The unfortunate collapse of the I-35W bridge in Minneapolis, Minnesota last week has put attention on the country’s bridges. Many of the Washington, D.C. bridges are quite old, and well documented in the Library of Congress’ Historic American Engineering Record, available online through the American Memory website. The images here and more are all available on the site – simply click on the image and then the permalink in the description on Flickr.
Although the Washington region’s largest and best known bridges cross the Potomac and Anacostia Rivers, for some reason I am more interested in the bridges that cross Rock Creek Parkway. Bridging the river, road, and the natural gorge, the bridges are easily overlooked, but possess a subtle drama.
One of the most dramatic is Connecticut Avenue’s Taft Memorial Bridge. When it was completed in 1907 for almost $900,000, it was thought the largest unreinforced concrete bridge in the world. A series of excellent Historic American Engineering Record drawing illustrates the bridge’s design and construction.
This much smaller bridge connecting Pennsylvania Avenue to Georgetown contains its own story. Encased in cement dating from 1916, the bridge still contains large cast iron pipes constructed by the Army Corps of Engineers in 1856.
Are these old bridges, some dating back over 100 years, safe? According to the Washington Post, bridges dating from the 1950s and 1960s that raise the most concerns, since that generation of bridges was built with less steel to save money. It turns out older bridges tend to be over-engineered since their designers new less about the strength of their designs. Nonetheless, last week’s tragedy shows we cannot take our infrastructure for granted.
> W. Post: “Generation of Bridges Was Built With Less Steel,” “Inspections Note Significant Flaws, But Officials Call Area Bridges Safe”
> Library of Congress – American Memory Architectural Records