A Bit Hard To Find

I’ve mentioned before the various possible locations for a bridge deck relative to the main structure, with the two most common being on top (for trusses, girders, and arches, mostly) and through (for trusses and suspension bridges, mostly). The thing about putting the roadway on top is that the bridge becomes invisible or nearly so for the people traveling over it. That’s the Crow Creek Bridge above carrying Grove Way in Alameda County, California, but you have to look to find it. The parapets are an indication, but you’ll see similar guard rails when there’s a hill nearby or a culvert; the real giveaway is that the curved and sloped road becomes straight and level.

Same bridge, more photogenic angle:

This is actually quite nice for an American concrete bridge constructed in 1913, when engineers in the US were still debating the basics of reinforced-concrete analysis and design. It was demolished and replaced in 1988, and more about that below. The roadway was 25 feet wide and the bridge total was 33 feet creating this narrow walkways/shoulders in the first picture. It’s hard to get the exact dimensions of the main arch: the overall span, including the girder spans on each end, is 100 feet. Judging by the photos, it looks like the arch is between 40 and 50 feet. Here’s a view including one end span:

And here’s the traditional 3/4 angle, from the side that hides the worst of the graffiti:

I’m not really an arch-bridge aficionado – I like them, but know far less about them than I do about truss bridges – but it seems to me that the description in the HAER text that accompanied these pictures that this was a “transitional” design is exactly right. If the big arch were masonry and the space above it filled solid, you have a nineteenth century arch bridge as built in the US. If the arch were thinner and segmented rather than semicircular – in other words, if the shape reflected the point loading from the piers between the arch and the deck – it would be a solid entry for mid-twentieth-century design. Moving closer to the present gets a bit depressing: the replacement is a concrete-girder bridge with a 125-foot span, and a deck that is 102 feet wide with a 72-foot roadway. In other words the road went from one lane in each direction to three lanes in each direction. My guess is that the old bridge was replaced not because of physical incapacity but because it was functionally obsolete: a narrow bottleneck on a much widened road.

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