By Aaron B. Colorito and Justin Bouscher, Contributing Authors
This fact seems so unexpected to most Americans that it could be a “Jeopardy!” question: This jewel of Appalachia is called the “City of Bridges” because it boasts 446 of them.
Answer: What is Pittsburgh?
For those who have never visited southwest Pennsylvania, the city needs each of these structures. The Allegheny River and Monongahela River converge downtown at Point State Park, where the French built Fort Duquesne in 1754, and form the Ohio River.
Pittsburgh’s three rivers are surrounded by so many hills and ravines that these crossings have come to symbolize the city. They are part of Pittsburgh’s identity and culture, and they are essential to the region’s transportation system.
The Three Sisters Bridges are perhaps the most iconic ones in Pittsburgh. Not far from
the Point, they span the Allegheny River and connect downtown to North Shore destinations like PNC Park, Acrisure Stadium, and the Andy Warhol Museum.
The trio of bridges honor local icons: the Roberto Clemente (Sixth Street) Bridge, named for the greatest right fielder in the history of professional baseball who played for the Pittsburgh Pirates; the Andy Warhol (Seventh Street) Bridge, named for the artist, director, and producer; and the Rachel Carson (Ninth Street) Bridge, named for the noted marine biologist, writer, and conservationist.
The current crossings were designed by the Allegheny County Department of Public Works and built between 1924 and 1928 in response to a mandate by the War Department to increase vertical clearance over inland navigable rivers.
These structures reflect historic bridge design and construction feats. They were the first suspen- sion bridges erected by the cantilever method, where the bridges were erected as cantilever trusses using temporary members.
They also make up the only trio of identical, side-by-side bridges in the world, are the first self-anchored suspension bridges constructed in the United States and are listed on the National Register of Historic Places.
In recent years, it was determined that the bridges required rehabilitation because of accelerating age-related deterioration.
The owner, Allegheny County, selected Michael Baker International for preliminary engineering, final design, and construction-phase services for the rehabilitation of each bridge. L.R. Kimball, now part of TranSystems, completed the final design of the Rachel Carson (Ninth Street) Bridge.
To effectively analyze the bridges, the team’s engineers devised an approach blending modern analysis tools with traditional design methods. The bridges were analyzed for the first time using a fully three-dimensional finite element analysis.
To verify the results, the structures were also analyzed by hand using techniques formulated
in the 1940s. Michael Baker performed the first Load and Resistance Factor Design analysis of the bridge. This uncovered a mistake in the original calculations, which was addressed during the rehabilitation.
An Individualized Approach
While the Three Sisters Bridges are identical, they have aged in different ways.
The team approached the rehabilitation of each structure separately as each had its own needs and posed its own challenges. The designers combined recognition of historical significance with modern engineering practices to complete a structurally superior, sustainable rehabilitation project that was also sensitive to the historic nature of the eye-catching bridges.
The existing hold-down mechanisms were replaced 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 2005, the former Seventh Street Bridge became the first bridge in the United States to be named for a visual artist. The Andy Warhol Bridge’s rehabilitation began in 2016 and was completed in 2018.
The updates to the structure involved the replacement of the concrete bridge deck, full repainting, steel repairs of the fascia beams and sidewalk framing, floor beam connections, stiffening girders and sidewalk brackets, substructure repairs, and installa- tion of scour protection.
The second bridge to undergo rehabilitation, the Rachel Carson Bridge, began work in 2019 and was completed in 2020. Renamed on Earth Day 2006, the bridge’s rehabilitation included repairs to the steel members and replacement of the concrete deck. Additionally, service lines for utilities and the expansion dams and drainage systems were replaced.
The bridge updates also included new sidewalk framing and sidewalks.
The Roberto Clemente Bridge was the final of the Three Sisters to be rehabilitated. Renamed after the Hall of Fame baseball player in 1998, the bridge is closed to vehicular traffic on game days for the Pittsburgh Pirates and Steelers, which makes it even more significant for local residents.
The rehabilitation began in 2022, and the bridge opened to traffic again on Sunday, Dec. 31, 2023.
Rehabilitation of the bridge required the replacement of the existing deteriorated concrete deck, structural steel repairs, including a complete eyebar hanger replacement, replacement of the sidewalk framing and concrete sidewalks, and replacement of the bridge hold-down mechanisms.
Opportunities and Challenges
Because of the bridges’ age, history, and importance to the city, materials not normally used in modern bridge construction were required to complete the rehabilitations.
These included unique tie-down anchorages combining high-strength post-tensioning bar and high-early- strength cement grouts; forged steel bridge pins and nuts; permanently lubricated bronze bushings and washers; lubricants to fill the roller “nests” at the expansion towers; and bronze dedication plaques cast to replace missing plaques.
Thousands of rivets were replaced with ASTM F3125 Grades F1852 and F2280 high-strength bolts with button heads to mimic the look of historic rivets, improving structural capacity while being sensitive to appearance. Bolts were installed with a high-tech electric wrench that allowed them to be inserted and tightened from one side, thereby providing uniform torque on the nuts and uniform pretension in the bolts. Fracture-critical tie-downs were replaced with new tie-downs to add redundancy.
Pittsburgh’s history is deeply engrained in its industrial past, and over time, the sandstone masonry substructure units of all three bridges had been obscured beneath the soot from the local steel mills.
The team cleaned this masonry to reveal its original appearance.
Various methods were tested in search of the desired results, including various chemicals, concentrations, pressure washing, and even a laserablation process, but in the end, a wet-abrasive blasting method using glass bead media was utilized across all three structures.
This process offered the best compromise between restoring the original masonry surface appearance and color while minimizing damage to the hand-finished masonry surfaces. The bridges were repainted with an Aztec Gold finish coat, the iconic color of most of the bridges in downtown Pittsburgh.
Continual collaboration was required among more than 20 key stakeholders to successfully complete this project. A major challenge was to find ways that such a large group could reach agreements that benefit the majority.
The team embraced this complexity, quickly identifying issues, coming to a consensus internally on a solution, communicating these issues to the stakeholders involved, and accepting and incorporating comments and feedback into the solutions.
When the Roberto Clemente Bridge reopened on New Year’s Eve 2023, the opening ceremony coincided with the launch of an enhanced lighting system. Specialized shows can be run during holidays and can be static or dynamic.
With the rehabilitation of all Three Sisters Bridges complete, Pittsburgh locals and visitors alike can enjoy a convenient, safe, and beautiful river crossing for decades to come. RB
Aaron B. Colorito is a technical manager for Michael Baker International. Justin Bouscher is a bridge department manager for Michael Baker International.
