The "Three Sisters Bridges" represent an adaptive engineering design response to political and technical concerns. County engineers successfully overcame challenges presented by federally mandated clearances, aesthetic and financial considerations raised by local agencies, and the lack of adequate anchorage points along the river banks. They were the first self-anchored suspension bridges built in the United States. The design’s solid plate deck-stiffening girder provided compressive support while lowering visual barriers between Pittsburgh and the historically distinct North side (formerly Allegheny City), annexed in 1907 in a contentious fight.