Wire breaks detected on Forth Road Bridge

Fifty wires have snapped in the Forth Road Bridge’s main suspension cables since 2006, according to its chief engineer.

An acoustic monitoring system, installed in August 2006, detects breakages among the 11,618 individual high tensile steel wires which make up each of the bridge’s main suspension cables.

Five of the fifty breaks happened on the west cable between September and December 2009, but despite this recent increase no immediate action is considered necessary.

While conceding that the number of breaks seen so far is not significant enough to merit a closer inspection, Chris Waite from the Forth Estuary Transport Authority told The Journal that information obtained through acoustic monitoring is limited: 

“It’s important to remember that acoustic monitoring can’t tell us how many wires had broken prior to the system being installed in August 2006, and it can’t tell us how many more are likely to break in future.

“Only by wedging open cables and making physical inspections of a wire sample can we make a judgement on this.”

Barry Colford spoke to the Forth Crossing Bill Committee, as MSPs continue to scrutinise plans to construct an alternative to the bridge by 2016.

Opponents of the new crossing, which is expected to cost around £2 billion, have highlighted the low breakage rate as evidence that the existing bridge is in better shape than was previously thought.

So far, two such examinations have taken place. The last, in 2008, showed that the cables had lost ten percent of their strength. Another is scheduled to take place in 2012.

Despite a number of measures taken against future cable corrosion, Mr Waite explained that past damage is likely to have caused micro cracks along the lengths of wire, which makes their durability uncertain:

“Cables will need to be monitored and inspected for the remainder of their service life.”