Feasibility study into the replacement or augmentation of the main cables of a long-span suspension bridge

Forth Road Bridge is a long-span suspension bridge with a main span of 1006 m and side spans of some 408 m. Approach viaducts to the north and south bring the total length of the structure to just under 2500 m. The bridge was opened in 1964 and is used by over 24 million vehicles each year. All of the maintenance work on the bridge that requires lane or carriageway closures is carried out overnight or during limited weekend carriageway or lane closures to minimise delays to users on this important strategic and commuter route. During the first internal inspection of the main cables carried out in 2004 (40 years after opening) significant corrosion was found, leading to the retrofitting of acoustic monitoring and a system of dehumidification on the main cables. In addition, a second internal inspection was carried out in 2008 to try to establish the rate of corrosion found in the cables. The inspections and retrofitting works are described elsewhere. This paper deals with a feasibility study to determine if the main cables on Forth Road Bridge could be either replaced or augmented in the event that corrosion continued following the retrofitting of the dehumidification system. Another key area that was examined as part of the study but does not form part of this paper was the long term structural integrity of the cable anchorages.