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Cable Guy: Mastering the Forth Road Bridge

Inspecting corrosion on the Forth Bridge Road

The Forth Road Bridge is at risk because of water corroding its cables. But that isn’t its only problem facing Bridgemaster Barry Colford.

Suspension bridges are like the triathletes of the structural engineering world. They have to be strong and tough enough to overcome a number of extreme obstacles – long distances, high winds and water.

And from the moment a suspension bridge is born, it has to be maintained to keep it at peak fitness.

The magnificent Forth Road suspension bridge spans 1005m across the Firth of Forth linking Fife and northern Scotland with Edinburgh. It opened in 1964 and at the time was the largest suspension bridge in Europe. Its personal trainer, dietician and doctor is bridgemaster Barry Colford. “Suspension bridges need a lot of looking after,” says Colford.

“There’s a danger that people will think that there’s always something wrong with them. But it’s because they move around – they are more like a piece of machinery. You wouldn’t think twice about putting your car in for a service, would you?”

Colford is also chief engineer for bridge owner, the Forth Estuary Transport Authority (FETA), and his day-to-day job involves ensuring that vehicles can cross the bridge safely, while the longer-term structural needs of the bridge are met.

The bridgemaster fraternity is a close-knit one and when Colford’s colleagues in New York discovered that Brooklyn Bridge’s suspension cables were corroding, FETA took the initiative to investigate its cables, under Colford’s predecessor Alastair Andrew in 2004 (NCE 6 January 2005).

Consultant was Faber Maunsell with Flint and Neill carrying out the peer review. Unfortunately when the west cable was unwrapped in 10 different locations, the wires were found to be in a much worse state than expected. Water had somehow entered and caused the wires to rust.

On the surface, however, there was no evidence of any ingress. “We still don’t know what causes one wire to break and one not to, or why a corrosion pit occurs where it does. Sometimes the wire appears to be in good condition and then there’s a pit,” says Colford.

He adds that there is no correlation between the position on the cable and where corrosion is worst. The cable inspection has sparked a 15-year programme of what-if scenarios, starting with installing acoustic monitoring to detect wire breaks and dehumidification equipment to dry out the cables.

Inspection of three locations on the east cable is now complete and results are due to be published imminently.

The results of this investigation “will give another point on the graph for strength loss versus time”, says Colford.

Acoustic monitoring was commissioned in August 2006 on the west cable and dehumidification began in February this year. The acoustic monitoring gives an indication of how quickly the wires in the cables are deteriorating, but no indication of how many have already snapped.

The £10.3M dehumidification process involves wrapping the cable in an airtight neoprene membrane and pumping dry air through in an attempt to prevent further corrosion.

Although the cables are made up of bundles of tightly wound wires, each cable contains 20% of voids. “But we can’t guarantee that dehumidification will work,” says Colford.

Nowhere in the world has dehumidification been applied retrospectively to a structure. Only one section of the system is operational and it won’t be until the end of next year that the system will be running.

After 18 months of dehumidification, conclusions can be drawn about whether the system is working after a third cable inspection is completed. All cable inspections are being carried out by contractor C Spencer.

If dehumidification fails, plan B is already under way – to replace or add more cables. This is being devised by consultant Fairhurst with Cowie, Amman and Whitney.

The options are to add extra cables to the side at £91M or above the existing cables at £120M, or completely replacing the cables at £122M.

“The big caveat on the first two options is whether the existing cables will last the lifetime of the bridge,” says Colford. “Whether this is possible, I’m not sure.”

In the meantime, consultant Arup is considering another Forth crossing – this time a cable stayed structure to replace the Forth Road Bridge or ease the pressure on it.

But it’s not just cables corroding. Protecting the whole structure from rusting is a major issue. Colford is overseeing the repainting of the towers and has encountered problems with 100mph gusts of wind.

“Wind always causes trouble with temporary works, cabins and sheeting; just working on an access platform is difficult,” says Colford.

The painting gantries for the tower are out of use because the floor of the gantry failed in high winds, despite being designed for gusts of up to 125mph.

“It could be due to a wind-tunnelling affect around the towers,” says Colford. The main truss on the suspended span also needs repainting. Until now they have been recoated, but the system has reached its limit and further layers could potentially cause the paint to fall off.

Instead, all the paint will have to be stripped off and a new, epoxy-based coating applied. To do this will require the bridge to be covered where the paint is applied, to prevent it entering the estuary. But this will be like creating giant sails on the structure.

“We can only do this in short sections because the wind loading on the 1006m span would be huge if it was all wrapped up,” says Colford.

Wind tunnel tests at Glasgow University reveal that the work can be carried out on five 18m long sections at any one time. This work will start after the third cable inspection results are known in 2012.

