Sunday Photo

Frank: How can you trust a man who wears both a belt and suspenders? The man can't even trust his own pants.

from Once Upon A Time In The West

In keeping with Dana Hunter's article on the new bridge at Hoover Dam, here's a panorama I took of it last year:
Image credit: Cujo359

This is from June of 2009. As she wrote, at the time we weren't sure what we were looking at. It looked like a suspension bridge with an arch, which didn't make much sense to me at the time. It had the towers of a suspension bridge, and there were already a set of cables. Yet, there was also an arch, which would have supported the same part of the span as the suspension cables. It was like seeing a man wearing a belt and suspenders. The more recent photo at Dana's place shows the completed central arch, after the deck on both sides had reached the towers. Despite what it looked like though, this was not a suspension bridge.

As this How Stuff Works article explains, arch bridges work thanks to a combination of gravity and materials that are highly resistant to compression:

Arch bridges are always under compression. The force of compression is pushed outward along the curve of the arch toward the abutments.

The tension in an arch is negligible. The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of tension on the underside of the arch. The greater the degree of curvature (the larger the semicircle of the arch), however, the greater the effects of tension on the underside.

How Bridges Work

The tricky thing about arches, though, is that they're only good for structural support when they're completed. Until then, they need support. A Nova article on arch bridges explains why they need the cables until the arch is completed:

Constructing an arch bridge can be tricky, since the structure is completely unstable until the two spans meet in the middle. One technique is to build elaborate scaffolding, or "centering," below the spans to support them until they meet. A newer method supports the spans using cables anchored to the ground on either side of the bridge. In situations where there is an active water or road way below, this method allows contractors to build without disrupting traffic.

Super Bridge: Arch Bridges

So, this bridge looked like a suspension bridge, because the cables used for it were run the same way - from anchors in the ground, over towers, and to the arch sections as they were added onto the bridge.

It's quite a bridge, too. As the London Daily Mail says:

Rising 890 ft above the Colorado River, when finished, its total length will be 1,900 ft, with its longest supported span running to 1080 ft.

Built in the shadow of the iconic Hoover Dam, which powers most of states Nevada and Arizona, the construction is the first concrete-steel composite arch bridge built in the United States.

A dam big project: Incredible images of construction work on 1,900ft-long Hoover Bridge

Be sure to follow that link for more photos, including this one of the just-completed arch.

Here's a photo of the just-completed bridge, from Wikipedia:
Image credit: Wikimedia/Central Federal Lands Highway Division

It's now named the Mike O'Callaghan-Pat Tillman Bridge, and should be opening soon.

UPDATE: Added the quote from the London Daily Mail.

UPDATE 2: Changed the narrative so the beginning matches the end. I'm done changing it now. This is why I don't write more articles about engineering.