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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Dressing Up the Three Sisters

By T.R. Witcher

The first of a trio of iconic Pittsburgh bridges is undergoing a comprehensive rehabilitation.

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Pittsburgh’s three Sisters Bridges are ongoing a comprehensive rehabilitation. Work on the Andy Warhol Bridge, in the center, began in August. Courtesy of Allegheny County Department of Public Works

September 13, 2016—In 1967 the Silver Bridge collapsed in West Virginia, killing 46 people; the following year Congress passed the Federal-Aid Highway Act, which established the National Bridge Inspection Standards program. The NBIS created a rigorous schedule for the regular monitoring of bridges across the country, and continues to be a critical element in ensuring the safety of the nation's bridges.

Among the cities most affected by the inspection standards was Pittsburgh. The American steel capital straddles three rivers; at the center of town, where the Allegheny and Monongahela rivers converge to form the Ohio River, there are more than a dozen bridges, all requiring regular inspection and maintenance. Among the most distinctive are three closely spaced bridges on the Allegheny River known as the Sisters Bridges due to their similar style and color. The bridges link downtown to the city's north shore, and one now also serves pedestrians heading to the Pittsburgh Pirates's riverfront ballpark, now known as PNC Park.

Though all three bridges have received regular maintenance, the last major rehabilitation was conducted in the 1990s. Now the Allegheny County Department of Public Works is embarking on an ambitious plan to overhaul all three bridges over the next five years.

The crossings date to the 1920s; the Seventh Street and Ninth Street bridges (now known respectively as the Andy Warhol Bridge and the Rachel Carson Bridge) were completed in 1926, and the Sixth Street Bridge (now called the Roberto Clemente Bridge) was completed in 1928. They are noteworthy in that all three are self-anchored suspension bridges with eye bar links. As described in the book Images of America: Pittsburgh's Bridges by Todd Wilson, P.E., and Helen Wilson, self-anchored suspension bridges use stiffening members along the deck, tying the cable ends together to counter the cable's outward pull. The deck must be completed for the bridge to function.

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The bridges’ steel has corroded over the years and will be repaired and replaced as needed. Future curb plates will be sloped to prevent water infiltration. Courtesy of Allegheny County Department of Public Works

The American Bridge Company, of Coraopolis, Pennsylvania, built the "Three Sisters" as cantilevered trusses, keeping the river channel clear. Eye bar chains were used because wire rope could not resist the compressive forces caused by the cantilevered construction. According to written description of the bridges provided to Civil Engineering online by the county, only a few other bridges of this type exist in the United States. "Because of their complex and unique nature, renovation work is needed periodically to keep them in a state of good repair," the document states.

The bridge decks on the three sisters have undergone regular maintenance but have continued to deteriorate nonetheless. "The decks don't look as bad as they actually are, and this is attributed to the ongoing repair efforts managed by our public works staff," says Stephen Shanley, P.E., the director of the Allegheny County Department of Public Works.

Planning for the multiple rehabilitations of the bridges began in 2008. Because the bridges receive most of their funding from the federal government (80 percent) and the state (15 percent), there were a lot of procedural steps to clear. Also, incorporating the three bridges together into one design package but three separate construction contracts added to the funding and planning complexity. "It may take a little longer but these are expensive, iconic structures to rehabilitate," Shanley says.

The bridges received in-depth inspections in 2013 to determine the scope of the repairs needed, and preliminary engineering for all three began in 2014. The county began work on the Warhol Bridge, the worst of the three bridges, earlier this month; Brayman Construction Corporation, of Saxonburg, Pennsylvania, received the contract to perform the work. The county will then move to the Carson Bridge and then the Clemente Bridge. The project will cost around $25 million per bridge.

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The Sisters Bridges are self-anchored suspension bridges with eye bar links. When the repairs are completed, all three will be repainted in their signature Aztec gold color. Courtesy of Allegheny County Department of Public Works

The Warhol Bridge restoration has many elements. For starters, the existing deck will be replaced with a new composite deck. Workers will "install shear studs on the existing floor system to make the deck composite with the beams," says Shanley, adding that this is a typical rehabilitation technique to help strengthen a deck's load-carrying members.

Extensive steel repairs are planned as well, including repairs to the floor beam connections, stringers, sidewalk brackets, and stiffening girders. New curb plates will be added along the stiffening girders, sloped away from the web plate to allow corrosive water to drain properly, thus reducing future deterioration in these areas. Currently water from rain or snow melt can seep into the built-up stiffening girder boxes and deteriorate the steel members. "Due to the design of the curb plates, water tends to collect at these locations and contribute to the deterioration of the existing curb plates and access hatches," Shanley explains.

The deck buckle plates, which span between the longitudinal beams, will be repaired where required and incorporated into the new concrete composite deck. The county will refurbish the suspension tower bearings, replace existing bearings where required, and install new tie-down linkages at the ends of the stiffening girders. "We are rehabilitating the anchorages of the stiffening girders at piers 1 and 4," says Shanley. "This detail was incorporated to enhance the connection at these locations and was one of the challenges due to the location."

Workers will also place scour protection (riprap) at some pier footings and point the masonry joints at the piers. In addition, expansion dams will be replaced with neoprene strip seals, which allow for ease of movement and expansion of the structure. Utility conduits and supports will be repaired or replaced, and scuppers and downspouting will be either cleaned and repaired or replaced as needed. New slabs and overlays will be added to the approach roadways as well.

The lights on the bridge will be replaced with lighting that replicates the 1920s-era lamps that were originally installed on the bridge. Decorative lighting may be expanded in the future.

Perhaps most importantly to Pittsburghers, the bridge's structural steel will be repainted in its signature color: Aztec gold. According to Brayman, this includes painting the towers, suspension eye bars, and 1,000 ft long deck system. The county actually conducted a poll to gauge citizen interest in changing the color of the bridges. "There were suggestions indicating an interest in painting these structures different colors, but the poll results indicated the majority [wanted] to maintain the existing Aztec gold," says Shanley. The girders will be sandblasted and then a three-part coating system will be applied.

Perhaps most critical to safety, the bridge's four narrow lanes will be restriped to carry three wider lanes while maintaining the existing turn lanes at the ends of the bridge. Shanley notes that all three bridges carried four lanes until 2015, when the Clemente Bridge was reduced to two lanes and a bicycle lane. "Following a traffic study of all three structures, it was determined that the level of service for the Andy Warhol and Rachel Carson Bridges were not impacted significantly by reducing to three lanes when turning lanes were maintained at the bridge ends," Shanley explains. "In addition, the wider three-lane section is considered safer and more comfortable for motorists."

The Warhol Bridge was shut down completely last month, traffic diverted to the Carson Bridge. The bridge deck will be removed by the end of this year, and the new deck will be placed over the course of next summer. Work on the Warhol Bridge should be completed by late 2017, at which time the county will move on to the other two bridges, where similar repair work will be conducted. Work on the Carson Bridge will take place from spring 2018 to fall 2019, and work on the Clemente Bridge will extend from spring 2020 to fall 2021.

Shanley says the bridges are a "vital part of the fabric of the City of Pittsburgh and Allegheny County."

"They are iconic structures," he says, "and it is a pleasure and honor to be a part of the ongoing restoration of these historic bridges."

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