The challenge
Originally constructed in the 1960s, Barton High Level Bridge (on the M60 near Manchester’s Trafford Centre) was widened in 1989 to create an additional lane of traffic in each direction. According to design guidance at the time, differential movement between the new and existing piers should be kept to a maximum of -3mm, to prevent damage being caused to either the girders or bridge decking.
After recent monitoring of the tolerances revealed that they were either at or beyond their limit, with the bridge sitting within National Highways’ top 20 ‘Structures Requiring Most Consideration’, the need for a jacking scheme was triggered. Required to lift up the deck and reintroduce tolerances back into the bridge, precise monitoring data would also be used to realign the bearings with additional shims.
Along with many other structures, the Barton High Level bridge, which rises 30 metres above the Manchester Ship Canal, is covered by National Highways’ Construction Works Framework and led to civil engineering specialist Balvac being awarded a Task Order for the necessary interventions.
Building on our previous experience of working with Balvac and other National Highways framework members, we engaged in Early Contractor Involvement, contributing to the pre-contract planning process. This included liaising with Amey - the project’s consultant structural engineers - and advising on the appropriate equipment required to detect movements in the structure, measured in fractions of a millimetre.
Our solution
The real-time monitoring consisted of 14 linear displacement transducers which were used to ensure that the bridge was kept within tolerance throughout the works. A total of 85 No strain gauges were installed on each side of the structure to measure the stresses created during the jacking works. 13 No Strain gauges were installed on top of the concrete bridge deck; these were installed through holes which were cored in the tarmac to allow access to the concrete. Underneath the deck, 40 No Tri-Axial gauges were used to monitor the stresses in the main steel beams. Arrays of 10 were installed on both the internal and external faces of the beams. A further 32 No linear strain gauges were used to monitor the stresses in the cross bracing between the outer and inner longitudinal beams.
Both the ambient and steel temperatures were recorded in order to correlate the movements due to thermal effect. All data was transmitted back to Insite, our web portal for round the clock visibility while alert thresholds were set to automatically notify of any breaches in movement levels to ensure that the bridge remained within safe working limits.
One of the challenges with the project was that the motorway had to remain live throughout the works. In order to lift the bridge, bespoke high-pressure jacks were required because of the limited space available while, due to regular expansion and contraction of the deck, bespoke temporary bearings were designed and installed to allow for the normal articulation of the structure.
Andrew Higson, the Contract Manager for Balvac, commented: “Balvac has worked with Mabey Hire to successfully deliver other significant projects for National Highways. This project provided an opportunity to continue that relationship, with confidence that the Mabey Hire team would provide the level of service that this complex scheme required.
“The adjustment work needed to be undertaken to fractions of millimetres, hence the accuracy of the monitoring being critical to successful completion of the project. Accurate measurements were also required to ensure that thermal expansion was taken into account, and to record strain movement of the structure to mitigate the risk of damage.”
He concluded: “Mabey Hire played an important role in the effective planning and delivery of these works. They used their experience to help plan around key project risks, and the level of support during the work was extremely professional, and well-aligned to our target of achieving technical excellence through a ‘right first time’ approach.”
