FEARS THAT internal corrosion could be eating away at the Forth Road Bridge's suspension cables are to trigger a £1.2M inspection programme in the New Year, the structure's owner said this week.
The Forth Estuary Transport Authority has invited bids for the work after corrosion was discovered on similar but older structures in the United States.
The 39 year old, 1km main span, Forth Road Bridge west of Edinburgh was the longest span suspension bridge outside the US when it was built in 1964.
'In the US, it was found that main cables on suspension bridges are corroding internally, despite appearing in prime condition on the surface, ' said FaberMaunsell technical director Charles Cocksedge, consultant for the Forth Bridge works The inspection method will demand that the 5mm diameter wires, which are compacted together to make up the cables, are held apart with wedges.
Contractors will wedge open 20m lengths of the main cables to create V-shaped grooves to aid internal inspection.
A bronze chisel will initially be hammered into position to separate the cable strands before plastic or hardwood wedges are driven in.
'This is the first time this method has been undertaken outside the US, ' said client Forth Estuary Transport Authority depute general manager, Barry Colford.
Each cable on the Forth Road Bridge is made up of 11,600 galvanised high tensile steel wires.
These are wrapped in wire sheath, which must be removed ahead of work. Heavy machinery is required so developing a safe platform is vital.
'When we open up the cable we expect to see some rusty wires. Some of these will be removed to assess the galvanised coverage. This will then give us a good idea of the state of the whole suspension cable using statistical guidance from the US report, ' said Cocksedge.
This procedure is only possible for aerially-spun cables, as the wires are thin and can be easily spread. It is not possible for lock-coil-stranded cables, like those on the Tamar suspension bridge, where the cables are 10 times thicker.
Contractors are tabling their final bids next week.