Boathouse Bridge Repairs

client: Christ Church, Oxford
engineer: Peter Corcoran
status: Completed Summer 2013
role: Structural Engineer & Contract Administrator

The bridge provides access between the Meadow and Boat House Island.  It was constructed some time in the 1950’s or 60’s and is an arched bridge constructed of precast concrete segments which have been linked together to form a single arched unit by post tensioning.

We were asked by Christ Church to carry out investigations to the bridge and prepare a method of repair.  The concrete to the underside of the bridge had spalled and a weight restriction imposed on the weakened bridge meant that no vehicles could cross to the island.

We carried out a number of investigations and opening up that confirmed that the post tensioning wires had corroded in a number of locations.  We looked into installing a cathodic protection system however the extent of the corrosion was too extensive.  We carried out finite element analysis of the bridge structure in the existing condition and also with an increased bridge thickness.  The aim of the exercise was to determine if by applying additional weight and mass to the bridge, we could justify that it is acting as a traditional arch with no tensile forces acting to the underside of the bridge.  It was confirmed that by increasing the thickness to the underside of the arch this was the case and that the post tensioning system could be ignored.  The added benefit of the additional skin to the underside was that this will further protect the remaining reinforcement and post tensioning bars.

A micro-concrete was used to the underside as it was poured through holes drilled into the bridge deck.  The concrete had to be self compacting to fill all the voids as access was not possible to vibrate the concrete.

Environment Agency approval was obtained for the works prior to commencement.

The bridge has now been brought back into full operation use.

Pictures kindly provided by Martech