That’s the Taft Bridge, AKA the Connecticut Avenue Bridge in northwest Washington, carrying the avenue over Rock Creek. It’s an impressive structure by any standard, over 1300 feet long and with its deck about 130 feet above the creek at the bottom of a steep valley. It’s a spectacular location, with the gorge and Rock Creek Park below; the multiple-arch bridge fits the site and is well deserving of its National Register status.
Structurally, it’s a rather conservative design in most ways. The main arches span 150 feet, which is pretty good but there are plenty of masonry arch bridges with longer spans, many of which have more difficult, shallower curves rather than the semi-circular arches here. The piers are high but not exceptionally so. The interesting fact about its engineering and construction is that it’s mostly unreinforced concrete rather than masonry. The deck is reinforced concrete spanning across the spandrel sub-arches (and cantilevering out past them for the sidewalks), but the main arches and the spandrel arches are all cast-in-place unreinforced concrete. The bridge took ten years to build, and it was an important ten years in the history of concrete use in the United States. When work began in 1897, there were vanishingly few reinforced-concrete buildings or bridges in the country, with most concrete being used in mass and without reinforcing. By 1907, when the work was complete, flat-slab construction (as promoted by people like C.A.P. Turner) was on its way to becoming the standard for multistory industrial buildings.
We think of concrete as a structural material, so it’s easy to forget that in the last thirty years of the nineteenth century it was seen as a way to cheaply approximate stone. It wasn’t (at that time) as strong as the best stone available, but it didn’t have to be carved and the base material was less expensive. A few years earlier, and this same bridge could have been built in stone; a few years later and it would likely have been reinforced concrete. The National Register nomination, tellingly, speaks of its structural aesthetics and describes it as the largest concrete bridge in the world when built (which may well have been true). Its structural engineering was a dead end before the work was completed, which has the advantage of leaving it as unsurpassed in its type.