Published on 21 Oct 2009
The engineer in charge of the Forth Road Bridge today warned that the bridge’s corroded main cables will almost certainly need to be monitored and inspected for the remainder of their service life, and that there will always be some degree of uncertainty as to how much more strength the cables will lose.
Barry Colford , Chief Engineer & Bridgemaster at the Forth Road Bridge, makes the comments in a new report to the FETA Board, which is responsible for scrutinising the work of the bridge authority and representing the views of local people.
Mr Colford writes: “Unfortunately, there seems to be some expectation that the 2011/12 inspection will produce significantly better data than we currently have in predicting the strength of the cable. This is not the case.
“We will have another point on the strength loss versus time graph but it will be another indicator only. Of course, it will be important and will give more confidence but should not be described as significantly better data.
“Further points in the future, post 2011/12, will need to be obtained to further increase confidence levels.
“It is almost certainly the case that some degree of uncertainty concerning the magnitude of future strength loss will remain. The cables will need to be continually monitored, and subject to a regime of internal inspections and strength evaluations, for the remainder of their service life.”
The Chief Engineer’s report comes as the bridge authority prepares to switch on the final section of the dehumidification system designed to dry out the cables and prevent further corrosion. This project is set to be completed on schedule by contractor C Spencer Ltd. Both cables have now been wrapped and dehumidification plants and monitoring sensors have been installed. The drying out process is expected to have begun on both main cables by the end of the month.
The report notes that dehumidification of the west cable, which began in March 2008, is producing the expected slow and steady fall in the relative humidity within the cable. However, Mr Colford cautions that there is no guarantee this will prevent further loss of strength:
“Dehumidification is a well-tried system of preventing corrosion of steel. However, its application to main cables of suspension bridges is relatively new. Whilst there is good reason to have confidence that dehumidification can slow down or halt corrosion there is no body of evidence yet available to allow an unconditional assurance to be given that this will prevent a further reduction in strength in the main cables of the Forth Road Bridge.”
The report also provides an update on each of the projects listed in the bridge authority’s long-term Capital Plan. This 15-year programme of strengthening and improvement works lists the projects considered essential for the continuing safe maintenance and operation of the bridge.
Other notable projects include the replacement of approach viaduct bearings, currently out to tender, which is expected to cost in the region of £15 million and take three years to complete, with work on site due to begin in April 2010.
- No restrictions on bridge (19:34 UTC 04/02/12)