Monday, January 31, 2011

Stompin At The Savoy?

Not for nothing do the armies of the world forbid their troops from marching across bridges. The rhythmic shock waves from many feet pounding in unison on the structure set up destructive 'aggregated' shock waves which bring about fatal collapse - no matter how well engineered might be the bridge.

So, dear readers, have a look at this story and ask yourselves some questions.

  • Of how many decks do you know which might withstand the pounding of thirty pissed youngsters engaging in what today passes for dancing?
  • How many decks on houses at Omaha do you imagine might be substandard?

20 comments:

Murray said...

Natural selection doesn't like to be second guessed.

kevin said...

Interesting... The Millenium Bridge crossing the Thames in London, suffered shake/wave issues from folk just walking over it... marching/dancing would have been the end of it.

WWallace said...

It's "resonant response" that is the problem.

Many small impulses at just the right time interval (frequency) can cause frighteningly large displacements -- I remember doing that on a swing bridge as a lad.

If the youngsters were inebriated, they wouldn't notice anything wrong with a heaving deck.

Adolf Fiinkensein said...

WWallace Many thanks for finding the right terminoloy for me.

stephen said...

Oh, the humanity!

(smug prick)

Lou Taylor said...

From an engineering pov just google Tachoma Narrows. It was a famous bridge that was destroyed by wind that caused the structure to reasonate. It was quite a tourist attraction for a while.

Lou Taylor said...

Once I was in SF and they opened up the Golden Gate to walkers.
Thousands turned out and jammed the whole bridge. I got about 200 metres out and realised this was a bad idea. The movement was incredible.

Falafulu Fisi said...

[I suspect that my post went directly to the spam filter perhaps too long - unless I'm being moderated. Here it is in 2 parts]

Part 1:
======

Adolf, the general physics concept is called SHM (simple harmonic motion) and it is universal. It's applicable in various domains from:

- atomic physics such as atoms only absorbs when the incoming radiation's (or photon's) frequency matches it own natural frequency (or resonant as Wallace mentioned above). This explains why certain materials absorb at certain wavelengths but completely transparent (pass thru) in certain other wavelength domains of the spectrum.

- Digital electronics such as digital filters. When you tune your radio to say, 86 FM station, you're turning the knob in the radio so that its own natural frequency can match that specific external frequency (86 FM), designers uses the half-bandwidth mean-square-power as a cut-off. When the 2 matches it then absorbs, i.e., you listening to it, while all other frequencies are not being absorbed.

- Mechanical example #1 as in the kid's playground swing sets. When the kid's frequency of pumping or humping (slowly standing & slowly sitting) matches the natural frequency of the rope/pendulum, then you notice that it absorbs and the swinger gets higher. Notice that the swinger goes highest (@ resonant frequency) a specific frequency. If the swinger goes slower or faster which is out of sync (you can clearly see this at the playground) then the swinger won't go high. WHY? Because the swinger is not humping @ the resonant frequency of the swing/pendulum itself.

Falafulu Fisi said...

[My first 2 comments disappeared or perhaps went straight to the spam folder, may be too long - otherwise I'm being moderated. I'll split them into smaller parts]

Part 1:
======

Adolf, the general physics concept is called SHM (simple harmonic motion) and it is universal. It's applicable in various domains from:

- atomic physics such as atoms only absorbs when the incoming radiation's (or photon's) frequency matches it own natural frequency (or resonant as Wallace mentioned above). This explains why certain materials absorb at certain wavelengths but completely transparent (pass thru) in certain other wavelength domains of the spectrum.

- Digital electronics such as digital filters. When you tune your radio to say, 86 FM station, you're turning the knob in the radio so that its own natural frequency can match that specific external frequency (86 FM), designers uses the half-bandwidth mean-square-power as a cut-off. When the 2 matches it then absorbs, i.e., you listening to it, while all other frequencies are not being absorbed.

Falafulu Fisi said...

Part 2:
=======
Continue on...

- Mechanical example #1 as in the kid's playground swing sets. When the kid's frequency of pumping or humping (slowly standing & slowly sitting) matches the natural frequency of the rope/pendulum, then you notice that it absorbs and the swinger gets higher. Notice that the swinger goes highest (@ resonant frequency) a specific frequency. If the swinger goes slower or faster which is out of sync (you can clearly see this at the playground) then the swinger won't go high. WHY? Because the swinger is not humping @ the resonant frequency of the swing/pendulum itself.

