What was the height of the office floor trusses?

OneWhiteEye,

I used the landmark from the video below called "the third tower", this is a quite good hi-res xvid codec video. I have to say that I didn't do it very precisely, there was not an antenna to take, it was rotated a little bit (virtualdub has a free filter to rotate it back) and it depends of course where you take it, it probably had no hat truss that leads to a uniform collapse, I don't know.

http://www.megaupload.com/?d=I0V3WV66

E.

offline now and changing diapers...

The collapse curve for a crush-up is expected to be close to parabolic and the collapse duration is expected to be close to freefall duration when the value of E1/mh is a small fraction of g. E1 is the energy to collapse a story, m is the falling mass at any given time, and h is the story height.

If E1/mh is constant throughout a crush-up collapse, then the acceleration will be constant throughout the collapse, although smaller than freefall acceleration g, and the collapse curve will be parabolic for the entire collapse. This would require that E1 decrease with tower height such that E1/mh is the same for any position in the tower. m is the total mass above a particular story and E1 is the energy needed to collapse that particular story. The acceleration for crush-up in that case is g-E1/mh.

The total crush-up duration for constant E1/mh can be determined using the equation x = 0.5 (g-E1/mh) t^2 and solving for t knowing tower height x. If E1 is zero, then t is the freefall duration. When E1/mh is a small fraction of g, t will be close to the freefall duration.

For an E1 that is relatively constant with building height, the acceleration will decrease during crush-up. The amount of falling mass m decreases during crush-up, which increases E1/mh, which slows the collapse. The collapse curve would start out approximately parabolic but would become more linear with time as E1/mh approaches g (or E1/h approaches mg). In theory the curve would eventually tail off to a horizontal (collapse halted) at some time after E1/mh became greater than g. That tail effect would be more noticable for large values of E1/h.

The situation is different for crush-down due to the slowing effect of momentum transfer, particularly near the beginning of the collapse. The collapse curve for a crush-down would be flatter at first but would tend to approach a parabola as the collapse progressed and a large amount of falling mass accumulated at the collapse front. More mass at the front decreases the slowing effect of momentum transfer, and the large KE of the falling mass overwhelms E1. Both of those tend to make the crush-down collapse curve parabolic later in the collapse. That's roughly the opposite of what happens for a crush-up with approximately constant E1, where the acceleration decreases later in the collapse.

The above assumes no shedding, no air resistance, etc.

wcelliot
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A 10microsecond pulse reflecting off multiple surfaces will generate a "pulse" that's made up of the summation of those reflections, and the summation *is* long enough that it has features that can be heard at bandwidths below that of the original 10 microsecond pulse. The downward-slope of that "N" wave is precisely that.
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The downward slope is not precisely "that," and you are confabulating again.

The downward slope maps the return from compression to equalibrium and beyond into rarefaction.
That's a fact even Trippy could understand, despite his posting of an INFRAsonic pulse in support of your whacky ULTRAsonic assertion.
Do you understand the difference between ULTRA and INFRA sonic yet?

Arthur posted a 3.3 millisecond pulse, the amplitude of which would have destroyed the diaphram of any microphone in its proximity, despite the fact that its frequency was within the discriminable range. This also failed to support your unreferenced assertion.

On the other hand, my 5 points, contrary to your defamatory claim, were entirely correct!


=========quickandthedead==========
http://www.americanscientist.org/template/...ge/1?&print=yes

Shock waves were recognized as a natural phenomenon more than a century ago, yet they are still not widely understood. They are responsible for the crash of thunder, as well as the bang of a gunshot, the boom of fireworks, or the blast from a chemical or nuclear explosion. But these are not just loud noises.
Sound waves can be thought of as the weaker cousins of shock waves in the air:
They are both pressure waves, but they are not the same.
... but they are not the same.
... they are not the same.
... not the same.
=================================
Thanks, q&d!



====Arthur====
If you listen to the various sound tracks there is none of [i]the classic BANG BANG BANG of HE going off [/i]
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====Grasshopper====
That kind of makes some of the discourse here quite comical then.
=================
"We can control noise levels."

Are you still amused?



These spectral distribution graphs have a play button attached for the associated audio:
http://history-bytes.blogspot.com/2007/10/comparitor.html
compare, y'all.

