Foyle Road Bridge - Northern Ireland

Team effort ensures successful outcome during bridge strengthening

The 20 year old bridge, which has a total length of 866 m, crosses the River Foyle near Londonderry and carries two separate carriageways of the A515. The three river spans comprise twin welded steel box girders of varying depth, with an orthotropic steel deck. The work has been part of a major project being undertaken by Northern Ireland‘s Department for Regional Development (DRD) to increase the load bearing capacity in order to bring the structure up to current assessment standards. At present the bridge carries 30,000 vehicles a day of which 2,700 are HGV's and it is estimated these figures will rise to 36,000 vehicles with 3,200 HGV's over the next 10 years.

Due to the original construction sequence of the bridge, and the lowering of the abutment ends to relieve moment at the middle of the main span, large moments were developed at the piers. Under dead load, these large moments use more than 60% of the capacity of the bottom flange. The overall project has been managed and supervised by Hyder Engineering who have developed a ground-breaking and highly innovative technique for strengthening box girder bridges. Their solution has involved placing a series of huge compression struts along the bottom flange of the bridge. By adding controlled amounts of load into these struts, a tensile stress has been created in the box girder's bottom flange, reducing the 'locked-in' compressive stresses resulting from the dead load. This removes the requirement to add local strengthening to the bottom flange with all its associated welding and fitting problems.

Our part of the project involved an innovative jacking system comprising 48 individual high capacity jacks, arranged into 8 sets, one per strut section. These were used to pre-stress the tubes to relieve the large compressive stresses in the bottom flanges of the girder. Controlled loads in the region of 1000 tonnes were applied to the tubes and this jacking actually lifted the centre of the bridge spans by around 100 mm. Pressure readings from the jacks were fed into the overall monitoring system and when the required pre-stress level was reached, the precision machined tapered steel locking-off wedges were finally welded in place.

During the application of pre-loads real-time data from over 450 strain gauges and displacement sensors was vital to enable the Stressing Controller to assess the response of the structure as the loads were increased in carefully controlled increments using the network of 48 computer controlled jacks. The struts are constructed from 508 mm diameter by 50 mm thick steel circular hollow tubes. Around 1200 m of tube, weighing over 1000 tonnes, has been fabricated in sections off-site and then lifted inside the box girders through holes cut in the carriageways, before being welded together. Twin parallel tubes are supported just above the bottom flange with steel frames pinned to additional steelwork welded to the box webs. The tubes extend from both sides of the main pier diaphragms into the spans either side. Anchorage at the diaphragm is via a cast- in-situ concrete block. At their other end anchorage is provided by fabricated steel jacking anchorages, which provide restraint to the jacking loads. The successful outcome of the work on the Foyle Bridge has been very much a team effort, involving the close co-operation of personnel from Bill Boley, AV Technology, DRD, Farrans Construction and Hyder Consulting.