Forth Road Bridge inspection finds slowdown in corrosion

A report by the bridge's chief engineer and bridgemaster Barry Colford outlining the findings of the third main cable inspection is due to be presented at a meeting of the Forth Estuary Transport Authority on 25 February.

Although wire corrosion appears to have been slowed down there are still existing cracks in some of the wires that might propagate from corrosion pits that existed prior to installation of the dehumidification system, Colford said. These cracks may eventually lead to the wire fractures but the rate of breakages is expected to slow significantly.

The authority had commissioned Flint & Neill, in conjunction with Ammann & Whitney and Cowi, to carry out the internal inspection of the cables. The inspection was carried out between April and September last year. Following inspection, wire sampling and laboratory testing, an analysis of the condition of the cables was carried out by Flint & Neill and a review of that assessment carried out by Aecom.  

It was the first internal inspection following installation of the dehumidification system, which was completed by the end of 2009.

"A degree of uncertainty concerning the magnitude of future strength loss of the main cables will  always  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," says Colford in the report. "However, the results of this latest inspection, albeit reduced in scope, are encouraging. The dehumidification system applied to both cables appears to be slowing down the rate of deterioration.

Broken wires were found in six out of eight panels inspected. The largest number of broken wires found was16 and no correlation could be made between broken wires and the number of 'Stage 4' wires - the most heavily corroded.

The highest percentage of Stage 4 wires was found in the panel near the mid main span (12%). Significantly high numbers of Stage 3 wires were found in six out of eight panels and a particularly high percentage of Stage 3 wires (71%) was found near mid main span.  A total of 55 broken wires were found.

The  inspection revealed that in total 45% of the test specimens failed below the specified minimum tensile strength of 1,544N/mm².

Fatigue tests  at an elevated stress range  revealed that only 27% of the total number of specimens tested passed 2 million cycles. "It is reasonable to conclude that the fatigue strength significantly decreases as the steel corrosion advances, the majority of failures were due to a corrosion pit and presence of a pre-existing crack," says the report.

The findings confirmed the likelihood of the areas of greatest corrosion being found at the lowest points of the cable and the report recommends that future inspections should be targeted in these regions.

Flint & Neill has recommended that a further intrusive inspection of a few lower panels is tentatively planned for 2017 but confirmed nearer the time when the on-going monitoring have been taken into account.