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In full swing

The engineers of the past were not afraid to experiment with new ideas, and the canal system gave them a great opportunity to test their theories. The Cheshire town of Northwich is home to the first electro-mechanically operated swing bridge in the country - the 99 year old Town Bridge which carries traffic over the River Weaver.

Its designer - a Colonel Saner - was keen to come up with a support system which put the least possible stress on the electrical and mechanical equipment. As a result, the enormous 360t lattice truss deck is supported one third of the way across the river by four quadrant-shaped pontoons, which originally floated inside a restraining ring of concrete piles. These were replaced by a circular concrete caisson during a refurbishment of the bridge in 1926. An arrangement of fan girders is fixed to the top of the pontoons, and links to a triangular support frame which spans the caisson, with a set of 76 rollers sitting on top to enable the deck above to move.

The entire deck and the pintle arrangement between the deck and the pontoons was built in cast iron and riveted together. It is an enormous structure which has been successfully carrying heavy traffic for most of its 100 years with only a few repairs.

Now, however, Town Bridge is undergoing a major strengthening programme to meet the 40t load requirement for highway structures. At the same time its owner British Waterways is taking the opportunity to refurbish and repaint it ready for its centenary next year.

Consulting engineer Parkman undertook the original survey, and is now managing the repair and strengthening. The major items in the project are replacement of the upper portion of the pontoons, which have become corroded; painting the pintle framework and strengthening the fan girders; replacing rivets with high strength bolts at the weak points on the bridge trusses; a new deck at the nose of the bridge, and replacement of the transverse beams; replacing the hanger bolts in the tail, and providing access for inspection; overhauling the mechanical system and replacing the electrical system; and replacing the surfacing with a 40mm mastic asphalt to reduce dead load.

The £1.5M contract was won by Kvrner, which is doing all the work on the river bank. A 1,000t crane was brought in to lift the huge deck off its bearings - a dramatic sight in this small town, which will be repeated when the deck goes back next month. Once it is back in place, a similar refurbishment programme is likely to be carried out on the adjacent Heyhurst bridge, built using the same design, and three other swing bridges on the River Weaver.

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