Falafulu Fisi said...

Part 3:
======
Continue on...

- Mechanical example #2 is the bridge collapse you have mentioned here. Well-known example is the collapsed of the Tacoma-Bridge in the US in the 1940s mentioned by Lou Taylor. It was caused by gale. Watch the collapse here on YouTube:

Tacoma Narrows Bridge Collapse

The wind frequency matches exactly the natural frequency of the bridge itself; therefore the bridge keeps absorbing this wind energy until its mechanical structure couldn't withstand this huge energy absorption leading to breakage. Engineers today have learnt to build bridges where they design its natural frequency to be outside any possible wind frequency that we're likely to have experienced (or physically possible to exist). Note that the gale that brought Tacoma was no way devastating as hurricanes we have observed in modern times. But we don't see bridges collapsed every time after some hurricanes. The reason is because those bridges have been designed to have natural frequencies outside those wind frequency ranges.

Adolf Fiinkensein said...

Fullafula

There's no moderation practiced here I have no idea whether there is any limit to the length of comment.

Perhaps you have been singing too many Welsh Methodist hymns while drinking whisky?

Falafulu Fisi said...

I forgot to add that the natural frequency of a playground swing set pendulum is shown below (the formula is under the sub-heading Mass on a simple pendulum from the http://en.wikipedia.org/wiki/Simple Harmonic Motion:

T = 2*PI*sqrt(L/g)

where:

T - period of oscillation
PI - the number pi
sqrt - short for square root
L - length of the pendulum/swing
g - gravitational constant (~9.8)

The frequency is (f = 1/T):

f = 1/(2*PI)*sqrt(g/L);

Lest say if a playground swing has a length L = 2m (hanging or pivot point to the swing's seat), then its natural frequency (using formulas above) is roughly:

f = 0.3523 hz (hertz)

or

T = 1/f = 2.8385 sec (seconds)

The swinger pushes him/herself up (until he/she reaches his max height) in roughly 2.8385/2 or 1.4 seconds, then start lowing him/herself until he/she reaches the min (which is the swing's seat level) in the same amount of time, i.e., 1.4 seconds. This is the natural frequency. If the swinger goes slower or faster than 1.4 secs (which is out of sync) then he/she won't swing to the highest point. Note that the natural frequency formula doesn't involve the mass of the swinger. This means that a fat person and a skin person will swing as high as each other as long their swing pumping frequency matches (resonates) with the swing's natural frequency.

The formula for natural or resonant frequency is pretty much the same in the varieties of domains that it is being applied to.

Here is the formula of resonant from electronics.

gravedodger said...

So now we know that thirty of the species inebriatus stupidous banging on a deck built during a previous deck party can overcome superficial good looks after the said deck has matured.
Man was that a rage or what, awesome, like monster, or whatever the current lexicon bastardisation of the language would describe. Maybe Bash is the word .
Dancing! I think we need a modern word for the activity that has replaced dancing as I knew it, dancing it sure aint.

baxter. said...

I note the house was rented for the weekend. I wonder if the owner knew he was renting it for a party crowd, and if his Insurance Co. was aware he had rented it.

Anonymous said...

Can we have some facts before we wander off claiming idiocy/youth/drinking is a factor. Don't forget a certain viewing platform built by the govt either.

Tinman said...

It must be the day for bloody stupid posts at No Minister - an unusual occurrence I admit.

Of the few decks I have some knowledge of that would hold thirty dancing people I doubt any of them would fall off.

In fact I suspect they would all stand thirty young people jumping up and down, in unison, on them.

The story mentioned "under thirty" as an age bracket and you can be damned sure if they were kids it would read "teenagers" or similar.

In all a post bloody near as pathetic as the anti-Key one immediately prior to it.

Incidentally Mythbusters did a program on the marching-on-a-bridge thing - be interesting to know their findings.

DS said...

Also neighbours said there was no noisy party. If it had been 30 old folk standing on the deck people on this blog would have been singing a different tune.

Why do so many oldies hate youth?

Andy C said...

Re the bridge, mythbuster Episode 14 revisted the question promoting it from busted to plausable.

REVISITED: Using one’s cell phone while pumping gas/petrol can cause an explosion. Busted...again.

Anonymous said...

Pics of the aftermath showed distinctive tell tale signs of damp looking rot on the wall side, from what i saw.