away" the lower parts, only to get them to move outwards a couple of feet.     
« Last Edit: 10/12/2013 20:19:44 by alancalverd »
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Offline Aemilius

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Re: What is free fall?
« Reply #77 on: 10/12/2013 20:25:42 »
So the failure was all due to buckling.... None of the columns were actually blown away?

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Offline alancalverd

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Re: What is free fall?
« Reply #78 on: 10/12/2013 23:21:51 »
Try sitting on a corrugated cardboard box, then get someone to kick the side of the box! Minimally stiff structures live up to their name.
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Offline Aemilius

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Re: What is free fall?
« Reply #79 on: 12/12/2013 10:19:27 »
So, none of the columns would have actually been blown away by the explosive pressure that had built up.... but some of the columns would certainly have been weakened and buckled by the blowout.
« Last Edit: 12/12/2013 10:26:51 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #80 on: 12/12/2013 11:32:49 »
Pretty much the point. Once one column has buckled, the weight of the floor around it and all the structure above and outside it will exert a torque on the neighbouring columns  and they will begin to bend. As soon as the next one has gone beyond its elastic limit you will have more floor area exerting torque on the next nearest neighbours....Hence the gentle initial acceleration building to near-free fall.

It's a mistake to think of modern buildings as composed of independently stable parts - the whole structure is only rigid if it retains full integrity. There were several instances of half-built "box girder" bridges collapsing in the Sixties: these structures were brilliant on paper and in the laboratory but only met their design strengths when complete - which by definition doesn't happen on a building site until the last day. And like office blocks, they were also vulnerable to serious weakening through minor damage. We live with stressed-skin aeroplanes and racing cars because they can be inspected easily, withdrawn from service if damaged, and repaired under non-stress conditions, but civil structures can't.   
« Last Edit: 12/12/2013 19:35:38 by alancalverd »
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Offline Aemilius

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Re: What is free fall?
« Reply #81 on: 17/12/2013 00:06:43 »
Cool theory, but it doesn't add up. It seems to me that for your "aerostatic blowout" model to work here it would have to take into account how all the remaining perimeter and core columns could all have essentially failed at once (starting with the descent of the East Penthouse), rather than progressively like the scenario described above, over a span of about 8 stories in order to match the conditions that we know must have, or very nearly must have existed....


....beneath the falling visible part of the building during its observed descent at gravitational acceleration for 105 feet in 2.25 seconds....


The shockwave/air blast in your model that would have been created by the initial collapse of the West Penthouse within the building by the "piston effect" would not have been powerful enough (left) to account for the creation of those conditions (center), and even if it was powerful enough to have blown out all the cladding and windows and buckle all the remaining perimeter and core columns too (right), which I very much doubt it would have been powerful enough to do, the building still wouldn't have gone into free fall....


Buckling columns don't just go from a hundred percent to zero percent when they buckle, they go from a hundred percent to zero percent while they buckle and that takes time. Whether one column or a hundred, no free fall....

« Last Edit: 01/02/2014 13:45:57 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #82 on: 17/12/2013 16:22:25 »
Have it your way if you wish - magic?

The rate of stress propagation throughout the structure is the speed of sound, which in steel or concrete is 5 to 7 times its speed in air - about 1 mile per second. And remember that the video does not show immediate free fall but quite slow acceleration in the initial few seconds.
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Re: What is free fall?
« Reply #83 on: 18/12/2013 16:45:37 »
Have it your way if you wish - magic?

The rate of stress propagation throughout the structure is the speed of sound, which in steel or concrete is 5 to 7 times its speed in air - about 1 mile per second.

Right. Looks like you found a novel (if highly improbable) "Newtons cradle" effect to explain the failure/buckling of so many columns at once. Again though, even if that were possible the buckling of all the columns at once will still not result in free fall....


And remember that the video does not show immediate free fall but quite slow acceleration in the initial few seconds.

The initial few seconds? Unless I'm interpreting this improperly, it's not a "few seconds" but only about 1.75 seconds from the beginning of the descent of the East Penthouse to the entire building going into free fall like a rock being dropped off a cliff....



