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Foyle crossing pioneers flange tensioning to minimise disruption

POST TENSIONED compressed steel tubes are to be installed within the deck of Northern Ireland's Foyle bridge as part of a £8.5M strengthening programme.

It is hoped that by placing the 508mm diameter, 50mm thick tubes inside and along the length of the steel box girder, the bridge will be strengthened with minimum disruption to traffic above.

The bridge crosses the River Foyle in Derry and is a major link from Northern Ireland to Donegal in the Irish Republic. It was built in the 1980s and uses pairs of parallel box girders to form a main span of 233m and two 144m side spans.

The innovative work was designed by consultant Hyder for the Northern Ireland Roads Service and will enable the bridge to carry 40-tonne trucks and handle higher volumes of lorry traffic.

Contractor Ferrans is expected to start work next February to lay tubes in pairs longitudinally along the bottom of the steel box girders, which are up to 9m deep.

Twenty concrete jacking points at the bottom flange will transfer the load When in place each tube will be jacked to apply a tensile load into the bottom flange of the box. This will reduce the compressive stress in the bottom flange by 22MN.

Comparable conventional strengthening would have required the bridge to be closed while stiffeners were welded inside the box girders.

This week, engineers were busy on site surveying the inside of the steel box girder in preparation for the tube installation.

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