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Joint effort

The 45 year old Silver Jubilee Bridge between Runcorn to Widnes is having a facelift. Bernadette Redfern reports.

Next month a spruced up Silver Jubilee Bridge, complete with new jonts and surfacing, will be freed of traffic cones after a £2.1M refurbishment.

As the only passage across the Mersey for almost 100km, the bridge carries a over 90,000 vehicles a day - approximately 50% over its design capacity despite being widened from two to four lanes in 1977. It was in urgent need of an upgrade.

Work began in May and has seen the replacement of two major expansion joints and a further eight minor joints. 'The principal joints were at the end of their serviceable life and we had spent a lot of money over the past few years carrying out repairs, ' says Halton Borough Council section leader for bridges Jeff Chrimes.

The 13.5m wide, 550m long bridge consists of a concrete deck supported by longitudinal and transverse steel beams. This deck unit is suspended by steel cables that hang from a pin jointed steel arch truss, which continues down past the deck, creating a centre span of 300m and two 125m approach spans. To prevent traffic chaos during the refit, tough restrictions have been enforced, with all lanes open between 6am and 7.30pm, and a minimum of two lanes open at all other times.

'As a result the works are generally carried out during midweek night-time closures or weekend closures, ' says Mott MacDonald project manager Gary Simpson, who is supervising contractor Mowlem on site.

Replacement of two, 2.2m wide bespoke Maurer expansion joints which sit 300m apart at either end of the main span has been done progressively. Contractors have had to carry out each activity involved in replacing each joint in three phases, starting with the section of the joint under the northbound carriageway, moving to the centre section and finally the southbound carriageway.

The first move was to install four shock absorbers or shock transmission units (STUs) below the deck to absorb any impacts caused by sudden breaking or other dynamic activity on the deck.

This type of movement was accommodated in the original joints, which were held constantly in compression even at maximum expansion. Each was made up of a concertina of 42 rubber and manganese steel plates, spanning 2.1m that sat on a phosphor bronze bearing strip on top of longitudinal steel beams.

The new STUs, by contrast, are large pistons which will absorb energy induced by any sudden movements. New Maurer joints will provide a 120mm tolerance for temperature changes.

Following installation of the STUs Mowlem brought on two sets of temporary bridging units to span both main expansion joints. These have allowed traffic to drive over the bridge whatever stage the joint replacement had reached.

Removal of the old joints and the concrete around them created a 5m wide gap across the deck and exposed the steel reinforcement.

As a new joint is 480mm wide compared to the 2.1m of the previous joint, this gap had to be filled in with a 650mm thick C40 reinforced concrete slab.

Additional shear bolts were drilled into the longitudinal steel beams supporting the deck to bond the beams to the new concrete.

The new joints were tied into position with insitu concrete stitches.

The deck is now being waterproofed and resurfaced with the whole operation scheduled to finish mid September.

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