Capital projects:

Main Cable Inspection

Last updated on 1 Dec 2011

Current works on site:

A £2.6 million contract has been awarded to C Spencer Ltd to carry out a third internal inspection of the main cables on the Forth Road Bridge.

Consulting engineers Flint & Neill Ltd have been appointed to oversee the inspection on behalf of the bridge authority.

Preliminary works are now under way and work on site is expected to begin in spring 2012.

The inspection will allow engineers to assess the current strength of the cables and gauge the effectiveness of the dehumidification system that has been installed to address the problem of corrosion in the 11,618 high tensile steel wires that makes up each main cable.

The main cables will be unwrapped at eight carefully chosen locations. Hardwood wedges will be driven into the bundles of wires, allowing engineers to see right into the centre and assess the condition of a representative sample. The condition of the wires will be recorded and samples taken for laboratory testing.

Background

In 2004 the Forth Road Bridge became the first suspension bridge in Europe to have its main cable opened up to check for signs of corrosion. Although the main cable showed no exterior signs of deterioration, the concern was that corrosion might be present inside – as had recently been discovered in the cables of older American long span suspension bridges.

When the cable was opened up, the bridge authority was surprised to find that 8-10% of the cable’s strength had been lost as a result of corrosion – despite the cable at that time being just 40 years old. Although such a loss is significant, it hasn’t necessitated traffic restrictions. It is crucial, however, that the corrosion is halted to prevent this happening in the future.

Engineers have now protected the cable with waterproof elastomeric wrap and fitted a dehumidification system in a bid to stem the progress of the deterioration. Dehumidification has already been used on the Forth Road Bridge in other areas and on in the main cables in of newer bridges in Japan and Sweden, but this is the first time it has ever been used inside a main cable that’s more than four decades old.

Three panels on the east cable were opened in early 2008 (two of which were opened previously in 2004) and, while the main cable was found to have suffered further strength loss due to corrosion, the rate of loss of strength was not as great as had been predicted in 2004.

This second internal inspection in 2008 indicated that the envelope of the predicted future strength of the cable, determined from the first inspection, was still valid. However, the second inspection also indicated that the timescale when intervention, in the form of loading restrictions, might be required now extended from 2014 to 2021 as results showed that it appeared more likely that the rate of deterioration was predicted to follow the more optimistic line.

Therefore, from the 2008 results it was considered more likely that any restriction in loading would not have to be considered until between 2017 and 2021.

The third internal inspection of the main cable will allow another point to be plotted on the cable strength loss versus time graph and be the first indication of the effectiveness of the dehumidification system, but further points in the future, post 2012, will need to be obtained in order to further increase levels of confidence in the capacity of the cables. It is almost certainly the case that some degree of uncertainty concerning the magnitude of future strength loss will remain and the cables will require to be continually monitored, and subject to a regime of internal inspections and strength evaluations, for the remainder of their service life.

Related documents:

The Bridge:

Facts & Figures

Opened 1964, 2.5 km long, Main span 1006 metres

Find out all the vital statistics
of the forth road bridge here

divider

No restrictions on bridge (19:34 UTC 04/02/12)