An old road bridge over the rail route on the A814 Dumbarton Road in Old Kilpatrick, Scotland, had deteriorated and was deemed inadequate for the volume of traffic using it daily, and was consequently scheduled for replacement. A complete bridge rebuild would take some time and minimising interruptions on the busy route was a priority – so a modular steel structure was commissioned to ‘overbridge’ the existing one to provide a reliable and safe diversion for traffic.

A 12-bay Mabey Compact 200® (C200®) bridge, 36.58 metres long and 6.3 metres wide with anti-skid decks, was selected for the project, partly for its modularity and strength, but also because of its speed of assembly and installation. It was to be installed on two new concrete foundations, set back from the old bridge, eliminating the need to introduce further weight restrictions. New approach ramps would be built at either end, with the road realigned to allow better sight lines.

The active rail track below the old road bridge required work to be completed during a possession, so a closely timed, well-controlled process was established with all stakeholders to ensure deadlines were achieved safely. Assembly of the bridge took less than one week, despite delays from two major storms.

Using the temporary structure ensured the quickest, safest and most efficient method to keep route closures to a minimum and it remained in place for approximately 18 months until the reconstruction of the old bridge was completed. Once dismantled, the components were reused for a permanent overbridge at another location, in Knagshill.