Three Sisters Bridges, Sixth Street Bridge, Spanning Allegheny River at Sixth Street, Pittsburgh, Allegheny County, PA
Significance: The Three Sisters Bridges represent an adaptive engineering design response to political and technical concerns. County engineers successfully maneuvered around federally-mandated clearances, aesthetic and financial considerations raised by local agencies, and the lack of adequate anchorage points along the river banks. The structures are the only trio of nearly identical bridges and among the few surviving examples of large eyebar suspension bridges in the U.S. They were the first self-anchored suspension bridges built in this county.
The Sixth Street Bridge is at the site of Pittsburgh's first Allegheny River bridge, Lothrop's 1819-20 wooden covered St. Clair Bridge. Replaced by a John A. Roebling suspension bridge in 1860, the bridge was rebuilt again in 1892 using a Theodore Cooper bowstring truss design to support growing railway traffic between Pittsburgh and adjacent competitor, Allegheny City, north of the river. The current three-span structure measures 995' with a main span of 430'. The design's deck-stiffening girder provided compressive support while lowering visual barriers between Pittsburgh and the historically distinct Northside, annexed in 1907. The American Instititute of Steel Construction named the Sixth Street Bridge "The Most Beautiful Steel Bridge of 1928."
Unprocessed Field note material exists for this structure: N582
Survey number: HAER PA-490-A
Building/structure dates: 1926-1928 Initial Construction
National Register of Historic Places NRIS Number: 86000017