Protecting the lifeline of a suspension bridge
March 21, 2022
In late 2021, the Nova Scotia Utility and Review Board (NSURB) approved a toll increase to accommodate a 10-year, $280 million capital and maintenance budget for Halifax Harbour Bridges (HHB). As a public sector organization committed to transparency, HHB is publishing a series of articles outlining the details of each of the major budget components. In this article, we tackle the MacKay Bridge Main Cable Dehumidification initiative.
When you are rappelling down the side of a cliff face, the last thing you want to find is your climbing rope beginning to fray.
That’s why seasoned professionals inspect their rope for soft or flat spots before every climb. If it’s not functionally optimal and safe, it’s time to retire the rope.
In the same way a climbing rope is integral to safe rappelling, the main cable of a suspension bridge is key to holding a road deck safely in place so vehicles can cross over challenging geography, rivers, or in the case of Halifax, Halifax Harbour.
The engineering of a suspension bridge is a mind-boggling combination of physics and gravity, but in the simplest terms, the road deck hangs on vertical suspender cables (hangers). With minimal direct support from beneath the deck, the weight of the bridge deck and traffic is transferred by the main cables to the towers, the cable bents, and the anchorages at both ends of the bridge.
CABLES ARE CRITICAL
On the MacKay Bridge, each main cable is about 15 inches in diameter and composed of 61 galvanized cable strands arranged in a hexagonal pattern. The cables are covered with a complex protective coating to bring the hexagonal shape to a round shape. The cable is wrapped with galvanized wire and then sealed with a rubberized coating.
When the MacKay Bridge was opened in 1970, the main cables were inspected annually for the first two decades. When those inspections began showing evidence of corrosion resulting from moisture, the frequency, and level of detail of the inspections increased.
“In our most recent inspections, the cable is showing notable signs of moisture, especially at the low points. We’ve seen some surface corrosion, and a rare broken wire, but the main cable remains safe,” says Chief Bridge Engineer Ahsan Chowdhury.
“But if, moving forward no action is undertaken, the capacity of the cable will decrease – and protection of capacity is considered the most efficient and reasonable way to maintaining the cable over the life of the bridge.”
As it is impossible to strengthen or replace the main cables without major traffic disruptions, HHB is looking at installing a cable dehumidification system to protect the existing cables from further deterioration. Dehumidification of enclosed steel cables on suspension bridges has been accepted and employed as an active corrosion protection method on landmark bridges worldwide for 50 years.
Just over $11.5 million has been budgeted for design, build, and installation.
HOW DOES MAIN CABLE DEHUMIDIFICATION WORK?
When dehumidified (dry) air is injected into the cable, it absorbs trapped moisture from the cable and cable wrapping. The saturated air is then exhausted through ports further up the cable. Over time, moisture in the cable is reduced to levels that eliminate the possibility of cable corrosion. As steel does not corrode when the relative humidity (RH) is below approximately 40 per cent, the cable is protected from corrosion. The system also produces pressure inside the sealed cable to prevent further water and moisture from entering the cable.
Dehumidification systems typically include three primary components: an effective sealing system applied to the span’s main cable; a dehumidification plant that creates and blows dry air through the cables and a monitoring system that automatically adjusts the system to real-time conditions.
The Macdonald Bridge had a cable dehumidification system installed during the Big Lift project.
As HHB has been considering options for the future of the MacKay Bridge, engineering staff have looked at the successful implementation of dehumidification systems at Maryland’s Chesapeake Bridges, Delaware’s Memorial Bay Bridges, and elsewhere.
“The structural integrity of the bridge is not in question. The safety of our users and our staff is not in jeopardy, but this is an important project to ensure the continued structural integrity of the bridge until we move forward with a major rehabilitation or replacement option that must be in place by 2040,” says HHB CEO Steve Snider.
In February 2022, HHB issued a tender for the preliminary design, drawings, specifications, and engineering services for the system. By June, HHB expects it will be able to issue a tender for the building and installation. The installation and testing are projected to take two years with minimal disruption to traffic.