Increase in wire breaks to be investigated

A special inspection of the Forth Road Bridge main cable has been commissioned after an increase in wire breaks was detected by bridge authority FETA.

Each of the main cables is made up of 11,618 individual steel wires, wrapped into a bundle 60cm in diameter. An acoustic monitoring system to detect new breaks among these wires was installed by FETA in 2006 after the discovery of corrosion within the main cables. This system uses special microphones along the length of the cables to identify the unique sound of a wire snapping inside.

Prior to 2015, a total of 93 wire breaks had been detected, relatively evenly dispersed along the length of the two main cables. However, since late January 2015, 24 wire breaks have been recorded near the top of the south east tower leg on the east cable. Twelve wire breaks had previously been recorded in this location, but these occurred in smaller clusters over a number of years.

Barry Colford, Chief Engineer & Bridgemaster, said “The main cable acoustic monitoring system is designed to give us early warning of potential problems within the cable. I would therefore stress that, while this increase in wire breaks does merit further investigation, there are no immediate safety concerns and the cables still have more than enough strength to do their job.

“The inspection that we are now carrying out is purely to determine whether the recent increase in the rate and concentration of wire breaks is indicative of a longer term problem.

“Once we know the results of this inspection we will recommend any further action that is required to ensure the bridge remains safe and open to traffic.”