Forth Road Bridge – Main Cables and Anchorages

The Forth Road Bridge is a long span suspension bridge with a main span of 1006 metres and was opened in September 1964. The bridge crosses the Firth of Forth some 15 km west of Edinburgh and is a vital link in Scotland’s strategic road network carrying over 24 million vehicles each year. The bridge deck supports a dual two lane carriageway without hard shoulders or strips. There is a separate footway/cycletrack on either side.

The main cables were regularly inspected externally and no leaching of water or moisture staining had ever been recorded. However, it was acknowledged that the condition of the individual wires could not be determined with any certainty from external inspections. Using techniques developed in the USA a first internal inspection on selected panels of the main cables was completed in 2004/5. Fairly extensive corrosion and wire breaks were found in some panels. Given the relatively young age of the bridge these results gave cause for concern.

From the results of the first inspection at Forth there does now seem to be serious doubts over the use of paint systems to try to protect suspension bridge cables. It was concluded that if the rate of deterioration due to corrosion could not be halted, consideration would have to be given to introducing loading restrictions on the bridge. Action was required to try to halt or limit the corrosion and to try to monitor the cables. The following works and studies were commissioned:

• Installation of Acoustic Monitoring on both cables.

• Installation of a Dehumidification System on the cables.

• A feasibility study to determine whether or not the cables could be replaced or augmented if the corrosion continued and to investigate the condition of the main cable anchorages.

  Dehumidification is a relatively new technique when applied to the main cables of suspension bridges. Therefore, there can be no absolute guarantee that it will prevent further deterioration and wire breaks. Acoustic Monitoring was installed on both cables by August 2006 and a system of dehumidification completed in October 2009. Prior to the dehumidification system being commissioned a further internal inspection was carried out on a limited number of panels to benchmark the condition of the cable. This inspection confirmed that the loss of strength was continuing but appeared to be following a flatter curve against time. The main cables are anchored into concrete filled rock tunnels. These tunnels were pre-tensioned using galvanised strands in grouted ducts. There is concern over possible corrosion of the strands. The investigation will comprise excavating down to the top of the anchorage tunnels on the south side behind the anchorage chambers. The ducts exposed will be carefully cut open and the strands inspected and tested. Work is currently being planned to start on site in late summer 2011.