THE CONTROVERSIAL strengthening of Kingston Bridge was the topic of Dundee Area Branch's joint meeting with the Institution of Structural Engineers last month.
The project was hit by lengthy delays and increased costs when it was revealed that methods being used were markedly similar to those adopted in repairing the Koror-Babeldaob Bridge in Palau, south-east Asia, which collapsed shortly afterwards, killing two people (NCE 4 June 1998).
Kingston Bridge, which carries the M8 over the River Clyde at Glasgow, was built 30 years ago to carry 30,000 vehicles a day. 'It now carries five times that number, ' said the Scottish Executive's Andrew Brodie, 'and it had suffered some serious structural defects.'
A detailed survey revealed the bridge was under strength in the deck, pier shafts and pile caps.
The deck had flattened by 30mm at mid-span and tension cracks had appeared underneath, Brodie explained.
With the aid of a simplified model Brodie explained the potential collapse mechanism, and how it had become vital to deal urgently with the problems.
Glasgow City Council engineer in charge of the strengthening work, Ian Telford, then illustrated how the deck was strengthened by installing external pre-stressing cables in the hollow, concrete box girder .
With the project team acutely aware of the disaster in Palau, the work was carried out in stages, carefully monitored by remote reading deflection gauges accurate to 0.1mm.
The second stage, Telford explained, was to lift the entire cantilever deck to allow the defective steel rocker bearings to be removed, and the existing piers to be demolished and rebuilt.
The delicate operation called for a sophisticated control system for the 1,000t lifting and lateral restraint jacks, as well as a second set of jacks ready to take the load should any failure of the main jacks occur.