By Leeza Clark

A £12 MILLION dehumidification system may be the way ahead to halt corrosion problem found in cables on the Forth Road Bridge.

Bridge operators, the Forth Estuary Transport Association (FETA), is set to appoint consultant engineers Faber Maunsell to carry out a feasibility study addressing the problem of corrosion inside the main cables.

A report, which will come before the FETA board next week, calls for an immediate study—carried out by experts from around the globe—to take place to investigate the prospect of installing a system of dehumidification on the main steel suspension cables.

This follows a detailed two-year internal inspection, completed last year, which found evidence of corrosion inside the cables resulting in some reduction of their strength.

It warned that if nothing was done to halt the corrosion then the bridge could close by 2019.

Dehumidification is a well-tried system of preventing corrosion of galvanised steel, but its use on bridge cables is less common.

The proposal is to wrap the main cables in a neoprene membrane, then dry them out by pumping in dried air at various points.

Faber Maunsell will bring together a team of world-wide experts to carry out the feasibility study—this will include the Nippon Steel Company of Japan, which has designed and installed dehumidification systems on Japanese suspension bridges including the Akashi Kaikyo, or Pearl, bridge which has the longest bridge-span in the world.

The consultancy will also link up with New York-based Weidlinger Associates, who prepared the industry’s first code of practice for the investigation of suspension bridge cables, following an American-sponsored research programme involving the Forth Road Bridge.

Bridgemaster and general manager Alastair Andrew said, “The bridge has always enjoyed the very highest standards of safety and maintenance.

“We are confident that by being pro-active in our investigations, following a very thorough and systematic approach to operational safety, we have caught the corrosion at a time when we can now do something about it.

“The team Faber Maunsell has put together to carry out this study brings together the very best in experience across the world in dealing with similar situations. They have an excellent track record in understanding the complex issues involved in tackling corrosion in suspension bridges, and, most importantly, providing the right solutions.”

Initial cost estimates to install the humidification system stand at around £12 million, which will be funded from a capital maintenance plan funded from the current £1 toll.

The first phase of the study is likely to be complete by the summer, and will prepare the way for a tender to be awarded for contractors to install the dehumidification system by the end of the year.

Construction work, to be supervised by Faber Maunsell, could start as early as spring 2007, and will take around two years to complete.

Once the dehumidification system has been in place for five years, engineers will open up the cable and assess its effectiveness.

There is still a risk that, even after the dehumidification system has been installed, corrosion might still exist in the main cables.

FETA has already committed to fitting acoustic monitoring, enabling the entire length of the cables to be monitored for any further wire breaks.