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Contractor slammed for Forth delays

THE DANISH contractor struggling to replace ageing hangars on the Forth Road Bridge has been slammed for the four month delay in starting operations.

Monberg & Thorsen, whose £7.8M bid for the contract to replace all 192 hangars on the 35 year old bridge marginally undercut British rivals Laing and Kvaerner, hopes to start replacement work this week.

But the company admitted that 24 hour working and a greatly enlarged workforce will be needed to meet July's target completion date. It blames problems in designing temporary hangars - an argument dismissed by supervising structural engineer W A Fairhurst & Partners.

'We accept the design has proved difficult,' said Fairhurst project director Alan Simpson. 'But this is a very experienced bridge contractor and we are very disappointed the learning curve is taking so long.'

Problems centre on how the bridge will react to the installation of temporary hangers either side of the main ropes while the originals are being replaced, as it is thought secondary bending moments could be induced in deck trusses.

'Structural analysis has proved much more complex than we expected,' said Monberg & Thorsen senior vice president John Jensen. 'We will probably start a night shift in the spring and remain confident of on-time completion by July.'

The contract was triggered by the discovery that several hangars were up to 25% understrength. Inspection revealed frayed and pitted wires near the bases of nine cables, due, it is thought, to fatigue and insufficient protective lubricant (NCE 4 September 1997).

Design of temporary fixings needed existing structural analysis software to be 'refined', claimed Jensen, especially as 'increased traffic loading over the years had used up much of the bridge's structural reserves'.

The first of six prefabricated sliding work platforms, to be hung from the main two suspension cables, arrives on the bridge this week and most of the 26km of Italian-manufactured wire rope is already on site. The shorter central hangars will be replaced first using full height scaffold access.

Hangars have been replaced on several US suspension bridges, although the operation remains rare in Europe. This is Monberg & Thorsen's first such contract.

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