D.C. is a city of many bridges, and also a city that does not rank high in bridges that are structurally sound. 13% of the District's bridges are "structurally deficient," which amounts to 2,353 bridges.
Some of these are bridges you probably take all the time. For example, the 9th Street Bridge that crosses I-395 is labeled structurally deficient by Transportation for America. That bridge was built in 1961 and carries 8,900 cars a day.
The 14th Street Bridge that crosses Outlet Channel, which was built in 1942 and carries 55,700 cars a day was also labeled structurally deficient.
Here is a map of the bridges in D.C., a red icon means the bridge is structurally deficient (click to enlarge). Though there may be problems with the others as well. You can get more details on specific bridges here.