This article was published in 2004, in Newsletter 52.
We asked Gareth Guest, Area Bridge Engineer at Cambridgeshire County Council, to explain why this bridge is closed and what might happen to it next.
Earlier in 2003 Cutter Ferry footbridge was subject to a Principal Inspection. These inspections are carried out on our behalf by Atkins and they are programmed for every six years. This report identified a deflection to the top of the steel chord and also confirmed what we already knew, in that the timber decking was becoming unserviceable and increasingly difficult to repair.
A bid was made to include Cutter Ferry footbridge in a package of works to redeck and refurbish six Cambridge footbridges. So as part of this refurbishment work our consultants were asked to fully assess the bridge for the loading of the new ekki timber deck. It was this assessment report that identified a 120 mm deflection to the top chord of the footbridge due to inadequate cross bracing. The calculations also indicated that under normal pedestrian loading conditions the top chord could become over stressed and the deck would twist and catastrophically fail. The risk of this occurrence was high and therefore with this knowledge we had no other option but to close the footbridge in the interests of public safety. In liaison with the police, a diversion route was set up via the dual use paths on Elizabeth bridge just along Cutter Ferry path. This ensured there would not be any potential crowd loading or congestion on the other nearby Fort St George footbridge.
From the date of closure our consultants were then asked to undertake a full study of the footbridge to determine the actual mechanism of failure. This can then be used to determine if a cost effective repair could be carried out. However, the structural analysis of the steel footbridge to BS5400 and current DfT standards is an involved operation. Temporary scaffold propping was not the answer, as what is required is bracing to the top chord of the bridge at all node points, including the ends where of course we need to maintain a headroom.
So a permanent bracing system is now being designed, costed and evaluated against a whole replacement new span across the river, to ensure the prudent use of the limited bridge maintenance budget that we are allocated. Of course we recognise that should a new span prove the best solution, then we should consider constructing this and adapting the existing bridge to current cycleway standards, to further enhance this valuable crossing of the River Cam.
The results of this study, and if funds are made available for this work, should be known towards the end of January 2004. A press release will be issued and the web page1 will also be updated.
Gareth Guest, Area Bridge Engineer, Cambridgeshire County Council
1 Cambridgeshire County Council’s Current Cambridge Transport Projects page: