Monitoring bridge project with 458 strain gauges

Structural monitoring specialists AV Technology has successfully completed the first two stages of a complex monitoring programme during strengthening and refurbishment of the Foyle Bridge.

Structural monitoring specialists AV Technology has successfully completed the first two stages of a complex monitoring programme for Farrans Construction during strengthening and refurbishment work on the spectacular and elegant Foyle Bridge in Northern Ireland.

AVT's 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 behaviour of the structure as the work is carried out.

Over six kilometres 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 metres, crosses the River Foyle near Londonderry and carries two separate carriageways of the A515 road.

The three river spans comprise twin welded steel box girders of varying depth, with an orthotropic steel deck.

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

The work is 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 HGV's and it is estimated those figures would rise to 36,000 vehicles with 3,200 HGV's over the next 10 years.

The overall project is managed and supervised by Hyder Engineering who developed a technique for strengthening box girder bridges.

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, those large moments use more than 60 per cent of the capacity of the bottom flange.

The Hyder solution involved placing a series of huge compression struts along the bottom flange of the bridge.

By adding controlled amounts of load into those 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.

That removed 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 behaviour during the installation and stressing of the struts was of vital importance.

The AVT data logging system has capacity for over 300 channels and is based on multiple Campbell Scientific CR10X data loggers with ten 32 Channel multiplexers and a multi-drop network.

That enabled several loggers to be 'daisy-chained' to the central PC running the real-time display software.

In addition, the system has 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.

Those are 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 1,000 tonnes were being applied to the tubes and this jacking actually lifts the centre of the bridge spans by around 100mm.

Pressure readings from the jacks are fed into the overall monitoring system and when the required pre-stress level is reached, the precision-machined tapered steel locking-off wedges are finally welded in place.

During the application of pre-loads, the real-time data is vital to enable the Stressing Controller to assess the response of the structure as the loads are increased in carefully controlled increments using the network of 48 computer controlled jacks.

To assist with that, the real time display includes colour coded alarm indications to highlight any imbalance between pairs of sensors on either side of the structure.

Graphs are also displayed showing the linearity of the structural movement as a function of applied load, to verify that all load carrying components are behaving elastically.

The real-time data also enables the stressing controller to apply a calculated amount of overload, to allow for relaxation as the strut loads are transferred from the jacks to the locking-off wedges after each loading increment.

The struts are constructed from 508mm diameter by 50mm thick steel circular hollow tubes.

Around 1,200 metres of tube, weighing over 1,000 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.

Duncan Stringfellow and Phil Bailey are two of the senior Hyder engineers responsible for interpreting the results during the jacking process.

They explained: "Our calculations of how the bridge is expected to perform are based on theoretical analysis and computer generated predictions for the structure.

During the jacking procedure, we are totally reliant on the results from the AVT network of sensors to give us a three dimensional picture on what is actually happening.

Huge jacking forces are being generated and we can see through the real-time graphs exactly how the bridge is reacting." AVT's divisional manager Neil Parkinson is very pleased with the results so far: "The key to successful structural monitoring is being able to collect, analyse and interpret large amounts of good quality information and then to present this information clearly and concisely for others to make informed decisions relating to the structure with high levels of confidence.

To put this into perspective, during the stressing operations, we recorded and displayed around 750,000 data values." All round good communication and the ability to work closely with other interested parties are also vital ingredients.

The successful outcome of the work on the first two pier sections of the Foyle Bridge has been very much a team effort, involving the close co-operation of, AV Technology, DRD, Farrans Construction, Hyder Consulting and Bill Boley personnel.

"The data we provided showed close correlation with the theoretical results and this has boosted everyone's confidence level.

Because all the strengthening work is taking place inside the box girders, from the outside there is little evidence of the major work being carried out.

As a result there is minimum disruption to traffic and the work is not weather dependant.

On completion of the work, the overall external appearance of the bridge will remain the same, save for a new coat of paint and new deck surfacing," added Parkinson.

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