By Aileen Robertson

A £500,000 system, already used in America, to assess damage to cables holding up bridges could be in place on the Forth road bridge by summer.

Installation of the system would cause weekend closures between April and November, excluding July and August.

It means motorists should be prepared for more disruption, after work last year to resurface the southbound carriageway, which meant a contraflow was in place at weekends.

Forth Estuary Transport Authority has already held meetings with police, Network Rail, Fife and Edinburgh councils and the Scottish Executive to discuss the closures.

An inspection of the bridge’s main cable last year, under the advice of Faber Maunsell and American consultants Weidlinger, showed that in one section of the road bridge’s main cable 31 wires had broken as a result of corrosion.

However, the broken wires represented less than 0.3% of the total and bridgemaster Alastair Andrew said, “There is no cause for bridge users to be concerned.”

FETA is due to discuss Mr Andrew’s report into the main cable inspection on March 3, when it is expected to approve the installation of acoustic monitoring equipment.

The cost of designing, supplying, installing and commissioning the system has been estimated at £500,000.

Mr Andrew said, “The inspections have shown that there is a small number of wires which have broken, and that some of the more corroded wires contain cracks that will propagate with time causing these wires to break.

“The internal inspection work being carried out offers a snapshot of the current condition of the cable. However, Faber Maunsell and Weidlinger have recommended installing a proprietary acoustic monitoring system of the type which has proved invaluable on a number of US suspension bridges.

“The acoustic monitoring system will provide information that will assist in the determination of the rate of deterioration of the cables and will be used to predict the likely remaining service life of the cables.”

Results from the initial inspection showed a “considerable variation” in the condition of the cables and in the number of broken wires at different sections.

Contractors Spencer, who are carrying out the work on site, have been instructed to “unwrap and wedge open” cables at two further low-level locations so FETA can get a better picture of the condition of the cables. This should by the end of this month.

Mr Andrew’s report continued, “So far the worst section of cable had thirty-one broken wires representing 0.27% of the total. The visual inspections are being supplemented by laboratory testing and analysis, which is still ongoing.”

The bridgemaster said FETA has an ongoing programme to assess the loading in the main cables to check their safety.

“It is still not possible to predict the full extent of the deterioration in the main cables due to corrosion,” Mr Andrew went on.

“A clearer picture of the integrity of the cables will only emerge as the inspection programme nears completion towards the end of 2005.

“Installation of a system of acoustic monitoring will give a much clearer indication of the rate of deterioration of the main cables. There is no cause for bridge users to be concerned at this stage in the investigation but equally the need for major remedial works cannot be ruled out.”