The 9/11 Forum

of the designed sort after a drop of about 0.5 m with a speed of about 0.4 m/s.

The resistance offered thereafter was (almost) solely due to

(1) inertia, i.e., conservation of momentum, and

(2) vertical avalanche term, described more fully later.

Using the crush-down equation and OneWhiteEye's antenna mast data to estimate parameterrs via Powell's method, the calculation completely removed the resistive forces described in BLGB, including the force proportional to mass above a height (measured down from the top); this last force being the designed resistance to the force of gravity. In addition, air resistance and concrete communiution, both at the crushing front, as described in BLGB, were found to be neglectably small forces for the first 3.76 seconds of collapse.

Air --- Each story contained almost 12 acre-feet of air, which had to escape somehow. Some could go down the voids in the core; some could escape through broken windows; some could go up into the forming zone B of crushed materials. I hypothesis that the air going up partially fluidized the zone B materials, giving rise to the vertical avalanche resistive force term proportional to the mass of zone B times the square of the material speed.

There is no surprise that crush-down takes about 15 seconds to complete. Of mild surprise, perhaps, is that the terminal speed is reached by 10 seconds into the collapse, according to the calculation.

Why no (designed) resistance at all after a quite short initial interval? The tilt of the upper zone C ment that no impacts would be co-axial. Moreover, a translation of the core of zone C to the south by as little as 10+ cm would be enough so that core column members would not impact end-on at all.

Fascinating.

David B. Benson wrote:I hypothesis that the air going up partially fluidized the zone B materials...

Brilliant. I think you're on to something.

Lets the very (strong) top slide off like snowball sliding across pavement, too.

Why no (designed) resistance at all after a quite short initial interval? The tilt of the upper zone C menat that no impacts would be co-axial. Moreover, a translation of the core of zone C to the south by as little as 10+ cm would be enough so that core column members would not impact end-on at all.

Puts it into perspective.

David B. Benson wrote:
Why no (designed) resistance at all after a quite short initial interval? The tilt of the upper zone C means that no impacts would be co-axial. Moreover, a translation of the core of zone C to the south by as little as 10+ cm would be enough so that core column members would not impact end-on at all.

I assume the same applies to the perimeter walls?

Result!



The lower section A core columns slice the upper section C floors.

Heiwa --- As Major_Tom has so capably shown, large sections of perimeter walls, west north and east sides (south unknown), simply broke off to fall outside the structure. Furthermore, at least the west and north walls of WTC 1 peeled away from the descending materials. Finally, almost all perimeter wall columns and sections were found outside the building footprint during cleanup.

As OneWhiteEye suggested, a fluidized crushing front could indeed move the whole upper zone C slightly in some direction; there was certainly plenty of excess energy to do so. So your picture might partially apply to the east side (initial tilt to SE, more or less). However, I would then expect futher large sections of perimeter walls to peel away from the upper zone C. AFAIK there is no evidence of this; instead there are one or two massive sections of east wall from floors 92 through 97 falling away rather early on.

Edited to also add: NIST put together a fairly sizeable section of the center south perimeter wall in the vicinity of stories 97 and 98. These clearly show that the truss seats failed vertically below floor 98 but with more varied orientations above there. I take this as definite evidence that the upper zone C south wall fell inside the lower aone A south wall.

The windows were 22 inches wide (0.48 m), 1/4 inch thick but I don't know how high. I need that to plug into the overpressure failure graph on page 5 of
http://handle.dtic.mil/100.2/AD687294.

By assuming windows 6 feet high, the graph states that 50% chance of failure at only 0.65 psi for so-called free field overpressure. Not much for a structure designed to stand up in 98 mph winds.

well, if you're now willing to admit the air pressure is significant enough to allow for skyscraper air hockey, than you must take the medicine with the sweetener.
every single floor had the same air cushion. that has never been factored in ANY collapse calculation that i've seen.
please provide femr2 any numbers you might have so he can add them to his spreadsheets.

David B. Benson wrote:Heiwa --- As Major_Tom has so capably shown, large sections of perimeter walls, west north and east sides (south unknown), simply broke off to fall outside the structure. Furthermore, at least the west and north walls of WTC 1 peeled away from the descending materials. Finally, almost all perimeter wall columns and sections were found outside the building footprint during cleanup.