Anyway it wouldn't be when it went into free fall that's amazing, what's amazing is that it went into free fall at all. For example.... Is there some point during this collapse (below right) where one could say "The columns have all clearly buckled and the level of resistance offered by them to the upper part of the building that's descending should be roughly equivalent to that of air (below left) at this point in the collapse."....


I don't think the progressive collapse of a building (below right) can achieve gravitational acceleration (below left) over time through the path of greatest resistance in a manner indistinguishable from air.... Is there an equation that describes how that would work?

   

Thanks again for responding Mr. Calverd.
« Last Edit: 22/12/2013 00:45:49 by Aemilius »

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Re: What is free fall?
« Reply #84 on: 19/12/2013 04:46:48 »
The rate of stress propagation throughout the structure is the speed of sound, which in steel or concrete is 5 to 7 times its speed in air - about 1 mile per second. And remember that the video does not show immediate free fall but quite slow acceleration in the initial few seconds.

Just thinking that's interesting. I was aware of the speed of sound in steel being greater than the speed of sound in air, but.... Stress propagation? Does that mean that if one bends a 5 foot long iron bar at one end that the other end will undergo some measurable similar type of stress a fraction of a second later? Maybe I'm not clear on the definition of "stress propagation".... Is that seen by physicists as being essentially synonymous with "sound propagation"?
« Last Edit: 24/11/2015 00:58:25 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #85 on: 19/12/2013 18:48:41 »
The cross bracing and floor reinforcements of a modern building are such that there may be several "rigid" paths, allowing stress to be propagated by tension and compression, between any two points. Tension and compression forces are transmitted at the speed of sound - by definition.

Imagine a simple cubical box frame. It has tension and compression rigidity along the axes of the struts but is very weak in shear and torsion. So we add diagonal tensioners, or fill the faces with compressively rigid panels, and now any force applied to a corner will be transmitted rapidly to all the others.   

Bending a single bar beyond its elastic limit at one point won't transmit much stress to the other end, but in a real building the ends are connected  by a whole lot of other bits of rigid structure. Hence you have to look at your collapsing building as an entity, not a bunch of disconnected components.
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Re: What is free fall?
« Reply #86 on: 19/12/2013 20:38:35 »
Thanks Mr. Calverd.... understood.

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Re: What is free fall?
« Reply #87 on: 21/12/2013 06:14:48 »
Hi Mr. Calverd. As you outlined earlier....

Once one column has buckled, the weight of the floor around it and all the structure above and outside it will exert a torque on the neighbouring columns  and they will begin to bend. As soon as the next one has gone beyond its elastic limit you will have more floor area exerting torque on the next nearest neighbours....Hence the gentle initial acceleration building to near-free fall.

....in a progressive collapse due to buckling, the buckling of one column leads to the buckling of the next, and the buckling of that column leads to the buckling of the next and so on. In that scenario (starting with column 79 which would be on the left) we should, at least to some degree, expect to see some recognizable signature of that failure mode during the collapse. It should have progressed across the 300 foot wide facade of the building from left to right during the buildings descent, but we don't see anything like that. Buckling can't account for how the conditions required for free fall that we know must have existed, or very nearly existed, could have arisen beneath the visible upper part of the building as it descended either. That scenario doesn't match observations....

Post blowout (slower than free fall) progressive column
failure due to buckling.

The rate of stress propagation throughout the structure is the speed of sound, which in steel or concrete is 5 to 7 times its speed in air - about 1 mile per second.


With the "speed of sound stress propagation" approach, by which (theoretically) all the remaining columns throughout the building could've been buckled at roughly the same time following the buckling of column 79 and subsequent collapse of the West Penthouse, any "aerostatic shockwave" that resulted, just as with the first scenario, would only result in buckling at best which, though it may account to some extent for the initial uniform descent of the facade, again, doesn't account for the conditions required for free fall we know must have existed, or very nearly existed, or how those conditions could have arisen beneath the visible descending upper part of the building....

