In this fun video, the length of the bridge shows waves traveling (I think in only one direction).
What allows a solid material like concrete to deform in such a smooth manner and then for it to return to its original form?
Or are some sort of strain relievers placed up and down, sort of like mini -hinges?
*By-the-by, the sound of the wind in the vid sounds extraordinarily like some of the compositions of Ligeti used in Kubrick’s 2001, when the monolith is touched and where that guy begins his trip to Jupiter and Beyond. But the tones are all sung in those compositions!
Everything is flexible, at some scale. And the force restoring the bridge to its original shape (and beyond, to distortions in the other direction) is gravity: The bridge’s normal position is that which puts its center of mass as low as possible, subject to the constraint from the cables.
As a professional videographer I would like to point out that long shots (zoomed in compressed video) tends to exaggerate movements to look far more dramatic than it actually is.
Yes the bridge is moving, but it is deforming over a much longer distance than is readily evident.
That’s not a concrete bridge, it’s a steel suspension bridge. If I saw a concrete bridge do that, I’d stay away. Well, I’d probably stay away in any case. But, like Chronos said, it’s just the natural elasticity of the material. Concrete also cracks a lot. You wouldn’t be able to see them from a distance, but they’re there, and allow sharper deflections.
I am not a civil engineer, but I know that bridges aren’t built as fixed, rigid structures; they are designed to bend, flex and move a certain amount based upon anticipated/modelled conditions. For a simple span, one end of the bridge may be fixed, but the other is on “rollers”, allowing it to move back and forth along its length a certain amount. A fully rigid/stiff bridge would break rather rapidly.
I’m not sure how suspension bridges are built, but it looks to me like this bridge is bending at the joints between segments of concrete/steel/span - the concrete itself isn’t rippling. It’s more like the joints in a toy snake like this one. The suspension design allows for more freedom of movement, I think.
My dad still talks about the Strength of Materials course he was taking at that time. The next day, the classroom bulletin board had a newspaper picture of the bridge with a caption that said “Damn that decimal point!” (only funny if you have used a slide rule)
Yes, if you watch the video closely, it seems there are solid segments (about 4 suspension wires apart?) and the flexing is happening at the joints between the segments. In a long shot it looks like a smooth ripple, but it is segments.
As a resident nearby, in our defense, the original Tacoma Narrows bridge was built before bridge wind-tunnel tests were standard, and before the conceptual framework for avoiding resonance in large construction projects was in-place.
And I’ll note that the replacement bridge, with only minor changes to the design (it uses the same towers, in fact) is still standing tall and proud.
