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Bishop's move

Bridge replacement Replacement of Bishop's Bridge in west London is posing a series of tough and unusual engineering challenges. Andrew Mylius reports

In west London this weekend, lift maestro Mike Wade faces one of his biggest challenges yet, with work entering a critical phase on 'one of the two or three most interesting jobs I've worked on'.

As lift master of specialist jacking and moving contractor Dorman Long, Wade has taken charge of some spectacular feats, including hoisting Wembley Stadium's new arch (NCE 15 January 2004) and raising Heathrow Airport's new air traffic control tower (NCE 24 February 2005).

The 48-year-old is a veteran of Hong Kong's Tsing Ma Bridge, and is off to Venezuela next month to launch four 2,600t sections of a new cable stay bridge across the Orinoco. Wade routinely performs stunning feats of strength and dexterity for clients in the power, oil and gas, and petrochemical industries.

So it seems incongruous that the launch of a modest, 101m long road over a rail bridge just north of London's Paddington Station ranks at the top of Wade's list of all time best projects.

What makes Wade's role in the £24M replacement of Paddington's Bishop's Bridge so engrossing is the requirement to keep railway lines into the station open while work is carried out, combined with the new structure's complicated underside geometry.

The new Bishop's Bridge is a five lane structure, flaring to six lanes wide at the northern end (NCE 19/26 August 2004).

The original 170m long structure was a traffic bottleneck. It consisted of a mish-mash of structures: brick and cast iron spans lurched from the north abutment across the Grand Union Canal, London Underground's Hammersmith & City Line, and three Network Rail lines.

A hefty steel Parker truss then carried the road over 10 sets of mainline rail tracks to the south abutment.

The brick canal spans were broken up, with the iron girders lifted out by main contractor Hochtief. Removing the Parker truss is altogether trickier.

The plan, drawn up by Hochtief with subcontractors Cleveland Bridge and Dorman Long, was to jack it into the air with the aid of four temporary steel lattice towers. Raised out of the way, the replacement bridge will be slid into position below the Parker truss, which will then be lowered onto the new structure before being hauled off to the north abutment for demolition.

Wade oversaw the raising of the Parker truss last August. In the intervening eight months Hochtief has built new abutments and piers to the north of the railway tracks and on Paddington Station's platform 10/11. Cleveland Bridge has installed the canal span on which it has subsequently built the first 70m of the railway spans, which will start to be edged out across the tracks on Saturday night.

Wade says that the bridge's tapering width, its skewed alignment and its variable depth set it apart from a run-of-the-mill deck launch.

To resist bending moments, the bridge's longitudinal steel girders are some 1.3m deeper at mid-point - where they will be supported on the pier on platform 10/11 - than at either end. Were fixed rollers used, the deck would rear up as it advanced, explains Wade.

The bridge's 30º skew across the rail lines, meanwhile, means that the deck is not perpendicular to its supporting piers - so the deepest ection of each girder comes at a slightly different point along the bridge's length.

And for added complexity the bridge's taper means that part way along its length the number of longitudinal girders increases from six to eight.

'There are two girders we'll need to 'pick up' as we go along, ' Wade says. 'And the girders' flange widths change as we go, so we'll have to constantly adjust the roller guides.' To tame the bridge's bucking bronco tendencies Dorman Long has mounted the rollers over which the girders run on adjustable frames controlled by 400t capacity hydraulic jacks (see diagram).

There are eight parallelogram frames across the width of the bridge at each pier. To manage them, frames on each pier have been linked to form groups of four, either side of the deck's centre line.

'There's a [shared] reservoir system in each group of four jacks, ' explains Wade. This enables the height of each frame within the group to adjust passively to the changing pitch of the girders, ensuring that loads are distributed evenly. Wade can also raise and lower the groups of four as 'global units'. The system enables him to compensate for differential bridge deck depths by making adjustments at four points rather than 16.

Jacks are calibrated and constantly monitored on computer. Dorman Long has calculated by exactly how much each girder will fall and rise over the course of the launch. 'The geometry of bridge is pre-determined, so at each increment of the launch we know what level it needs to be adjusted by. For example, at 51.6m we have to lift up by 97mm on one side and lower down by 20mm on the other, ' Wade notes.

At the platform 10/11 pier a different support system is called for. There is a 5m height difference between the north and south abutments. The deck will come across the railway tracks to its intermediate support at a higher level than that at which it will eventually sit.

The support system must also allow for the 1.3m difference in girder section depth from end to mid-point. 'We have significant height differentials, and it's right up in the air - there's a 3m jack down at end of the launch process to set the bridge on its bearings, ' Wade says.

Eight climbing jacks clamped to the pier provide Dorman Long with the reach needed to engage and support the nose of the bridge as it reaches its mid-way support point. As with the parallelogram frames, the climbing jacks are linked into Dorman Long's computer control system, allowing for millimetre-precise vertical adjustment.

Supporting the bridge's twisting, turning geometry is not the whole of Wade's challenge. Its curvaceous underside means that at different stages of the launch operation he will variously have to overcome friction and restrain the bridge from running off.

'Depending on what the roller friction is, sometimes the deck wants to slip away and sometimes you have to apply force to haul it. We are using two 400t jacks to pull and two 400t jacks to hold the bridge back, ' Wade says.

Strand jacking cables for propulsion and restraint are attached to the rear of the deck.

Wade will apply a preload of 900kN-1,800kN to the restraining cables. He estimates that between 650kN and 2,250kN of pulling force will be needed. As with the jacks controlling support height, pulling and restraining jacks will be computer controlled. 'You set maximum and minimum loads into the system for both sets of cables, and if those are exceeded the process will stop.' Launching will take place during Network Rail's scheduled night-time possessions.

Wade is expecting to achieve up to 5m progress a night, spanning the 40m to pier 10/11 in about 10 days. There will then be a two month intermission while the 30m of steelwork making up the tail of the bridge is added on. The launch is scheduled to begin again and complete in July.

What will make the next 10 days such a highlight in Wade's working life is the degree of finesse he is being required to deliver using a standardised kit of parts. 'This is all existing technology but it's combined here in probably a unique way.

That's the beauty of it - rising to the challenge and deriving the engineering solution.'

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