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Defective action

Eight years ago the bridge's then owner Strathclyde Regional Council logged an armful of defects. A 300mm mid-span sag and tilting northern pier was due, it claimed, to a combination of understress and incompatible articulation.

This three pin asymmetrical portal frame articulation - pin bearings beneath each pier plus a rocker bearing above the northern end - had produced single point vertical loading resulting in piled piers being understrength. Additionally, the northern pier's 165mm tilt outwards left a similarly displaced central slab leaning on now damaged approach viaducts (NCE 21 March 1996).

Repair solutions included increasing deck prestress by 90MN through 32 now installed tendons. Lack of space in deck box slabs and webs has forced stressing subcontractor Balvac Whitley Moran to thread the 200m long tendons through the boxes themselves.

Each 3.5m wide understrength hollow concrete pier is being replaced by a 7.9m thick solid version. Narrow sides for the new piers are built up just 180mm outside the original. They will support main lifting jacks so, when the slab is raised, the old pier can be slid out between them and the gap infilled with concrete to create a new wider support.

Revised articulation will include pin bearings above each pier but none beneath. While the deck is raised it will be pushed back 65mm southward.

Yet to come are extensive repairs to much of the 4.2km approach viaducts; spalled concrete replaced and the deck waterproofed. Strathclyde Region claimed most of the defects were caused through errors by bridge designer WA Fairhurst & Partners and sued the consultant for £16.5M damages. The case was settled out of court last summer for an undisclosed sum, although Fairhurst did not admit any liability.

Ownership passed to the Scottish Office when Strathclyde Region was abolished in April 1996.

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