9 Other aspects considered during the audit period.
In undertaking this audit further items have arisen which justify additional comment by the audit team.
It should be noted that the intrusive inspection resulted from experience in the USA. Routine inspections of the Forth Bridge cable undertaken using the standing procedures for such inspections gave no indication of any areas of concern over deterioration of the main cable. Other European bridge owners are now likely to be aware of this 'failure' of existing procedures to identify deterioration within the main cables although the only alternative authoritative guidance for inspection is for intrusive inspections under the NCHRP document.
We have not identified within the Guidelines any requirement to inspect and record the condition of all the external wires around the circumference of the cable. The Guidelines require inspection only of the wires on the faces of the wedged openings and that condition is then assumed not only for the unseen wires between the wedge lines but also for the visible wires around the circumference. Inspection of the external circumference wires would be expected to provide additional 'actual' condition data rather than 'projected' data. 5
In the US, when similar conditions have been discovered, action has been taken which has been intended to slow or stop the corrosion. This has entailed unwrapping the cables for their full length and pouring oil (usually linseed oil with or without additives) into wedged openings in each panel, followed by installation of a complete new wrapping system (wire wrapping with or without neoprene overwrap).
Recent thinking favours installation of dry air injection systems that force dehumidified air directly into the cable to keep the interior of the cable dry, thus eliminating any further corrosion. This was first developed and adopted in Japan by the Honshu-Shikoku Bridge Authority and has recently been installed on the Aquitaine Bridge in France, the Hoega Kusten Bridge in Sweden and the Lillebaelt Bridge in Denmark. Studies are currently being carried out for its applicability on some US bridges.
We would endorse FETA's decision to study the feasibility of dehumidification.