Sunday, September 19, 2010

Sunday Photos: A Real Arch Suspension Bridge

A few weeks ago I wrote about one of the larger arch bridges on the continent:
Image credit: Cujo359

As I mentioned, I wondered at the time if it could have been an arch suspension bridge (perhaps more commonly referred to as a through arch bridge), but it later proved to be simply an arch bridge, the Mike O'Callaghan-Pat Tillman Bridge. It was only a matter of time, though, before I actually did find one.

And here it is:
Image credit: Dana Hunter (used by permission)

It's a foot bridge in Bothell, Washington, over the Sammamish River:
Image credit: Dana Hunter (used by permission)

Wait a second, you're probably wondering, where is the suspension coming from? Well, just as a bridge that has cables isn't necessarily a suspension bridge, a bridge without them isn't necessarily not a suspension bridge.

Recall that an arch is a structure whose pieces are forced together by gravity. Of course, there are other forces acting on a bridge besides its weight, like wind and earthquakes. The "suspension" part of an arch suspension bridge is how the weight of the bridge's deck is supported. Typically, that's done with suspension cables, as in a regular suspension bridge. Instead of being suspended from large cables strung over towers, though, an arch suspension bridge's deck is suspended from the arch.

That's why the arch pieces need to be tied together with some form of fasteners like the plates and bolts in this picture:
Image credit: Dana Hunter (used by permission), with notations by Cujo359

This is the top of the bridge, showing the arch, the wooden beams, and the plates that hold them in place. Note the plate that is labeled "plate" has four bolts in it to hold the vertical beam in place, as does the plate on the outside of the arch (see the first photo of this bridge). The vertical beams aren't holding up the arch; they're actually in tension. They're what this bridge is suspended by. Clever, huh?

Why choose an arch suspension design for this sort of bridge, rather than a suspension bridge? One reason might be that it's simpler to put together. One of the unique things about a suspension bridge is that it needs large, heavy anchors at each end for the main cables. A cable stayed suspension bridge can avoid that difficulty, but looks to me like it would be tricky, too. This is the footing of the arch suspension bridge:
Image credit: Dana Hunter (used by permission)

Just simple chunk of concrete with a couple bolts in it. In fact, they're usually termed "abutments" in this type of bridge. That's a far cry from the sort of anchor a suspension bridge requires.

So, that's an arch suspension bridge, albeit a much smaller one than I thought we'd found in Nevada last year, and one not using cables for suspension. Another interesting thing about this bridge is that it appears to be a standard design, rather than one that's just done for one spot. It's a foot bridge, and it could probably support light vehicles, so it's clearly not something that needs to be designed for each application. While researching this type of bridge I ran across this photo at a site that sells these sorts of things:
Image credit: Enwood Structures

While the bridge we saw doesn't look like one of their designs, I'm sure there are other companies that do this sort of thing.

Wonder if they sell kits?

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