Except Al, what he means by "With the USE OF DELAYS ... We can control sound levels" is EXACTLY the reason you get the BANG BANG BANG BANG associated with HE demolition.

Consider the ALTERNATIVE.

No Delay

Then ONE MUCH BIGGER BANG.

Which of course is MUCH MORE OBVIOUS.

Both VISIBLY and AUDIBLY

Which of course, as the sound tracks from multiple sites have shown, the collapse begins with the slow pulling in of an exterior wall and when the motion becomes obvious its well BEFORE any EXPULSIONS of high speed debris or before any loud sounds are generated by the falling structure.

As to your comparator, BFD.

One is a shaped charge set off in the open, the other is an UNKNOWN sound from an UNKNOWN source at an UNKNOWN distance.

Al, you get more desperate with each passing post.

I mean first it was 50 pages of "Dude, it wasn't 19 Arabs, the joos did it, didn't you read about the DANCING ISRAELIS"

Then it was 50 pages of "Dude, it was GLOWING YELLOW. I mean WHAT IN THE WORLD BESIDES THERMITE could POSSIBLY cause GLOWING YELLOW MATERIAL."

Now you are using as evidence the equiv of: "Dude, it was LOUD SOUNDS. I mean WHAT IN THE WORLD BESIDES EXPLOSIVES could POSSIBLY cause such LOUD SOUNDS."



Arthur

Al -

A point (of many) that goes straight over your head, is that the acoustic spectrum of a 10microsecond spike goes all the way from DC to a MegaHertz, so it includes spectral energy in all bands, including the audio band (40-20,000Hz). So I'm not saying that an explosive detonation is silent, I'm saying that you can hear it go off, but you can't hear anything distinctive between that and anything else that has a similar characteristics in the same audio band. It WILL have energy in the 20,000Hz to 1,000,000Hz part of the spectrum that you won't hear, though, and ordinary recording equipment won't capture.

Any notion that explosives have a characteristic waveform would require the contributions of this part of that spectrum, and ordinary recorders simply undersample this part of the "characteristic waveform" and won't reproduce it. You can still hear the 40-20,000Hz part of the waveform, but that's less than 2% of the "characteristic spectrum" of the detonation.

But, let's consider that part for a second - the spectrum of a real explosion goes all the way up to MHz frequencies, 20kHz is only 2% of the way up in frequency, but all those sonograms you provided seemed to peak-out at 10-11kHz. What happened to the 10-20kHz components?

See, with your expensive speakers, the top-end frequency response is limited by the mass of the speaker cone and the power of the speaker coil - the coil's force has to accelerate the speaker-cone mass to reproduce the high slew-rates associated with high-frequency sounds. Sound familiar? That's why the expensive speakers brag about their power and the REALLY expensive speakers brag about what their speaker cones are made of - low mass/high-rigidity materials.

But as you've pointed out, concrete floors, even ones 600x bigger across than they are thick (like a playing card) are still concrete, and that's fairly massive. Then again, the structural steel connected to that concrete was stressed to the breaking point, so the energy available to move that concrete was a lot greater than you speakers' voice-coils.

The point is, that a sudden release of mechanical energy coupled to a big floor will make a pretty good approximation of a woofer reproducing an explosion. It'll have some upper limit to its frequency response, though, so the top-end of its frequency spectrum will tend to drop off. I wouldn't expect it to go as high as 20,000Hz, would you?

But a block of C-4 definitely WOULD have higher-frequency components all the way up to the MHz.

So show us a sonogram from a WTC collapse that has spectral components in the 10kHz to 20kHz range and let's talk.

Oh, and find me a microphone that has a DC response. Without the DC component, the waveform captured MUST be balanced at zero, so that high-pressure shock wave has to go negative to balance-out the response. Otherwise, it'd be capable of detecting barometric pressure. You want to impress me, grab a book on Fourier Series and read a few chapters, you'll learn a lot about more about acoustics than you currently do.

shagster --- Thanks, but for crush-down that appears to depend upon assuming a constant stretch. I'm getting the best results using a constant force together with a stretch which starts at about 0.3 and decreases with the square of the increasing speed. This gives an amazingly precise fit to the data, the standard deviation is only 20 centimeters.