Wednesday, December 9, 2020

Think of a guitar string, or a tuning fork


This video shows the Verrazzano Narrows Bridge in New York vibrating in high winds a couple of weeks ago.  Listen closely.

It's just like a guitar string or a tuning fork.  Get it vibrating at the right frequency, and it sings to you.

If the bridge is properly built, it'll withstand those stresses.  If it's not, like the infamous Tacoma Narrows Bridge known as "Galloping Gertie", things can get very interesting . . .

I guess things have come a long way in bridge design since 1940.  Even so, it's kinda scary to see the Verrazzano Narrows Bridge sway so ominously.



Coyote Hubbard III said...

Slightly funny in that the way blogs thread vid links, off the main page your vid here show as vids from elsewhere, So this singing bridge vid and the Tac Narrows bridge link as Bohemian Rhapsody and Bohemian Rhapsody for Symphony. In a weird way, kinda fitting.

Unknown said...

I have sat in traffic on Verrazzano bridge while it swayed. Not fun for someone who does not like heights.
We did call the bridge by it's politically incorrect nick name.


Old NFO said...

Wind has some 'interesting' effects on structures. Also, the bridge roadbed 'effectively' forms an airfoil, which does react to the the wind appropriately.

Jonathan H said...

Any structure moves under load; we feel uneasy when a large one does it.
You can feel the movement of any bridge larger than a small highway overpass - it seems odd when you realize that even concrete can bend and sway.

Ben Yalow said...

We actually learned a lot more about bridge design from what I was first being taught in physics / engineering classes in the 60s, when I first watched the film.

At that time, we were being taught that it was a regular forced resonance, that matched the resonant frequency of the bridge, and therefore amplified the disturbances until the bridge failed. A simple answer -- but wrong.

Further studies and simulations showed it was aeroelastic flutter, which caused entirely different oscillations (and from different causes) than the simple resonance explanation. The vortices being shed from the wind weren't causing the up-down motion enough for the bridge to fail, although that effect had been present since the beginning. It was the vortices along the upper and lower surfaces of the deck that were twisting the bridge (as is visible in the film). And a significant cause of those was the solid sides of the bridge edges.

If you look at the Verrazano (not in the film you show, but in any of the photos from the side), you'll see a gridwork, rather than vertical sides, on the bridge. That's to prevent the sort of flutter that destroyed the Tacoma Narrows bridge.

There's a good explanation of this failure at (both in written and video form).

Well Seasoned Fool said...

Passed under that bridge while it was under construction in 1964 on the deck of the General Maurice Rose. Was driven across it in 1966 from Ft Hamilton to Ft Wordsworth to be discharged from the Army.

Greg said...

It does take me back--engineering classes in 1971. I don't remember if it was physics or statics/dynamics, but it was impressive then, and still is.