Design of strengthening works for an 1890s steel rail bridge

Abstract

The project involved the strengthening and deck replacement of a five-span lattice truss viaduct that carries the West Highland Line across a steep-sided gorge near Crianlarich. The bridge was designed and constructed for the lines opening in 1894. Railtrack Scotland awarded a design-and-build contract to Edmund Nuttall Ltd with White Young Green as Design Consultants to assess the options to strengthen the bridge and to meet current design standards, and to upgrade the capacity of the line for future increases in the weight of freight vehicles. White Young Green's input also involved detailed design, site supervision and the design of enabling works for free-standing and suspended scaffold structures. Innovative truss-strengthening measures included the installation of an external post-tensioning system deflected from the bottom members of the bridge truss. This increases the live load capacity of the truss by reducing permanent load stresses The system allowed the capacity of the trusses to be increased without the practical difficulties associated with adding additional metalwork to the top and bottom truss members. The scheme adopted comprised full deck replacement (still to be carried out), strengthening of the main truss members, and replacement of tertiary bracing members. The predicted response of the bridge under load was determined by developing a space frame model, which included all members of the truss and post-tensioning system. The analysis was used in conjunction with Freyssinet Ltd to develop the stressing system of deflected bars and strand tendons. Real rail vehicle loading, both stationary and slow moving (to minimise dynamic effects), was used in conjunction with strain gauges to confirm that the behaviour of the bridge before and after modification was as expected by the analysis. Monitoring proved that actual and predicted results were within 10% of each other. Some of this variation was attributed to the loads detected by strain gauges in secondary members. In conclusion, the post-tensioning process proved to be rapid and straightforward, with one span being post-tensioned per night shift. Other repair methods would have required much longer ‘no train’ periods to construct. The post-tensioning option was considered to offer the best value for money to Railtrack, and has the potential to be used on other rail bridge strengthening schemes.