Main cable inspection

An internal inspection of the Forth Road Bridge main cables is programmed to be carried out during 2018.

A tender competition to appoint a suitably qualified contractor has been carried out and a contractor is expected to be appointed during September 2017. The contractor will then have until March 2018 to either refurbish the existing main cable compacting and wrapping machines and the high level access gantries or to manufacture new items. Site works are due to commence in April 2018 and should last for approximately six months.

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.

The process involved unwrapping the cable at carefully selected locations and driving hardwood wedges into the bundle of wires, allowing engineers to see right into the centre and assess the condition of a representative sample. The condition of the wires was recorded and samples taken for laboratory testing. Specially designed access gantries were used that could crawl up and down the cable independently, eliminating the need for traffic restrictions while work was in progress.

When the interior of the cable was inspected, engineers were surprised to find that 8-10% of the cable’s strength had already been lost as a result of corrosion – despite the cable at that time being just 40 years old. Although such a loss was significant, it did not necessitate immediate traffic restrictions. It was crucial, however, that the corrosion was halted in order to prevent this happening in the future.

A limited second internal inspection was carried out in early 2008. Three panels were opened up, two of which were opened previously in 2004.

This second internal inspection showed that the projected envelope of the predicted strength of the cables, determined from the first inspection, was still valid except that the second inspection extended the timescale for potential loading intervention.  In addition, the second inspection results indicated that it appeared more likely that the rate of deterioration was predicted to follow the more optimistic line.

Following the initial discovery of corrosion, the bridge authority had taken the decision to protect the cable with a waterproof elastomeric wrap and design a dehumidification system in a bid to stem the deterioration. Dehumidification had 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 was the first time it had ever been used inside a main cable that was more than four decades old.

The dehumidification system was fully installed by 2008 and switched on soon after the second internal inspection. The drying out process was completed by the end of 2009. Monitoring of the dehumidification process showed a fall in relative humidity within both cables over the majority of their length from an initial value of close to 100% to below the critical value of 40%.

A third internal inspection of the main cables was carried out in 2012 - the first such inspection to be carried out following dehumidification.

Due to budgetary constraints only eight panels could be opened during the third internal inspection. Although not ideal, this still provided sufficient data to allow an evaluation of the strength of the main cables which could then be compared to the results from the previous two inspections. 

This showed that the factor of safety in the cables had not materially diminished since the previous inspection in 2008, giving strong comfort that the dehumidification system has successfully slowed down corrosion.

Following the third inspection, the cables are not expected to lose significantly more strength in future so long as the dehumidification system continues to function effectively. However, further calculations of cable strength will need to be obtained in future years to increase confidence levels. It is expected that some degree of uncertainty will remain concerning the magnitude of future strength loss, and the cables will require to be continually monitored and subject to a regime of internal inspections for the remainder of their service life, but this uncertainty will diminish with continual monitoring and after each future inspection.