Concerns as inspections take place for broken cables on Forth Road Bridge

Concern has been raised over wires snapping on the Forth Road Bridge
Concern has been raised over wires snapping on the Forth Road Bridge

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.”

Corrosion was first discovered in the main cables of the Forth Road Bridge during an internal inspection carried out in 2004 and 2005. Extensive corrosion and a number of broken wires were revealed, with engineers estimating a total loss of strength of between 8 and 10 per cent.

As a result a dehumidification system was subsequently installed with the aim of preventing further corrosion and loss of strength. The first sections were switched on in 2008, when a second internal inspection showed that the cables had continued to lose strength since 2004/5.

A third internal inspection in 2012 indicated that the rate at which the cables were losing strength had been reduced and the factor of safety had not materially diminished since 2008. This gave strong comfort that the newly installed dehumidification system was successfully impeding further corrosion of the bridge wires.