At a Glance
The Philip Murray Bridge, also known as the South Tenth Street Bridge, is known in Pittsburgh as the bright yellow bridge crossing the Monongahela. To maintain its important role in local transportation, the city looked for a long-term solution to lower cable maintenance costs while preserving public safety.
The South Tenth Street Bridge was built in 1933 by the American Bridge Company, representing the longest bridge crossing the Monongahela River and one of only three parallel wire suspension bridges in all of Pennsylvania. Not only is it known for its vibrant yellow color but also for the key role it plays in Pittsburgh’s transportation network. It connects downtown to the South Side neighborhood and carries more than 18,000 vehicles daily onto the city's busy South 10th Street.
Because of the bridge’s significance to the community, protecting and extending its life is of upmost importance to the Allegheny County Department of Public Works. As the primary load-carrying element of the structure, the bridge's cables are especially relevant for safety maintenance. The bridge is supported by two main cables made up of 4,864 steel wires each. With each wire being about the size of a No. 2 pencil, they are securely banded together to ensure stability.
The biggest problem aging cable bridges face is cable corrosion caused by water infiltration. If there is severe corrosion, cables deteriorate and lose strength. This poses the concern of eventual breakage, deeming a bridge unsafe and unusable. In the case of complete wire breakage, a city would have to replace these cables, creating severe traffic delays and costing the city hundreds of millions of dollars.
Traditional methods for prolonging bridge cables included caulking, wrapping, and painting, but were rather costly and eventually rendered ineffective. Up until the 1990s the the cables of the Philip Murray Bridge were protected with wrapping wire and paint, when a neoprene wrap was added. The city was looking for a more long-term solution to preserve the bridge.
Allegheny County, PA used/is using dehumidification to address this/these challenge(s).
With the Phillip Murray Bridge holding such an important place in the heart of Allegheny County and its cables' corrosion, the department's engineering consultants, AECOM, made a push for innovation. They explored ways to not just rehabilitate the bridge, but also to extend the amount of time between future projects.
“The bridge is constantly exposed to water and severe weather conditions, making preserving its cables a challenge. We believe the most effective, cost-efficient, and environmentally responsible way to overcome that obstacle is dehumidification,” said Allegheny County Department of Public Works Bridge Engineering Manager Richard Connors, P.E. From here, the department shifted its efforts towards a dehumidification system.
The new dehumidification system is expected to almost completely stop cable corrosion. It works by drawing in outside air, removing the moisture to a targeted relative humidity, and storing the dry air in a plant room built into a bridge anchorage vault. A fan forces dry air between the two bridge cables along pipework to ports located on the cables. The dry air then permeates between the voids in the cable wires, collects moisture, and exhausts the now moisture-laden air out of the cables.
System controls are stationed in the plant room, but can also be accessed remotely via a website, allowing the public works department to continuously monitor and adjust the system’s relative humidity, air pressure, and fan speed to ensure optimum efficiency.
- The county anticipates saving $4 to $5 million in cable maintenance costs over the next 40 years
- The system helped the bridge meet targeted relative humidity (at 40% or below) in a little more than four months
- With remote website control, the public works department can monitor humidity levels without being at the plant room - saving the county time and money
- With the system's seasonally adjusted set points, the dehumidifier can optimize system performance while minimizing power consumption
The use of dehumidification to mitigate corrosion originated from research conducted in the 1920s and ’30s. But the first time it was used on suspension bridges was in the early 2000s, with the first project being completed in the United States in 2015. The project makes the Philip Murray Bridge the third bridge in the United States to use a dehumidification system!
Who Should Consider?
Cities looking to preserve and protect aging cable bridges.