According your BLGB paper and figures the upper part C is supposed to remain intact and one-wat crush down lower part A into rubble part B. Only at contact with ground/rubble heap B the part C walls should be crushed. Pls clarify.

If upper part C shifts sideways and then drop, evidently two C walls must drop inside part A.

As I observe developments part C is blown apart from inside even before colliding with part A.

newton wrote:every single floor had the same air cushion. that has never been factored in ANY collapse calculation that i've seen.

See BLGB.

Heiwa --- I have yet to see any evidence contradicting the hypothesis that (most of) zone C remained on top most of the way down. Clearly the B&V crush-down equation is an approximation, one which works quite well for WTC 1 but not for WTC 2.

On each floor once the crushing front descended below the windows there was no completely open air path. Supposing this happens three feet above the floor, there remains about 3 acre-feet of air which can only escape through voids in the core (about 6% of floor area) or up through the crushed materials or begin breaking up the floor below or even breaking apart the perimeter wall..

Now
2 psi = 288 psf,
4 psi = 576 psf
and an acre is 43560 ft^2, so the entire additional loading is
288x(0.94x43560) = 11,792,563.2 lb = 5,350 tonnes @ 2 psi
and twice that @ 4 psi, so about the mass of an ordinary trussed office story, I think. Appears to be possible to fluidize the crashed materials rather thoroughly with around 3 acre-feet of air (stp) per story.

A strange thread title given the discussion. If you are going to invest time looking at the effect of air between flat and 'air-tight' masses...

I would suggest the air should be treated as a very effective, full building footprint, velocity dampening spring.

femr2 --- Catchy title, is it not?

BLGB considers air resistance. That force is proportional to the square of the crushing front speed and as my first post indicates that is totally unimportant for the first 3.75+ seconds of the collapse; BLGB states something quite similar.

To be more complete, I am not assuming anything at all approaching air-tight. My hypothesis is that there was a nearly continuous turbulent flow of air up through the crushed material zone B and out through the sides and top. This fluidized the crushed materials to result in the calculated best-fit force, proportional to the zone B mass times the square of the material speed.

As Major_Tom has so capably shown, large sections of perimeter walls, west north and east sides (south unknown), simply broke off to fall outside the structure.

From the upper block. Not so simple. For WTC1 I also show that some of these large pieces appear cut along a straight horizontal line on their lower edge.

I prefer the word "kick-out" to describe how large sheets of the upper block displace outwards and over the lower perimeter during the earliest moments of local perimeter failure.

This initial perimeter failure seems to happen with very little observed buckling, spitting out large unbuckled sheets in every case documented so far.


So it's not so simple.



The surviving core columns (spire) with horizontal bracing still attached tell us that the debris flowed away from the core and towards the chutes, whereas peeling (of different degrees on different sides, note almost no peeling for WTC2 west face, another freakin' miracle) tells us it was mostly trapped within the "cage".



The surviving columns means there is a big, gaping hole right up through most of the entire building for air to escape. This air flow is actually visible in photos.

Major_Tom wrote:This air flow is actually visible in photos.

Could you post one?

I can show you but you'd probably just disagree with me.

Can you please explain how such tall surviving sections of the core can exist with horizontal bracing still attached without the need the debris to go around it, not through it (hence a gaping hole up the middle of the debris distribution)?

I believe we spoke of this a few hundred posts ago. If progress could be made I'd like to help, but these posts seem to repeat themselves over the months and that diverts my interest and energy.

Major_Tom wrote:I can show you but you'd probably just disagree with me.

Probably. I don't think on can see even the antenna mast after a descent of only 50 meters.

Can you please explain how such tall surviving sections of the core can exist with horizontal bracing still attached without the need the debris to go around it, not through it (hence a gaping hole up the middle of the debris distribution)?

The west and north walls peeled away sufficiently rapidly that deebris tended to move west and north near the spire. Similarly, but to a lesser extent, to south and east. There actually wasn't a gaping hole, just less density and in particular no structural steel to break connections.

One could attempt to model this, but it seems such a minor aspect for which qualitative explanations, such as above, suffice for me.