The bridge is not designed to resist wind loading, although the possibility of fitting “wind shields” to protect high-sided vehicles from toppling over is being investigated.

Currently, when high winds are forecast, drivers are notified before crossing to use other routes. However, this can take up to three hours, so it’s no wonder some lorry drivers take the risk, says Colford.

There are further concerns with the suspended deck truss. An initial assessment has revealed that it is overstressed in places, so it will have to strengthened with plates and friction grip bolts before repainting at a total cost of upto £65M.

As with anyone getting older, the Forth’s 44-year-old expansion joints are beginning to suffer. These are located on the decks near the towers.

“We’ve inspected them and they have to be replaced,” says Colford. Initial estimates by consultant Atkins were that the carriageways would need to be closed off for eight weeks while work was carried out.

“We worked hard to get a solution that would work. In the end we came up with the idea of building mini-bridges across the joints,” says Colford. Each carriageway would only need to be out of action for 1.5 weeks while the deck steelwork is strengthened locally to carry traffic over the joints and repairs take place.

This method saved £20M and will start on site at the end of 2009. At the bridge’s extremes, the anchorages are also a cause for concern.

Assessing the condition of the anchors is extremely difficult since the ends of the cables are incarcerated in concrete filled, post-tensioned tunnels cut into rock.

“What we don’t know is the condition of this post-tensioning and how we go about digging a trench to load test the upper strands,” says Colford.

A £5M investigation by consultant Fairhurst to access the anchorages has begun and the first job will be to design the load testing equipment with a fail-safe shield should any of the tendons snap.

“Since we’re in quite a saline environment, we’re very concerned,” says Colford. “Maintaining a structure is different to designing one,” concludes Colford.

As a bridgemaster, he is constantly thinking about how his decisions will affect the bridge in 50, 100 and 200 years time. “I think about what it takes to keep this structure going; that it has to be looked after with a budget”.

As a designer, you’re the custodian for a very short time.

The Bridgemaster

Barry Colford joined FETA in 1996 and earlier this year became Forth Road Bridge bridgemaster.

Colford joined with bridge maintenance and design experience under his belt from the Erskine and Connell bridges and bridges on the M80.

Engineering is in Colford’s blood – his father is a mechanical engineer. After being inspired by the construction of the Erskine Bridge as an 11-year-old, he was spurred on to pursue a career in civil engineering. After completing a BSc at Strathclyde University, Glasgow, Colford worked for consultant Babtie, which took him road building in Saudi Arabia and designing water supply schemes in Nigeria.

His career in bridges kicked off upon returning to Scotland, working for Strathclyde Council designing bridges on the M80, M74 and M77, and maintaining the Erskine and Connell bridges, and the White Cart viaduct near Glasgow airport.

When wires in the Forth Bridge’s main cables were found to have corroded in 2004, FETA had to think quickly.

The investigation cost £2.8M. A subsequent study into determining whether the cables could be replaced or new ones added cost £900,000. Acoustic monitoring at £929,000 was installed in the cables in 2006 to examine for wire breaks. But the real money is being spent on dehumidification costing £10.3M between now and 2012. This brings the total spend to £14.9M. If the cables can be sufficiently dried out, then further corrosion can be stopped. If the process does not work then cable replacement or additional cables can be installed to the tune of up to £122M.

SUSPENDING BELIEF: CARING FOR THE FORTH

The past decade has seen the structure go through some major surgery from hanger replacement to tower strengthening.

“There’s very little redundancy in a suspension bridge. Take the hangers. They take such stresses, constantly being moved back and forth, that they have to be replaced,” says bridgemaster Barry Colford.

Hangers connect the suspended span decks to the main cables on a suspension bridge. On the Forth, these steel rope hangers are between 44.5mm and 52.4mm in diameter and between 2.5m and 90m long, and carry up to 224t each. Between 1998 and 2000 all were replaced without interruption to traffic.

“Our hangers lasted 33 years and we are painting them this year to keep them going. You really have to spend money on maintenance or else you store up trouble for the future,” says Colford.

The main towers are of welded cellular high tensile steel construction, and rise more than 150m above high water level. They were strengthened in the late 1990s to support the increased weight of heavy goods vehicles crossing the bridge. But there is only so much strengthening a suspension bridge can take because beefing it up adds more weight to the structure.

When the Forth was first designed, it assumed lorry loads of 24t and 4M crossings annually. It is now subjected to 44t vehicles, and 24M crossings a year. No wonder it’s feeling a little worn out: over the years, the surface thickness has not been increased despite being worn down more quickly. The upshot is that the 38mm surface must be replaced more frequently.

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