[Aemilius (OP)]

Hi RD (nice to meet you).... very cool, I'll check it out.

[Aemilius (OP)]

I'm waiting with bated breath for the moment when Bin Laden's face appears in the smoke, or the lizard changes into George W Bush.

Did you have something on your mind Mr. Calverd?

[Aemilius (OP)]

So I want to look at the clamp release thing again. In this example, the conditions under which the scenario weight falls will be obscured by a 15 foot tall blind. The conditions below the weight as it falls aren't visible....

Another way to know what conditions the weight fell under, besides actually being able to see the scenario weight or into the space beneath it, is by just looking at how the attached red marker moves compared to the control. Putting the marker on the scenario weight makes the distance from the bottom of the scenario weight, at 15 feet, to just under the red marker, another 15 feet, a total of 30 feet (plus the red marker).

The scenario with the control included is....

Then put the 15 foot blind in front of the space beneath the scenario weight. If the scenario weight falls 15 feet, the marker will be visible just above the 15 foot tall blind at the same height the scenario weight was at before it was released.

As usual, the comparison begins with the opening of the clamp and the appearence of the control. In the animation, comparing the marker to the control shows that the scenario weight came down at free fall, there absolutely can't have been anything beneath the scenario weight that would've tended to impede it's progress ....

By the same token, if there's anything at all beneath the scenario weight that would tend to impede its progress the marker will (to one degree or another) descend at a slower rate than the control. In the animation, comparing the marker to the control shows that the scenario weight came down at less than free fall, there must've been something beneath the scenario weight that tended to impede it's progress....

[alancalverd]

So far, so obvious. I'm sure this is leading somewhere. Can we cut to the chase?

[Aemilius (OP)]

So far, so obvious. I'm sure this is leading somewhere. Can we cut to the chase?

Sure. Like I said earlier, it has to do with something I read about, a building collapse. The name of the building or where it was located.... I'm not really interested in that. I just want to know how it could have gone into free fall as a result of a progressive structural failure. The NIST has apparently confirmed that it went into free fall, after doing a formal pixel by pixel analysis of the video, for 2.25 seconds (8 stories, or approximately 105 feet).

[img[https://web.archive.org/web/20151125115847im_/http://picasion.com/pic76/6c7cd2005f1c75d081a720e434c5c713.gif[/img]http://

Just like my scenario though, in the video, we can't see into the space beneath the visible falling portion of the building because of other buildings (blue) in the foreground....

So that's what I'm curious about. The way the building came down is consistent with free fall acceleration....

But if free fall occured, and it was a progressive failure, it would seem to end us up in a rather awkward, even impossible situation like this....

We can't have it both ways can we? What's your take?

[Aemilius (OP)]

Am I missing something?

[alancalverd]

Difficult to comment without seeing the actual video, but a lot depends on the internal structure of the building.

Consider a simple brick-built shed with a pitched, trussed roof (I've just rebuilt one!)

If you had a gas explosion near the base of the building, the bricks would blow outwards but the roof would remain fairly intact as the trusses can withstand tension as well as compression, so the entire roof would fall like a parachute. Now blow away the roof tiles (which will happen after a few seconds' descent, because shingles are only intended to support forces from outside)  and the "parachute" approximates to your dense weight.

I can envisage a building where progressive failure in the lowest part of the walls becomes explosive as the upper part and roof accelerates downwards, with the lower walls bursting like an aneurysm under the increased internal pressure. In its simplest form the model is a cylinder whose walls are supporting a weighted piston. Once the cylinder begins to give way, the piston starts to compress the air inside and bursts the walls, which then allows the piston to fall. The total outward aerostatic force on the walls, once the building starts to collapse, equals the weight of every part that is no longer supported. Very few buildings (apart from nuclear power stations and the like) are designed to withstand outward force.   

[Aemilius (OP)]

Difficult to comment without seeing the actual video, but a lot depends on the internal structure of the building.

Right. Here's a schematic rendering of the building, the video of the collapse the NIST used, and also the arrangement of the 81 steel support columns that held it up showing where the progressive failure began. Column 79 fails first (due to heating), followed in rapid succession by the other 80* columns.

*Correction.... 12 columns were damaged prior to column 79 failing, leaving 68 columns.

       

Consider a simple brick-built shed with a pitched, trussed roof (I've just rebuilt one!)

Sounds like quite a project. You should post a picture.... I'd like to see that!

If you had a gas explosion near the base of the building, the bricks would blow outwards but the roof would remain fairly intact as the trusses can withstand tension as well as compression, so the entire roof would fall like a parachute. Now blow away the roof tiles (which will happen after a few seconds' descent, because shingles are only intended to support forces from outside) and the "parachute" approximates to your dense weight.

Makes sense, but no natural gas or other explosives were in the building. There was stored diesel fuel though, which is said to have contributed to a couple of the 5 or 6 fires that burned here and there on various floors throughout the building.

I can envisage a building where progressive failure in the lowest part of the walls becomes explosive as the upper part and roof accelerates downwards, with the lower walls bursting like an aneurysm under the increased internal pressure. In its simplest form the model is a cylinder whose walls are supporting a weighted piston. Once the cylinder begins to give way, the piston starts to compress the air inside and bursts the walls, which then allows the piston to fall. The total outward aerostatic force on the walls, once the building starts to collapse, equals the weight of every part that is no longer supported. Very few buildings (apart from nuclear power stations and the like) are designed to withstand outward force.

That makes sense too, but in this case, at the point the building went into free fall, it hadn't descended far enough to have developed the kind of extreme internal presurization that would have been needed to blow out the walls, windows and steel columns over a span of 8 stories. Even if it had, the built up pressure would have blown out windows to relieve/vent the accumulated pressure, not steel columns.

[alancalverd]

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. 

A lot of work was done on this sort of phenomenon in the early days of nuclear warfare, where most of the damage is caused by the compression and rarefaction waves, but most of the buildings I have seen tested were either wooden huts (inherently more burstproof structure) or concrete bunkers designed for the purpose of withstanding rarefaction.   

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.

My recent construction was actually renovating an old barn: we've replaced several wooden uprights with brick or steel columns and strengthened the roof timbers. This was only possible at an economic cost because the original roof was made from overdesigned tied trusses so the framework could be cut and patched piece by piece without the whole lot collapsing. Modern roof structures tend to be minimally rigid and more difficult to repair. It looks very much as though the video'd building was also minimally rigid.

[Aemilius (OP)]

Thanks Mr. Calverd. Busy day ahead.... will get back to this.

[Aemilius (OP)]

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.

[alancalverd]

Not nearly as remarkable as being asked a sensible question by someone who seems to care about the answer!

Keep drinking the good stuff.

[evan_au]

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.

[Aemilius (OP)]

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?

[Aemilius (OP)]

Just noticed your "Profile". Telecommunications.... Electrical Engineer?

[Pmb]

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.

[Aemilius (OP)]

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."

[Aemilius (OP)]

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!

[alancalverd]

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.

[Pmb]

That's incorrect.

Nope. In fact it's very correct.

It's not at all wrong to say that it's negligible in this case.

You don’t seem to have read my post very carefully. I said that you can’t talk about what is low speed in all generality because what works in one case doesn’t work in all cases.

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

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 I’ve been a physicist for over a quarter of a century you don’t 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 body’s spatial extension is large compared to the region over which tidal forces can’t be ignored then tidal forces will affect the body’s rate of fall. I.e. in a curved spacetime large objects don’t move on geodesics.