Wire Breaks Reach Milestone

The news comes on the day the Forth Road Bridge’s Chief Engineer & Bridgemaster gives evidence to the Scottish Parliament committee considering legislation to authorise construction of a new Forth crossing.

Installation of the Main Cable Acoustic Monitoring System was completed in August 2006 and since that date both main cables have been continuously monitored for breaks among the 11,618 individual high tensile steel wires that make up each main cable. Microphones placed at intervals along the cables’ lengths send data back to specialist contractor Advitam, who detect the distinctive sound signatures of wire breaks and provide monthly reports to the bridge authority.

The report states that there has been a recent increase in wire breaks on the west cable, adjacent to one of the bridge towers.  Five wire breaks were recorded between September and December 2009 at this location.  However, this is not considered significant enough to warrant action, except to continue to monitor the situation and review it over time.

Barry Colford, Chief Engineer & Bridgemaster, said: “The cables will have to be monitored for the remainder of their service life, even if corrosion is successfully halted.

“The reasoning for this is that individual wires that have already suffered from corrosion are likely to contain some micro cracks along their length. The number and depth of these cracks, and their potential to grow within the dehumidified cable, is unknown. Therefore, the determination of how many of them will in turn lead to wire breaks is unknown and, as loss of strength is directly related to wire breaks, the future strength of the cables is also unknown.”

The FETA Board will consider the report at its next meeting on Friday 26 February.