On Thursday 20 August Hammersmith Bridge will close to all traffic for 11 days. After months of near closure and controversy over the bridge's long term future and the implications for traffic congestion in west London, the most complex operation in a two year refurbishment must be completed in time to reopen at 6am on 31 August - albeit with continuing traffic restrictions. There will be many watchful eyes monitoring progress, and controversy is unlikely to end afterwards.
Most vehicles have been unable to cross the 110 year old structure since February 1997, when it was discovered that key bearings on top of the bridge's northern (Hammersmith) tower were effectively seized. These steel roller bearings were supposed to allow the saddles that carry the complex Victorian suspension chains to move horizontally under live loading, reducing horizontal forces on the tower, which was never designed to resist such forces.
With the bearings seized, Hammersmith & Fulham Council had little choice but to restrict traffic across the bridge to emergency vehicles, light buses, taxis and motorcycles - plus pedestrians and cyclists. The complete closure is to allow contractor John Murphy to jack up each set of Hammersmith tower bearings in turn and replace the rollers with modern elastomeric bearings - much as was done to the southern (Barnes) tower in 1984.
'It will also give us the chance to see if we can decide on a long-lasting replacement for the current surfacing,' says Hammersmith & Fulham development engineer Littleson Wijayasinghe. 'We are setting up two experimental areas on the bridge, one towards each side, and trying out two alternative types of surfacing to replace the existing bitumen over 24mm plywood, which is 11 years old.'
One trial will check the longevity of 10mm thick rubber-coated steel sheets, the other precoated plywood. Both materials will be bolted through oversized holes in the main 75mm thick tongue and groove timber deck into the 300mm square hardwood deck beams below.
This main deck structure dates back more than 20 years and is said to be in reasonably good condition, with only about 5% of the tongue and groove needing replacement. The wrought iron structure is another matter.
Since the bridge was partially closed, some 1,200 rivets on the cantilevered walkways have already been replaced with HSFG bolts. John Murphy's current near £1M contract also includes major strengthening work on the longitudinal trusses which transfer loads from the deck to the hangers.
These are 1970s steel replicas of the wrought iron originals. Steel plates will be welded on to the top chords of the trusses right across the full length of the bridge. On the backspans the bottom chords will undergo similar reinforcement.
Several of the wrought iron hangers need attention as well, as consulting structural engineer Hyder Special Structures project manager Simon James explains.
'Short hangers close to centre span suffer from high bending stresses because their lower ends are effectively locked on to the truss. We have to insert an extra pin-joint in the hanger to allow rotation to relieve the stresses. And we discovered that the longest hangers, next to the towers, are overstressed and have to be replaced.'
In all, 40 out of a total of 156 hangers will have to be modified or replaced with modern steel replicas before September 1999, when the phase two contract is due to be completed. The bearing replacement operation will be much faster.
An assembly of 12 jacks will be used to generate the 400t force needed to lift each chain saddle 5mm. This should be enough to allow the contractor to knock the 150mm diameter steel rollers out sideways and replace them with their modern equivalents.
Extra brackets will be added to the saddles to ensure vertical loads are transferred directly to the new bearings. Post-tensioned high tensile steel bars will connect the new brackets to the cast-iron saddle.
Unlike Westminster Bridge further downstream, no serious boat strike damage has been discovered. 'Luckily, there is an inspection cradle that runs on rails below the bridge,' explains Wijayasinghe. 'The rails seem to have taken what impacts have occurred.'
Even the 10m deep brick anchorage chambers turned out to be in remarkably good condition, needing only cleaning and minor repointing.
By autumn 1999, with the Hammersmith side saddles free to move and the longitudinal trusses strengthened, the local authority will be able to reopen the bridge to traffic up to 7.5t. Buses up to 12t will also be permitted. During the closure, however, not even emergency vehicles will be allowed across - and the local fire brigade will station appliances on the Barnes side of the river.