Post blowout (slower than free fall) simultaneous
column failure due to buckling.
 
In neither of those scenarios (or any scenario that only buckles the columns) would an aerostatic blowout have been powerful enough to have physically blown out all the remaining columns, and since they could at best only have caused the columns to buckle, knowing as we do that buckling columns cannot give rise to or match the conditions required for free fall to occur....



....neither the mechanism of buckling nor the outcome predicted by it can in any way account for or explain the prevailing conditions we know the upper part of the building must have fallen under....


In other words, it's inconsistent with the observation of free fall since we know buckling cannot give rise to the conditions required for free fall to occur, so the "areostatic blowout" model for how the building came down fails.

Nice try though!
« Last Edit: 11/01/2014 11:17:05 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #88 on: 22/12/2013 16:11:42 »
Quote
the initial uniform descent of the facade,

not seen on the video. You can see the propagation of a shockwave diagonally across the front of the building and the roof accelerates up to near-free-fall for several seconds. The acceleration only decreases when the collapse is almost complete.

My concern is that you are unlikely to accept any explanation other than the sudden magical and simultaneous disappearance of all the steelwork, which would be a proud first for the demolition industry, especially if it rematerialised on the far side of the moon.
« Last Edit: 22/12/2013 23:53:52 by alancalverd »
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Re: What is free fall?
« Reply #89 on: 23/12/2013 01:07:13 »
You can see the propagation of a shockwave diagonally across the front of the building and the roof accelerates up to near-free-fall for several seconds. The acceleration only decreases when the collapse is almost complete.

A moot point. Your catastrophic aerostatic blowout/speed of sound stress propagation shockwave (really reads more like it was hit by an asteroid!), as we discussed earlier, could have only buckled some of the columns, and even had it been powerful enough to buckle all the remaining columns, knowing as we do that buckled/buckling columns, whether weakened by heat (left) or by overloading (right), whether buckled in sequence or simultaneously, whether one or a hundred....


....cannot give rise to or match in any way the conditions required for the observed period of free fall the building verifiably underwent. In other words, whatever this "shockwave" did, it couldn't have created the required conditions for gravitational acceleration "The condition under which a body is, literally, free to fall under the influence of the local gravitational field with no resistance to its acceleration."

The model still fails.

My overall impression (not trying to be rude) is that somehow you'd like to minimize the significance of free fall by referring to it as "near-free-fall" and similar, maximize the impression of suddenness of some modes of failure that can "come down very quickly" and emphasize the inherent lack of structural integrity/flimsiness of construction in this case but, except for progressive structural failure involving things like bridges that pass through the air over rivers and such, you'll not succeed at marrying progressive structural failure of any building to gravitational acceleration.... if I'm not mistaken, it's right up there with perpetual motion. Haven't you some equation or formula that would bear any of this out?
« Last Edit: 09/12/2014 07:00:57 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #90 on: 23/12/2013 17:10:40 »
Nothing moot about it: the evidence is all on the video - or was that a fake?  Anyway here are the relevant equations

a = F/m  (Newton's Law, where F is the net force)

F -> mg (= GmM/r2) as the supporting structure fails in vacuo.

v = u + integral(at)dt  at any time

These equations describe the observed speed/time curve quite nicely. You can't "minimise the significance of free fall" because the curve pretty closely approximates to free fall over most of its length.

What, apart from structural failure and gravity, do you think could cause a structure to collapse at a rate approximating to free fall, apparently without damaging adjacent structures.? A giant vacuum cleaner, perhaps? 
« Last Edit: 23/12/2013 17:22:39 by alancalverd »
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Re: What is free fall?
« Reply #91 on: 24/12/2013 07:15:51 »
My concern is that you are unlikely to accept any explanation other than the sudden magical and simultaneous disappearance of all the steelwork, which would be a proud first for the demolition industry, especially if it rematerialised on the far side of the moon.

Any explanation? We've only discussed one theory so far Mr. Calverd, yours, so I'm not sure how your "concerns" made it all the way out there into left field. If it was magic I was looking for though, I would have instantly accepted your catastrophic aerostatic blowout/speed of sound stress propagation shockwave model.... it's a beauty!

