Michael Baker International provided preliminary and final design and bid- and construction-phase services for the rehabilitation of the historic Roberto Clemente (Sixth Street) Bridge, Andy Warhol (Seventh Street) Bridge and Rachel Carson (Ninth Street) Bridge over the Allegheny River in Pittsburgh, Pennsylvania.
The structures, built between 1925 and 1928, are collectively known as “the Sisters Bridges” because they are virtually identical in appearance. The Sisters reflect historic bridge design and construction feats and make up the only trio of identical, side-by-side bridges in the world. Pittsburgh’s geography – which notably includes three rivers that converge at the city’s “Golden Triangle” and countless hills and ravines – necessitates these crossings as part of the region’s transportation system. The bridges span the Allegheny River and connect the downtown business district to North Shore destinations like PNC Park and Acrisure Stadium. Because of their location in downtown Pittsburgh, the bridges are near numerous businesses and residences and are integral for multimodal forms of residential transportation.
While the Sisters Bridges are essentially identical and often referred to as a collective unit, they have aged in different ways. As such, the Michael Baker team approached the rehabilitation of each structure separately as each had its own needs and posed its own challenges. Due to the bridges’ age, history and importance to the city, materials not normally used in modern bridge construction were required to complete the rehabilitations.
The Michael Baker designers combined recognition of historical significance with modern engineering practices to complete a structurally superior, sustainable rehabilitation project that was sensitive to the historic nature of the bridges, while also being eye-catching and enhancing their beauty as well as the beauty of the surrounding areas. The existing hold-down mechanisms were replaced in their entirety using temporary systems to maintain the existing bridge vertical geometry. Structural steel repairs were provided as required to increase member strengths as well as repair damage from corrosion and vessel strikes on the Allegheny River. In addition, the project entailed cleaning the sandstone masonry substructure units of all three bridges to reveal the original appearance of the stone masonry, obscured for decades beneath the soot of Pittsburgh’s industrial past.
New highway lighting was installed to improve the road-user experience and an enhanced lighting system was installed on each bridge. This system can provide choreographed or independent lighting “shows” on the bridges using over 600,000 individual LED lights.
The completed project features many practical improvements to enhance travelers’ experience and safety, including wider travel lanes, bike lanes with buffers and delineators, improvements to highway lighting and three interpretive panels providing information about the history, design and construction of the Sisters Bridges. With the rehabilitation of all Sisters Bridges complete, Pittsburgh locals and visitors alike can enjoy convenient, safe and beautiful river crossings for decades to come.