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High wire act

A small £1.8M maintenance contract to replace access cables on the main cable strands of the Humber Bridge has flushed out some large scale innovative thinking from those involved.

A new rigging system and a crane constructed 144m above deck level have been created to increase the speed and safety of the job.

The hand rails, used when accessing the main cable for inspection and maintenance, consist of a pair of wire rope cables running parallel to the main cables with posts at 18m centres.

Originally workers would clip themselves onto the access system but the new one uses a thicker 32mm diameter strand with braced posts so a cradle can be used in future. This will allow safer and easier access to the main cables.

COMAG, the French rigging specialist which carried out the high level installation works, used Kevlar rope and a system of winches and pulleys at the bottom of the towers to hoist the new cables into place.

Kevlar weighs 54g per metre as opposed to the new handstrands, which weigh approx 10,000g/m. This meant that the Kevlar rope could be laid out by hand and then used to winch the handstrands into position.

The method avoided the use of winches at height and impressed Mark Thornton, the project manager for main contractor Balfour Beatty. “Hats off to the guys at Comag who put the sequence together – it worked fantastically well,” he says.

Once the strands were in place, the next stage of the works involved tensioning the strands to form a free catenary, above the line of the posts. It can then lift the new anchorages.

When the cables were tensioned and shaped, the workers were no longer reliant on the old handstrand system, which was useable but badly corroded. The cradle was then winched up to the top and worked its way down, installing posts.

A lifting arm across the top of the cradle allowed materials to be winched up and down when needed at post points rather than having to be carried in the cradle.

This allowed a lighter cradle design, enabling speedier operation. Existing anchorages also had to be replaced. “One of the biggest headaches was how to install the tower top anchors,” explains Thornton.

At 2m tall and weighing a quarter of a tonne each, they were too heavy and bulky for manhandling and could not fit in the small tower lift. “We really wanted a skyhook to hang things off . . .so I took it upon myself to invent one,” he says.

The solution involved building a crane on top of the tower from components that were small enough to fit in the lift. The crane was built horizontally on the top portal beam of the tower and then hoisted up to the vertical. It could then lift the new anchorages.

The 25 year old Humber Bridge is 15 years younger than the Forth and the Severn Bridges, both currently dealing with corrosion issues. Knowing that similar problems are likely in the future at Humber, some acoustic monitoring of the cables has already been done.

“Corrosion of the main cable is a problem globally and Humber Bridge Board are keen to catch it early,” says Mark The high-level access works, which started in March, are due to finish in September.

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