Nothing moot about it: the evidence is all on the video - or was that a fake?

Your model only buckles the columns, and since we know buckled/buckling columns can't account for or explain the creation of the conditions required for free fall, it's moot. You can buckle all the columns on every floor of the building at the speed of light if you wish.... no free fall. You must know this. 

Anyway here are the relevant equations

a = F/m  (Newton's Law, where F is the net force)

F -> mg (= GmM/r2) as the supporting structure fails in vacuo.

v = u + integral(at)dt  at any time

These equations describe the observed speed/time curve quite nicely. You can't "minimise the significance of free fall" because the curve pretty closely approximates to free fall over most of its length.

I get the free fall part. What I was really asking you for is some hypothetical scenario with a formula or equation, even a crude graph, anything that describes how any falling object, with a starting velocity of 0, could gently accelerate building to free fall speed while overcoming resistance in the process. For example.... How long would it take a 100 pound cannon ball dropped from a height of 1000 feet (with a starting velocity of 0) working against a resistance of 10 pounds to achieve (in vacuo if you prefer) gravitational acceleration? 

What, apart from structural failure and gravity, do you think could cause a structure to collapse at a rate approximating to free fall, apparently without damaging adjacent structures.? A giant vacuum cleaner, perhaps?

What apart from structural failure and gravity? My impression is that you're still including/considering gravity driven progressive structural failure as something that might be able to explain this.... it can't. There is no gravity driven progressive structural failure mode that can result in gravitational acceleration (except for bridges and other structures that pass through air), so I don't know. The only model I've seen so far that really displays a solid one to one behavioural correspondence with the video evidence is closer to your "brick shed gas explosion near the base of the structure", which you only touched on briefly. 

Originally the idea was to firmly establish what the conditions are that govern free fall and you did that.... "The condition under which a body is, literally, free to fall under the influence of the local gravitational field with no resistance to its acceleration."....


We agreed that this (below) was a fairly accurate and reliable empirical method of determining, by comparing distance travelled and rate of descent of the red marker with a control, that the scenario weight must have fallen under free fall conditions (nothing beneath it) despite not being able to see into the space it was falling through....


....which naturally led to the comparison....



.... establishing the conditions (nothing beneath it) under which the visible upper part of the building must have fallen....


Now, you're asking me to believe that there are other conditions under which free fall can occur, and even suggesting that the two fall time scenarios below, under certain conditions, can actually be the same even though in the one scenario (left) there's considerable resistance/mass occupying the space beneath the falling object, and in the other scenario (right) there's nothing but air occupying the space beneath the falling object....


Even after agreeing about the improbability of your catastrophic aerostatic blowout/speed of sound stress propagation shockwave being able to do anything more than buckle a few columns (or even all the columns, it makes no difference) you're continuing to insist that buckling, a mode of structural failure we know doesn't create the conditions for or result in free fall....
 

....somehow created the conditions for and resulted in free fall!


Abracadabra.... Mr. Calverd.
« Last Edit: 02/12/2015 08:01:16 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #92 on: 24/12/2013 09:45:42 »
Quote
I get the free fall part. What I was really asking you for is some hypothetical scenario with a formula or equation, even a crude graph, anything that describes how any falling object, with a starting velocity of 0, could gently accelerate building to free fall speed* while overcoming resistance in the process. For example.... How long would it take a 100 pound cannon ball dropped from a height of 1000 feet (with a starting velocity of 0) working against a resistance of 10 pounds to achieve (in vacuo if you prefer) gravitational acceleration? 

It can't. If F = mg - f, the mass can never accelerate* at g.

But then it didn't, so what's the problem?

I was very careful always to state "near free fall" because there was obviously always some resistance. Not a lot, but if you calculate the slope of the linear part of the velocity/time graph very carefully you will find that it is a bit less than g, and turns over at the top as the falling roof approaches a terminal speed with increasing f.   

