Strengthening of the Hammersmith Flyover, London (Phase 2) – design of the prestress

Hammersmith Flyover is an early precast segmental bridge located in west London which is now owned by Transport for London (TfL), the city's integrated transport authority. It has prestress which is described as external but runs in grouted ducts for some of its length. The prestress was deteriorating seriously and emergency strengthening to critical sections was installed in 2012. It was decided in anticipation of further deterioration that new prestress should be installed to make the structure independent of the original and allow for the possibility of this becoming completely ineffective. This is believed to be the first time that full replacement prestress has been installed in a bridge where it was not possible to remove the original. This paper describes the design, design approach and challenges, including use of innovative techniques which have won an award and bespoke design approaches. Much of the difficulty resulted from the original form of the bridge, which left very little space for the new prestress, and from the requirement to minimise disruption to traffic both below and on the bridge.