POST-TENSIONING of Glasgow's severely weakened Kingston motorway bridge has finally started nearly three years behind schedule after a major re-analysis of the safety of the operation.
Closer examination last month of a slipped deck bearing has triggered the surprise decision that the 90MN prestress is safe to start immediately, out of sequence with the rest of the long delayed £14M repair programme.
By the end of this month the first bundle of 200m long stressing cables will contain 25% of ultimate stress, transferred very slowly and monitored continuously by over 200 computer-linked instruments contained within the deck.
The three span, 268m-long bridge carries 10 heavily trafficked lanes of the M8 over the River Clyde. The 29-year-old crossing is riddled with structural problems, including a 300mm mid-span sag and a leaning river pier, with a slipped rocker-style bearing on top of it.
Plans to jack up the 143m main span for six months to allow new piers and bearings to be installed have been on hold for two years. Vital computer software has been repeatedly refined to ensure 100% safety of the deck during the 128 jack lift (NCE 22 October 1998).
Client the Scottish Office confirmed that all computer snags have been resolved and the lift is programmed for October.
Over 30 stressing tendons have been threaded through concrete deck boxes to help remove the mid span sag. But the stressing operation was to have taken place only after the deck had been lifted clear of its bearings.
'We were concerned that the slipped bearing would become more unstable while the deck moved up to 20mm across it during stressing,' said a senior Scottish Office spokesman. 'But on closer examination we now think the bearing may slide to a more stable position.'
Stressing by specialist Balvac Whitley Moran will continue until mid- summer, although main contractor Balfour Beatty Construction is unlikely to complete its originally planned 19 month repair contract until October 2000.