FORTH ROAD bridge operators were this week in talks with the Scottish Executive about how to fund vital repairs to corroded suspension cables.
Complete replacement of corroded cables is an option and the authorities admit that the 41 year old bridge will not last as long as its 120 year design life.
Operator Forth Estuary Transport Authority (FETA) admitted this week that it lacked the funds to carry out the massive programme. It ruled out funding the work by raising tolls.
Engineers discovered severe corrosion at low level locations of some of the wires in the 4km long suspension cables last year (NCE 6 January 2005).
The bridge was initially investigated after cables in American suspension bridges of similar age showed signs of corrosion.
The authority has already allocated £2.2M for cable inspection and for installation of acoustic monitoring.
It said it would be unable to bear further costs.
'We've already increased the bridge toll from 80p to £1 this month to cover long term maintenance costs, ' said FETA depute general manager Barry Colford. 'But there is no provision for the cables in our capital plan, ' he said.
Consultant FaberMaunsell has been investigating the cables and replacing 18m lengths of corroded wire.
'The worst we've found is 31 broken wires out of 11,618 in one section. It's an indication of a loss in strength making it highly likely that this trend will continue, ' said FaberMaunsell technical director Charles Cocksedge.
He also predicted that corrosion at the high level quarter points of the catenary would be worse. Inspection of these locations will begin later this month.
'At these points, suspension bridge cables are most flexible, so paint is more likely to crack. With more movement you get fatigue and water ingress which leads to corrosion, ' he said.
Cable corrosion at low level points has been attributed to water entering the cable via cracks in the paintwork higher up the catenary and collecting at the bottom.
Options for remedying corrosion include installing a dehumidifying system to remove moisture or lubricating the cables with oil.
Colford said that strengthening the bridge with two more cables would require the towers to be strengthened.
Replacing cables would be costly and logistically complex.
Traffi on the bridge has doubled since it opened in 1964 but this has not so far weakened the cables. 'But if the cables continue to deteriorate and traffic loads increase, we'll get to a point when the factor of safety is too low, ' said Cocksedge.