The Foyle Bridge is a spectacular man-made addition to the natural rugged terrain found to the north-east of Londonderry in Northern Ireland. The bridge was completed in 1984 and carries the A515 across the River Foyle; its elegant curve is enhanced by a minimum depth girder profile at the centre of the main span.
When rising traffic levels prompted an assessment of the bridge in 1999, it was found not to comply with prevailing design standards. It was not in a dangerous condition, however the Department for Regional Development Roads Service appointed Hyder Consulting to design and supervise the implementation of required long-term strengthening works.
The 866m-long bridge consists of three river spans, including a central span of 234m and side spans of 144m, made up of twin welded steel box girders of varying depths. The approach viaduct is of twin prestressed concrete box girders, with seven spans varying from 35m to 55m. Strengthening work is confined to the steel sections since the concrete approach viaduct meets current standards.
The bulk of the strengthening work is focused on the bottom flange. 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. Due to the amount of steel plate that would be required to relieve some of the load, this traditional form of strengthening was deemed economically unviable.
The solution that has been developed involves placing a series of compression struts along the bottom flange of the bridge. By adding controlled amounts of load into these struts, a tensile stress can be 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. Fabrication of the steel tubes can be performed off-site, which is cheaper and quicker than on-site working. Minimal traffic management is required as locked-in stresses from traffic on the bridge are not an issue.
Main contractor Farrans Construction recently started work inserting the 500mm diameter circular steel stressing tubes into the bridge structure. 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 and transmit the loads into the box girder, which in some tubes will be as much as 1,000t. Some additional local strengthening is required prior to stressing of the tubes, which will be carried out a pier at a time with no live loading.
The strengthening works are also designed so that as much of the work as possible can take place inside the box girder. This reduces dependence on the weather, the requirement for significant temporary works, and also the risk of working from gantries. To assist access into the confined space of the box girders for men and materials, improvements to box access and internal walkways have been undertaken.
Completion of the US$17 million strengthening project, including repainting and resurfacing, is currently scheduled for 2005.
Phil Bailey works for Hyder Consulting
Foyle's new coat
Protective coatings contractor Pyeroy is carrying out the repainting of the Foyle Bridge as part of the major refurbishment and strengthening works. Pyeroy's 12 month, US$2.5 million contract involves the provision of all access systems for internal and external coating works, spot blast surface preparation to SA2.5 and the application of high performance coating systems to Highways Agency specifications.
Refurbishment of the bridge includes a full repaint of all steelwork along with resurfacing of the roadway and footways, and necessary maintenance to parapets, safety fences and expansion joints.
