Ian Firth: Designing bridges for tomorrow

The British Columbia Regional Group were extremely fortunate in having legendary bridge engineer Ian Firth deliver a special guest lecture at the group’s annual dinner event. A Past President of the Institution of Structural Engineers, Ian currently chairs the British Group of the International Association of Structural Engineering.

Regional Group Chair David Harvey welcomed the attendees and introduced Ian, who kicked off his presentation with an image of the Wye and Severn Bridges. He noted the then revolutionary concept of the aerofoil deck section for both bridges, used for the Severn suspension bridge to achieve aerodynamic stabilization and low suspended weight. Assigned to evaluate and strengthen the Wye Bridge, Ian needed to first create access points to work inside the box deck to install new stay anchorages and other strengthening measures. With the pylons extended and new upper and lower cable stays installed, Ian describe his discomfort in cutting out the original stays with the bridge operational to effect load transfer to their replacements. Ian firmly credits his early career experience of existing bridge assessment and retrofit design as the best possible way of getting to understand structural behaviour.

Ian then moved to a gem of a bridge – Pont Schuman, Lyon, France. Conceived as two tied arch spans, buttressed over a central pier via drop-rib haunching forming two continuous spans of around 85 m. The unique structure features slender triangular arch ribs and an ultra-thin 600 mm roadway deck-slab comprising reinforced concrete topping on a composite steel plate soffit. This was necessitated by the very low road profile constrained by the tie-in points at each deck end. The slab effectively stabilizes the outwardly canted arch planes. With limited access to the river and no on-site laydown areas, the bridge was developed around prefabrication at a downstream site and barge installation. The main snag was fifteen city bridges between the fabrication yard and the site which would not accommodate the tied arches. The ingenious solution was to hinge the arch ribs into the flat position to minimize the height of the floating barge. Loaded via Self-Propelled Modular Transporters and careful barge ballasting, the two tied-arch units where first navigated up-river, the arches jacked into their planes, before the two tied-arch units we raised into position on falsework and the remaining structural work completed. The finished bridge is very elegant and looks impossibly slender.