No need for a hypothetical scenario - this is how loaded minimally stiff structures collapse: slowly at first, then more rapidly up to terminal speed. Try it with a house of cards! 

*Remember that free fall isn't a speed but an acceleration.
« Last Edit: 24/12/2013 09:48:28 by alancalverd »
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Re: What is free fall?
« Reply #93 on: 24/12/2013 16:08:11 »
Thanks, my error.... meant to say acceleration not speed
« Last Edit: 16/05/2016 02:31:44 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #94 on: 24/12/2013 17:20:30 »
OK, so your cannon ball weighs 100 lb = mg  (if you use the proper Imperial system of units, this gives the mass m of the cannon ball as 100/g, about 3.5 slug)

Resistance = 10 lb

So the accelerating force is a constant 90 lb

So from the instant is is released, the cannon ball accelerates at a constant 0.9 g.

In order to accelerate slowly at first, then gradually more rapidly, you need to replace the constant retarding force with a gradually decreasing one. Pretty much what happened to the roof of the building, in fact. The initial retarding force was obviously sufficient to prevent the roof from collapsing since the structure was built, then a bit of the structure gave way, the roof began to move, and the supporting structure progressively disintegrated, allowing the roof to accelerate more rapidly towards g, limited by increasing aerodynamic drag until it reached terminal speed or hit the ground, whichever came first.
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Re: What is free fall?
« Reply #95 on: 26/12/2013 18:36:34 »
Hi Mr. Calverd (hope you had a nice Christmas!)....

Your model seems to be growing more complicated, like a perpetual motion machine that really can work.... if we can just manage to add that one last gear!
 
Originally, we just had the brick shed "aerostatic blowout" gear to drive the whole thing, but when it came to light that the bursting event would not have been powerful enough to remove any of the columns creating the conditions required for free fall to occur, only buckling some of them at best....

The "speed of sound in steel stress propagation shockwave" gear is added to explain an even greater and more sudden weakening and subsequent near simultaneous buckling of all the columns, but when it came to light that even more forceful buckling of all the columns at once would not create the conditions required for free fall to occur....
 
"The roof began to move" gear is then added to explain how events occurring on the roof could have caused the columns to not only have buckled, but also to have "progressively disintegrated" (presumably fast enough to create the conditions required for free fall) 20 or more stories below where they would likely have been heavier and stronger....

And all of it, the whole "aerostatic blowout / speed of sound in steel stress propagation shockwave / roof began to move" contraption of catastrophic buildup of aerostatic pressure purportedly responsible for all this is powered by just the first 25 to 30 feet of descent of the West Penthouse, when as you say, the first evidence of a bursting event is seen along with an aerostatic shockwave texturally rippling across the buildings facade (apparently without much regard for any interior walls or flooring) along now with roof movement that purportedly causes immediate buckling and subsequent progressive disintegration (progressive disintegration?) of the columns many stories below....

It's almost as though you're attempting to conjure a nuclear explosion from a firecracker!
 
And we still have one more little gear left over that will help smooth things out.... We mustn't forget that the building was very badly designed (probably a junior architect at some nickle and dime outfit working out of a garage somewhere), and as anyone with eyes can see it was clearly a heavily loaded structure that was very poorly engineered and flimsily constructed (some say just barely able to stand!), and besides, free fall never really ocurred anyway making it all quite ordinary, so.... What is all the fuss about?
 
In a nutshell, the reason you're having to do all this is because of the inadequacy of the original precedent setting example you used for your model. The aerostatic blowout in the "brick shed" analogy, wherein an explosive aerostatic blowout following structural failure in the lower part of the walls can result in complete removal of support, allowing the structural components of the roof (without the shingles) to approximate free fall for a period of time under the conditions required for free fall to occur, ultimately couldn't be used to support an aerostatic blowout in the case of the building because, unlike the complete removal of support that could be expected to occur at some point in the case of the brick shed example, the same mechanism when applied to the building leaves the columns in place.... and you've been struggling with that ever since.
 
