Capital Projects:

Current works on site:

Work to install an acoustic monitoring system on both main cables was completedin August 2006. Since then the system has provided continuous monitoring for breaks among the 11,618 individual high tensile steel wires that make up each cable.

Background

Following the discovery of corrosion within the main cables in 2004, the Bridge Authority acted quickly to commission an acoustic monitoring system to detect new wire breaks within the 11,618 individual high tensile steel wires that make up each cable.

The system is designed to provide early warning of any potential problem areas or increases in the rate of deterioration. It uses a series of microphones positioned along the cables to listen out for the unique sound of a wire breaking. Possible breaks are recorded and sent over the internet to be analysed by experts in Canada, who report back regularly to the Bridge Authority on the timing and location of any new breaks.

What acoustic monitoring cannot tell us is how many wires had broken before the system was installed, or how many wires are close to breaking as a result of corrosion. The only way to determine this is to wedge open the cables and carry out an internal inspection.

Commissioning of the system was completed in August 2006 and both main cables are now continuously monitored.

To the end of December 2009, a total of 50 wire breaks had  been confirmed.  This is the sum of all confirmed wire breaks on both cables. There had been a recent increase in wire breaks on the west cable, on the south side span, at the panel immediately adjacent to the tower.  Five wire breaks were recorded between September and December 2009 at this location. However, given that there are 11,618 individual wires in each cable it is not considered that this activity is significant enough to warrant action except to continue to monitor the situation and review it over time.  Information on wire breaks will be useful intelligence when selecting panels to open up for the next internal inspection of the cables.

Some down time on parts of the system has again been experienced but the system remained live through the December 2009 monitoring period for 99.5% of the time.

Related documents:

• Main cable and anchorages update (17 February 2010)
• Main cables - update report (30 November 2006)
• Work on the main cables - update report (25 August 2006)
• Forth Road Bridge – First internal inspection, strength evaluation, acoustic monitoring and dehumidification of the main cables (Barry R Colford & Charles P E Cocksedge)