The A1 between London and Edinburgh was in March 2004 upgraded by the opening of the 13.7km Haddington to Dunbar Expressway. A key aspect of this scheme was the construction of a post-tensioned concrete bridge across the River Tyne valley. In keeping with the location, the Scottish Executive required that the crossing be of striking form; resulting in a 217m long three span structure, with a centre span of 89.4m and slender raking piers fully encastré with the deck and the foundations.

The design was carried out by Scott Wilson on behalf of Balfour Beatty, who constructed the bridge as a design-build contract.

The deck was fully cast on scaffolding prior to post-tensioning; this allowed a tendon arrangement without the heavy intermediate anchorages, required for a balanced cantilever type construction; it also much simplified analysis.

The structure was analysed using LEAP5 Classic, with post tensioning effects generated using the associated PRELOAD pre-processor. (Since completing the design the PRELOAD functions have been included in the SAM-LEAP5 integrated software).

Reinforced concrete sections were designed using SAM section modules. These were particularly useful for designing the slender pier legs, where high axial forces and moments required a very heavily reinforced concrete section with up to three layers of 50mm diameter bars in each face.

The flexible nature of the piers resulted in a complex interaction between the structure and supporting falsework; as prestress is applied load is transferred from the span regions to the pier and adjacent falsework. Modelling this interaction proved to be one of the most challenging aspects of the design process, structure and scaffolding were combined in a single model. By designing a staged application of prestress and associated lowering of the falsework it was possible to transfer load from the scaffolding to the piers without applying undue stress to either the scaffolding or structure.