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Capital projects:

Main Cable Inspection

Current works on site:

A third internal inspection of the main cables commenced on site in March 2012. This is the first such inspection to be carried out following the installation of the dehumidification system that was installed to address the problem of corrosion in the 11,618 high tensile steel wires that makes up each main cable.

The main cables have been unwrapped at eight carefully selected locations and hardwood wedges 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 has been recorded and samples taken for laboratory testing.

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

Despite the difficult weather conditions the Contractor has made good progress with all site work completed in September 2012.  The project is ahead of programme and forecast to be under budget with a cost saving estimated at £312,000. The remaining work outstanding is to complete the laboratory testing of the sampled wires and for the Engineer to carry out the analysis and strength evaluation.

There was a risk that the relatively low number of panels being inspected would need to be increased as the work was being carried out.  However, the knowledge and experience gained from the previous inspections has helped to reduce that risk and the condition of the six panels inspected to date has not generated a requirement to inspect more panels.

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 to allow an internal inspection of the wires forming the main cables (two of the panels were opened previously in 2004 during the first internal inspection).

This second internal inspection in 2008 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.

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.

Following the second internal inspection, the dehumidification of the main cables was carried out and completed by the end of 2009.  The monitoring of the dehumidification process has shown 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%.  There are still some limited lengths near the main tower tops where the relative humidity is still within the range 40-60% and further investigations are being carried out to determine the best method of addressing these areas.

A tender submitted by C Spencer Ltd., for the total sum of £2,573,310 to carry out a third inspection was approved by the FETA Board in October 2011. Flint & Neill Limited (F&N) were appointed as Engineers by the Authority to assist with contract procurement and supervision of the works.  

This third internal cable inspection 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.  This inspection will also increase confidence in the prediction of the future strength of the main cables.  However, it will not provide absolute values as there is a recognised degree of error in determining cable strengths from such a small sample of wires.  In addition, it is recognised that the determination of the condition of the wires which in turn affects predicted strength, is based on the subjective judgement of the engineers and inspectors carrying out the detailed inspection.

Further calculations of cable strength will need to be obtained in future years to increase confidence levels in the capacity of the cables.  It is almost certainly the case that some degree of uncertainty concerning the magnitude of future strength loss of the main cables will remain and the cables will require to be continually monitored, and be subject to a regime of internal inspections and strength evaluations, for the remainder of the service life of the bridge.  However, it is expected that this uncertainty will diminish with continual monitoring and after each future inspection. 

Related documents:

The Bridge:

Facts & Figures

Opened 1964, 2.5 km long, Main span 1006 metres
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  • Roadworks - Contraflow on bridge (22:46 BST 23/07/14)