The Three Sisters are the only trio of identical bridges in the world. Like the Union Bridge from the first post in this series and the Fort Wayne Railroad Bridge from the second, the Three Sisters were largely a result of the River and Harbor Act of 1899 (“Act”), passed by Congress on March 3.

The year after World War I ended, federal officials set a deadline for the Sixth, Seventh, and Ninth Street bridges to comply with the Act. By 1924, Allegheny County voters approved a $29.2 million bond for constructing the Three Sisters, known locally as the Sister Bridges.

This episode is also available as a blog post here.