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Contractors World  - UK & Ireland
2012 Vol 2 No 8
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Soon to celebrate the 50th anniversary of the opening of this iconic bridge, the Forth Road Bridge is beginning to show its age.

Forth Road Bridge Cable Corrosion

Inspection over recent years has shown that the cables on the Forth Road bridge in Scotland are showing signs of decay and have already lost some 8% of load capacity. Without replacement, the bridge could be required to impose load restrictions by the end of the decade. In 2014 the bridge celebrates 50 years since it was first opened, since which time traffic loads have increased significantly.

So much so that work is currently underway in building a second road bridge almost parallel to the existing structure.
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.

    For the first time in Europe, the inner strands of cable on a suspension bridge revealed unexpected erosion.Due to budgetary constraints only 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.

For the first time in Europe, the inner strands of cable on a suspension bridge revealed unexpected erosion.

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 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.

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