Monitoring the Foyle Bridge

By: Neil Parkinson

Structural monitoring specialists AV Technology have successfully completed a complex monitoring program for Farrans Construction during strengthening and refurbishment work on the spectacular and elegant Foyle Bridge in Northern Ireland.

AVT's crucial part of the project has involved the strategic installation of around 450 pre-wired full bridge strain gauges, 450 temperature sensors, 10 displacement sensors and eight jacking pressure sensors to keep a close eye on the behavior of the structure as the work was carried out. Over 6 km of wiring has been used throughout the bridge, connecting the sensors back to central data logging locations within the girders.

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 main span is 234 m, which is flanked by side spans of 144 m.

The work has been part of a major project being undertaken by the Roads Service Agency within Northern Ireland's Department for Regional Development (DRD) to increase the load bearing capacity of the bridge 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 HGVs, and it is estimated these figures will rise to 36,000 vehicles with 3,200 HGVs over the next 10 years.

The overall project has been managed and supervised by Hyder Engineering, which has developed a ground-breaking and highly innovative technique for strengthening box girder bridges.

The Hyder 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 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.

Keeping a close eye on the bridge behavior during the installation and stressing of the struts was of vital importance. The AVT data logging system had the capacity for over 300 channels and was based on multiple Campbell Scientific CR10X data loggers with 10 32-channel multiplexers and a multi-drop network. This enabled several loggers to be “daisychained” to the central PC running the real-time display software. In addition, the system incorporated a GSM modem to enable remote monitoring of data by AVT from their offices in Stockport.

AVT employed the services of controlled hydraulic jacking specialists Bill Boley Ltd. to fit and execute an innovative jacking system involving 48 individual high-capacity jacks, arranged into eight sets, one per strut section. These were used to prestress the tubes to relieve the large compressive stresses in the bottom flanges of the girder. Controlled loads in the region of 1,000 tons were applied to the tubes, and this jacking actually lifted the center of the bridge spans by around 100 mm. Pressure readings from the jacks were fed into the overall monitoring system, and when the required prestress level was reached the precision machined-tapered steel locking-off wedges was finally welded in place.

During the application of preloads, the real-time data 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.

To assist with this, the real-time display included color-coded alarm indications to highlight any imbalance between pairs of sensors on either side of the structure. Graphs were also displayed showing the linearity of the structural movement as a function of applied load, to verify that all load-carrying components were behaving elastically. The real-time data also enabled the Stressing Controller to apply a calculated amount of overload to allow for relaxation as the strut loads were transferred from the jacks to the locking-off wedges after each loading increment. The struts are constructed from 508-mm-diam. by 50-mm-thick steel circular hollow tubes. Around 1,200 m of tube, weighing over 1,000 tons, has been fabricated in sections offsite 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 on 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. Because all the strengthening work has taken place inside the box girders, from the outside there is little evidence of the major work being carried out. As a result there has been minimum disruption to traffic and the work has not been weather dependent. On completion of the work, the overall external appearance of the bridge has remained the same, save for a new coat of paint and new deck surfacing. The successful outcome of the work on the Foyle Bridge has been very much a team effort involving the close cooperation of AV Technology, DRD, Farrans Construction, Hyder Consulting and Bill Boley personnel. Campbell Scientific's instrumentation has allowed each party to obtain the data they need to operate effectively and safely.