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Giving a lift Bridging navigable waterways has always presented a technological challenge to structural engineers. Margo Cole reports from the north west of England on two very different solutions.

Britain's canal network is one of the greatest engineering achievements of the industrial revolution, and many famous engineers built their reputations by designing stretches of the canals or the elegant structures spanning them. Today canals still give engineers a chance to make a mark.

Nowhere is this more evident than Manchester, where redevelopment in recent years has centred around the canal network, and a series of spectacular bridges. The latest addition is the Lowry Centre footbridge, which will span 91m over the Manchester Ship Canal at Salford Quays. It will provide pedestrian access to the new Lowry Centre, currently under construction, housing examples of work by the artist LS Lowry, a theatre, arts centre and educational facilities.

The bridge will also be capable of being raised to a height of 22m above the canal to allow ships to pass underneath - a statutory requirement on the Ship Canal.

Like many of the new structures in the city, the Lowry bridge was the subject of a design competition, which was won by consulting engineer Parkman, in association with Spanish firm Carlos Fernandez Casado of Madrid. The firms' entry in the 1996 competition was a tied arch deck supported by four curved concrete towers.

Parkman project director Bill Middleton says: 'Part of our basic philosophy was that we wanted it to be educational, so we wanted the mechanism to be seen.'

The Lowry Centre Trust, which will own the bridge, liked the design, but thought a steel structure would fit in better with the new building, so the piers were redesigned as solid steel towers. However, when the tenders came in, the winning contractor's price was significantly higher than the £4.5M budget.

'We all agreed to enter into a value engineering process to review the design and look for cost savings - which we achieved,' explains Middleton.

That value engineering process included not just client Salford City Council and Parkman, but also the main contractor, Christiani & Nielsen, and the mechanical and electrical subcontractor Bennett Associates.

The resulting design retains the original philosophy, as the lifting mechanism is still on display, but the towers are now tubular steel lattice structures, and the lifting mechanism is hydraulic rather than electric.

Each of the four towers is 32m high and holds a counterweight of 50t. This offsets the 250t deck weight, as Middleton explains: 'The advantage is that the counterweights are used to reduce the forces needed to lift the bridge, so that a hydraulic force of only 50t will do the job'

A winch rope is slung over a pulley wheel, or sheave, at the top of each tower. When the rope is pulled down by a hydraulic pulley situated in a machine room in the tower abutments, the counterweights descend, and the deck goes up. When the bridge is down and sitting on its elastomeric bearings, there is a 5m clearance to the water level in the canal.

Work was just about to start on site in July 1997 when a member of the public told the site team there might be an unexploded Second World War bomb on the site of the north abutment. Christiani & Nielsen project manager Simon Laffan says: 'There was evidence that a 500kg bomb had landed there in 1941, but no record of it having been removed.

'One of our first actions on site was going to be to sink 20 piles on that north side, so we researched the records and unearthed photos from the time. Once we had that information we had to take precautions.'

The contractor brought in a bomb detection specialist to probe each pile location by sinking 1.5m deep boreholes ahead of the excavation.

No bomb was found - just some pieces of metal which might have been bomb casing - but with their equipment the experts were able to determine its exact shape and where it had landed. This process delayed the start of construction until September last year, cutting down the original contract period from 18 months to just 13.

Both abutments have piled foundations, with 750mm diameter auger piles socketed into the underlying sandstone. The remains of an old timber wharf were found during piling on the south side.

This wharf was replaced in 1930 by a precast concrete quay, built for shipping grain. The precast box sections sit on a caisson, and were designed to take load only along the two lines where dockside cranes would run. Christiani & Nielsen had hoped to construct the bridge towers on the quayside by welding together individual sections, and then lifting them into position in one go using a 400t crane.

This was done on the north side, but on the south abutment the old grain wharf would not have been able to carry the load.

Instead, the lattice towers were delivered by fabricator Fairfield Steelwork in smaller sections, lifted vertically - with a maximum lift of 15t - and then structurally welded in place. The crane was supported by structural steel spanning on to the quay's load lines.

While the towers have been taking shape on the quayside, the tied arch deck is being assembled by the Lengthline Ship Repair Company at a shipyard 1km away up the canal. The slender structure is 91m long and only 7m wide at the centre; the arches are inverted U-sections; and the entire structure is made of steel plate sections.

The deck is being assembled next to a drydock at the shipyard. Parkman project manager Charlie Parkinson says: 'One of the biggest problems is that it all has to be painted under cover because it requires high quality blasting and a high performance paint finish. So the whole thing is being formed in small lengths which can be transported to and from the painters.'

Once it is finished, the arch will be placed on a barge and towed to the site by two tugs. When it gets near to the final position, the barge will be attached by winches to the quays, and then navigated into position using a manoeuvre familiar to anyone parking a car - one end of the deck will be threaded in between two towers and reversed, to allow the other end to be lined up, and then pushed in between the opposite two towers.

To do this, the deck will have to be higher than the abutments, so once it is in place the barge will be slowly sunk to allow the deck to settle onto its bearings.

This tricky manoeuvre is planned for the beginning of October, after which the contractor will have to go through a series of testing and commissioning procedures, as well as finishing decorative work on the quayside and fixing a glazed panel on each side of the deck to provide comfort for pedestrians from the strong easterly winds which whistle up the canal. The structure is due for completion at the end of October.

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