See New Tacoma Bridge
http://www.nwrain.net/~newtsuit/recoveries/narrows/cb.htm
Gus

>
> On Jul 4, 2009, at 8:35 AM, Russell McMahon wrote:
>
> If nobody has mentioned the Tacoma Narrows bridge yet  ... .
> A fine early experience of resonance and the problems with cable  
> stayed
> bridges.
> Best of all, despite the fantastic film footage, nobody was killed  
> in the
> breaking of this bridge. Several people would have thought they were
> candidates.
>
> This is by far the best film footage of the event that i have ever  
> seen -
> well worth watching.
> Sound track is execrable. Can't have everything :-).
> http://www.youtube.com/watch?v=j-zczJXSxnw
> This may have been "post processed" and possibly colorised.
> This is more like what is usually seen.
> Has better footage of the actual moment of collapse.
> http://www.youtube.com/watch?v=HxTZ446tbzE&feature=related
> Script identical in most cases.
> Prof xxx is brave beyond belief (but, see below).
> Two men seen at end of footage MUST be engineers ;-).
>
> This version provides other color footage.
> it also says the dog rescue story is untrue and that the dog died.
> http://www.youtube.com/watch?v=vu4LPTsy_xY&feature=related
>
> _________________________
>
> The london Millenium pedestrian only bridge suffered from a subtler  
> fatal
> flaw which was discovered soon after it opened. It was closed again  
> soon
> after and took AFAIR about 2 years to sort out.
> The second stage of the problem is resonance if people walk in step  
> - the
> designers were well aware of this and didn't see it as a problem. BUT,
> The first stage of the problem was an effect discovered many decades  
> earlier
> and then largely forgotten - if a bridge with a natural oscillatory
> frequency is excited by random input (lots of pedestrians) it can  
> produce a
> forcing signal that is not large enough to be damaging to the  
> structure BUT
> which drives the exciters towards the resonance point. ie the  
> walkers resond
> to the bridghe's movements by altering thei stride and gait somewhat  
> and are
> driven into resonance with the bridge so that THEN they drive the  
> resonance
> more solidly. The MB had substantial dampers fitted which  
> preventthis - they
> are visible in photos if you know that there is something there to  
> look for.
>
> _______________
>
> Anti-resoant dampers for use in de-swaying tall buildings were  
> mentioned.
> Taipei 101was at one time the tallest building in the world by AFAIR  
> 5 of
> the 6 metrics used to measure building height as agreed to by the tall
> building builders association (names have been mutilated due to  
> imperfect
> memory, facts approx correct). One floor below the viewing deck  
> there is a
> large and very heavy and very high tech suspended weight -  
> approximately
> stepped spherical.
> Ah well ... Gargoyle says ...
>
> http://en.wikipedia.org/wiki/Taipei_101
> http://www.popularmechanics.com/technology/industry/1612252.html
>
>
> World's tallest at May 2005 (I was there ???)
> Burj Dubai is now taller BUT will not officially meet the definition  
> of a
> "building" until later this year (fwiw).
>
>
> 1667 ft. 660 ft. from a major fault line in Taiwan, ... could be  
> subjected
> to earthquakes, typhoons and fierce winds ... 730-ton tuned mass  
> damper
> (TMD). It acts like a giant pendulum to counteract the building's
> movement--reducing sway due to wind by 30 to 40 percent. Constructed  
> by
> specialty engineering firm Motioneering, the damper was too heavy to  
> be
> lifted by crane and had to be assembled on-site. Eight steel cables  
> form a
> sling to support the ball, while eight viscous dampers act like shock
> absorbers when the sphere shifts. Able to move 5 ft. in any  
> direction, the
> Taipei TMD is the world's largest and heaviest. This gold-colored  
> orb is on
> view from restaurants, bars and observation decks between the 88th  
> and 92nd
> stories. A bumper ring prevents the ball from swaying too far,  
> should that
> much swaying ever need to occur. Our recommendation, in that case,  
> would be
> an immediate egress to firmer ground.
>
>






2009/7/5 Carl Denk <cdenk@windstream.net>

> Oh, yes, it's the resonance frequency and the frequency of whatever,
> like horses or other things in unison.  Think of a child on a  
> playground
> swing. At the right time a small push and the amplitude is greater if
> that push is right after the extreme limit of travel. If the push is
> just before the limit, amplitude is reduced.
>
> John Gardner wrote:
>> 20-odd hoss & riders that different from 10-ton truck?
>>
>> Jack
>>
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