The Design of the New Medway Bridge

The New Medway Bridge comprises 12 continuous spans of twin cell box girder with an overall length of 953m and has the UK's longest externally post-tensioned span of 152.4m. It carries four London-bound lanes and a hard shoulder of the M2 and was constructed as part of the A2/M2 widening project in Kent for the Highways Agency. The need for external post-tensioning was imposed by the Client for durability and maintenance reasons. External post-tensioning can lead to a loss in structural efficiency when compared to internal post-tensioning, particularly with respect to anchor zones and overall flexure. There is also additional cost associated with the provision of deviators. The bridge was built under a Design and Build contract which made the need for economic and buildable details all the more acute. This paper identifies the various ways in which a cost-efficient durable design was produced and the way detailing problems specific to such a large externally posttensioned structure were overcome. Examples include consideration of composite action between formwork and deck in construction, optimisation of the permanent flexural design through overall non-linear analysis, design of diaphragms and spanning anchor beams, constraints on placement of external cables and simplification of the details for deviator blocks while maintaining adequate tolerances for construction.

• Introduction

• Global Deck Design

• Pier Design

• Cable Layouts

• Local Elements – Anchorages, Diaphragms and Deviator Beams

• Design for Temporary Conditions During Construction

• Tendon Replacement Using a Type 1 Tendon System

• References