Preserving Suspension Bridges through Dehumidification

With a significant number of U.S. long span bridges now more than 50 years old, the process of effective dehumidification systems has never been more critical in maintaining the overall lifespan and preserving suspension bridges’ most critical structural components.

The U.S. has nearly 50 major suspension bridges nationwide, according to the National Cooperative Highway Research Program, all of which serve as major connections for both regional and national transportation and commerce.

Cable corrosion occurs when water infiltrates suspension cables and reacts with oxygen and contaminants. Dehumidification addresses this concern by removing water, a required reaction agent, from the internal cable environment by reducing humidity levels.

“Cable dehumidification systems include air blowers, humidity monitors, controls and mechanical equipment installed in special housings called ‘plant rooms’ which are located either on the bridge or under it,” said Jonathan Morey, PE, assistant vice president and senior bridge engineer at WSP. “A single project could require multiple plant rooms depending on the volume of air the cable requires. We then monitor the system over decades, as they’re constantly running and pumping dry air into multiple injection sites on the cable.”

There are two main phases to the cable dehumidification process:

• Wrapping and sealing the cable from end to end, so that that the dried air can pass through the cable as a conduit.
• Creating a system that provides dry air to the cable in a controlled manner and at select points that allow the air to pass through the entire cable.

The efficiency of a cable dehumidification system is measured by the relative humidity at the exhaust location. If the humidity is high, then that’s a sign of a lack of effectiveness within the system.

“The two things that impact the system’s ability to dry the cables are the volumetric flow of air through the cables – the more air going through the cable, the more water you can purge – and how dry the air itself is,” Morey said. “Additionally, we account for the long-term performance of the system when we’re designing and installing, because it could work great for several years, but then become overtaxed and perform poorly later on.”

WSP has served as owner’s engineer and as design-build contractor’s engineer to many cable dehumidification systems. Work can include oversight of mechanical, electrical and controls engineering aspects of the system, as well managing the investigation and sealing of cables, where every seam gets focused attention.

WSP has conducted cable investigations and rehabilitations on 17 of the most iconic bridges across the U.S., including the Chesapeake Bay Bridge in 2015, which was the first installation of a cable dehumidification system within the U.S. 

In a demonstration of unsurpassed continual knowledge, the team has conducted 13 cable inspection projects for 10 different bridges in the past 13 years alone, opening 168 sections of main cable. Each project was conducted following the team’s detail-oriented approach.

The Walt Whitman Bridge in Philadelphia is a recent project that includes cable and anchorage dehumidification, cable inspection and strength evaluation. The team tailored its dehumidification techniques for this project as it does each unique set of cables.

“The general approach is the same, but the application of the specific components and the layout of the system is specific to each bridge,” Morey said. “That’s based on several factors, such as the space available for the plant rooms, the electrical capacity available for the mechanical equipment, the existing fiber network and how the owner wants to access and maintain the system virtually.

“But the most critical component is the amount of air that the cable will inherently receive,” Morey continued, “because different cables receive different amounts of air. That’s why one of the first things we did on this project was an airflow test, specifically at two locations on the cables. This allows us to determine the volumetric flow that the cable will receive.”

Suspension bridges across the country operate as critical infrastructure, so extending and maintaining their operation is crucial. One way to achieve this is by continuously improving cable dehumidification systems as we continue to learn more from the performance of prior installations, and our knowledge matures and evolves.

“Whatever system is going to be installed, it should be based on the objective, continuous improvement of prior systems,” Morey said. “There should be no cookie cutter repeats. It should always be about what worked on prior projects, and how can we make things even better.”

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