Thread: Tacama narrows

hi

How wind yes wind can make a birdge made up of concrete to oscillate . ?????
I mean TACAMA NARROWS?????

Perhaps Billco is more qualified to answer this one...
but as far as I understand, bridges are made in such a way so as to be flexible (imagine the concrete pieces not rigidly attached to each other).

As far as the Tacoma Narrows bridge, the wind was such that it created resonance (probably not a perfect resonance, but enough to collapse it).

Cheers,
william

Firstly, it was not a concrete bridge. It was a steel box-section suspension. I suggest rather than me going through it you Google it.

http://www.wsdot.wa.gov/TNBhistory/Machine/machine3.htm

Will get you started.

Wind is not perfect laminar flow as well, it swirls and does lots of other things around a bridge that would cause it to oscilate - think of how a strip of paper behaves when you hold it out of the window of your car.

And the narrows may have acted like a sort of wind funnel, where the wind is sqooshes through the narrows. And down goes the bridge.

The Tacoma Narrows bridge (not Tacama, btw) collapse was a pretty embarassing engineering failure, but the case is a nice "how-not-to" for civil engineers. Bridges are usually designed to avoid flutter or vortex-induced vibrations, VIV, (that's what brought Tacoma narrows down) but in this case some civil engineer must have been sleeping on the job.

Under certain conditions (depending on wind velocity, air density and viscosity, as well as bridge cross-section, or short: the Reynolds number) the downstream side of the bridge in cross wind shows vortex shedding, i.e. as the air passes across the bridge, vortices are dynamically shed alternately on the upper and lower sides of the bridge, creating a wake known as Karman vortex street. I am sure you have seen pictures of such vortex streets in other contexts. This vortex shedding causes a periodic (poistive and negative) lift on the bridge. So far so good, there is really not much you can do about this. However, this is only a problem when the frequency of the oscillating lift is close to any of the structural eigenfrequencies of the bridge (somebody mentioned resonance!). If that happens, your bridge will fail, regardless if it's made of marshmellows, steel enforced concrete or solid titanium. And there is where you have the flaw in the Tacoma narrows design. A differently sized cross section or different stiffness would have prevented this (as for most other bridges). Amazingly, the Tacoma bridge just barely got into VIV, leaving enough time to take a movie and even walk around on the bridge as the movie was made! If the vortex shedding frequency had been closer to the structural eigenfrequency, the whole thing might have happened in fractions of a second.

Not much to add after that excellent reply.

But I can add this:
Ahh. Galloping Gerty. What I would give to have taken her for a ride (not that last fatal ride, of course...)



How would you like to be the guy who owns that car?