Every day more than 180,000 vehicles cross Glasgow's Kingston Bridge and its adjoining roads. Since 1992 Glasgow City Council has been engaged in a marathon strengthening and upgrading exercise on the bridge and its associated access ramps. This will end with the replacement of the remaining 2,000m of unrefurbished copes and parapets.
So far, the biggest challenge has been replacing the main bridge piers and strengthening the main deck with extra prestressing. This was completed in 2000 (NCE 28 October 1999).
In January this year work began on the Stobcross on-ramp, a key link between the Clydeside Expressway and the west-bound M8 motorway which runs across Kingston Bridge. The on-ramp has had a 7.5t weight restriction since 1994.
Total length of the ramp is 400m, made up of two straight sections - totalling 250m - at each end of a central 150m curved and superelevated section spanning the main motorway, all supported by five concrete columns. Conventional insitu reinforced concrete was used for the box girder deck on the straight sections, but post tensioned prestressed insitu concrete box girder construction with internal tendons was chosen for the curved central section.
'During inspections of the on-ramp we discovered that the curved section had tilted over; therefore superelevation had dropped from the designed 7to 3- or less, ' explains Glasgow City Council project manager Ian MacGregor.
'The deck soffit was actually in direct contact with the outer shoulders of the column heads and was pushing them over.
This flexing induced cracking to the columns and had seriously damaged one of the movement joints.'
Like most of the structures associated with the bridge, the on-ramp's four columns had under strength foundations. It also shared their lack of deck waterproofing, although the substitution of glycol for salt in de-icing operations over the last 10 years had minimised the effects of chloride attack. And there were serious shortcomings in the copings and parapets.
Several solutions were considered, including total demolition and replacement of the entire deck and at least one column. This latter option was ruled out because the busy motorway would have to be closed while the central spans were demolished.
The City Council eventually opted to cut away and replace the deck on the straight sections but to keep the central curved section, and strengthen it with additional prestress applied by external post tensioning tendons outside the shallow and relatively inaccessible box girder.
Supporting columns would have their heads cut back and replaced - although one column was in such bad condition it would need complete replacement. Beefed up foundations, new stainless steel bearings, new movement joints and new copes and parapets would complete the transformation.
The proposal was radical, given the tight curvature of the bridge, says MacGregor.
'As far as we know it's never been done before, in the UK at least.' The inside radius is only 95m, so there have to be strong anchorages locating the tendon ducts on the inner edge of the curved section (see diagram. ) The outline plan involves jacking the curved deck up off its bearings once the tendon ducts and anchorages are installed.
While it is in mid-air, supported by falsework, it will be prestressed, then gently realigned by the jacking system.
At the same time the column reconstruction and bearing replacement would take place.
When the deck was lowered back down it would be linked to the two new straight sections.
The £11M contract went to Northern Ireland-based Graham Construction. Graham brought in Mabey Support Systems to handle the tricky deck lifting phase, prestresser VSL Systems (UK), and concrete removal specialist Core Cut (see box).
Central to Graham's plans for the curved section was the moveable eight frame gantry, developed jointly with Mabey.
'We could erect this over one end of the deck, well away from the M8, ' explains Graham works co-ordinator Des Millar.
'Then we could roll it out - it moves on wheels running in channel sections fixed to the deck - and work below the deck cantilever right above the motorway without needing any closures.' On its way 'out' the gantry was used to install the tendon ducts, deviators and anchorages, and provided access for VSL to install and ultimately stress the new tendons. During the return journey Graham will use it to break out the old coping and parapet and replace it with a new cope and galvanised steel parapet.