It is impossible for any period of free fall to occur in the case of the building in the same way it could be expected to occur in the case of the brick shed from an aerostatic blowout because the conditions required for free fall to occur are actually created at some point during the post blowout descent of the brick shed in the example, but the conditions required for free fall to occur are never actually created at any point during the post aerostatic blowout descent of the real building.
 
The aerostatic model fails.

I was very careful always to state "near free fall" because there was obviously always some resistance.

As far as whether or not it went into free fall, the NIST web page just says.... "Stage 2 (1.75 to 4.0 seconds): gravitational acceleration (free fall)" ....followed by.... "During Stage 2, the north face descended essentially in free fall, indicating negligible support from the structure below." I'm assuming they're as familiar with the precise use of language as you are so I'm going with that.

After your review of the graph though, how much less than g did you estimate it was.... Do you remember?
« Last Edit: 19/02/2016 08:14:42 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #96 on: 26/12/2013 23:25:55 »
Not sure what you are getting at here. On the one hand you say it is impossible to create the conditions for free fall, and on the other you are claiming that it was observed.

NIST was properly circumspect in its language   

Quote
descended essentially in free fall, indicating negligible support from the structure below."


Qualifying adjectives are not zeroes.

Far from adding gears (not the way to build a PM machine - simpler is better) I'm just trying to explain what you can see in the video, using known physics and engineering. A real building is a very complicated and interconnected structure so the best we can achieve from a blurred external video is bound to be an approximation, and the real thing will have a lot more hidden gears.

Difficult to estimate the actual acceleration from the image I have of the NIST graph. g is around 32.2 ft/sec2 and the maximum slope of the graph is certainly " a bit more than 30". I would be surprised if it exceeded 32, and the only way it could reach or exceed 32.2 would be if the air was being sucked out of the building. You could do that with an explosive but that would produce a different shockwave pattern from what we observe in the video.
« Last Edit: 26/12/2013 23:52:47 by alancalverd »
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Re: What is free fall?
« Reply #97 on: 26/12/2013 23:56:07 »
I didn't say it's impossible to create the conditions for free fall (the brick shed is a good example), what I said was it's impossible for any period of free fall to have occurred in the case of the building in the same way it could be expected to occur in the case of the brick shed example you've constructed your theory around because free fall was observed (during the descent of the building), and that since nowhere in the course of your model playing out would the conditions required for free fall to occur be created, your model fails.
« Last Edit: 27/12/2013 00:28:38 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #98 on: 27/12/2013 00:30:57 »
In what way? I haven't "constructed a theory around a brick shed" because we know that this was a steel frame building, and I've gone to a lot of trouble to explain how torsional stresses are incurred in a collapsing steel-and-concrete structure. The "brick shed blowout" is just a simple example of aerostatic collapse under an intact roof, which can result in loads beyond the design limit. A building with significant internal structure is bound to be a bit more complicated.

Nor have I criticised the design or construction of this building. "Just stiff" is optimal for civil structures and this one had presumably tolerated its design stresses of floor loading and windage. Building codes are particularly stringent with regard to fireproofing of steelwork precisely because it is prone to spectacular collapse under torsional load, which suggests that the initiating fire was particularly intense - sprinklers and passive protection should provide adequate time for evacuation under foreseeable conditions. 
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Re: What is free fall?
« Reply #99 on: 27/12/2013 04:53:35 »
In what way? I haven't 'constructed a theory around a brick shed' because we know that this was a steel frame building...."

Maybe I misunderstood (shouldn't surprise anyone, I'm an eighth grade dropout!), but I got the strong impression, at least at the time, that in reply 68 where I described my interpretation of your theory in some detail, including a schematic animated representation that very much resembles the brick shed example....


....and judging by your affirmative response in reply 69, it looked like that was exactly what you had in mind and had originally constructed your theory around, you even mentioned it as such in connection with the NIST graph in reply 58 saying that it was "....consistent with my aerostatic model of lower-floor blowout."
 
Apologies though.... if I was mistaken.
« Last Edit: 29/12/2013 03:47:20 by Aemilius »