Built in the 1950s during the reclamation of Hackney Marshes after WW2, Cow Bridge was a single-track traffic bridge connecting the marshes to the neighbouring Clapton Estate across the Lea Navigation Canal. The bridge was in use right up until 2002 when it was eventually closed to traffic due to defects and inadequate design capacity.
As part of the improvements plan for Hackney Marshes, the bridge was earmarked as central to the mobilisation of the project, providing essential access for works traffic to the site of a new community facility on the South Marsh. But initial studies carried out for the Transport Department at Hackney Council priced the demolition and re-build at £3 million – twice the intended budget – and so the project lay dormant for two years.
While working on the Hackney Marshes Centre project we were asked to appraise the existing bridge structure to see if it was feasible to reopen it with a pedestrian walkway. We carried out an inspection and recommended that the ramp and parapet walls were reconditioned and the defective central span replaced with a lighter construction meaning no verifications would need to be made on the substructure. This solution met the £1.5 million budget, and by retaining the original structure, the scheme also minimised the need for temporary works, making an additional cost saving of £250,000.
The existing span of the bridge is replaced with a new steel deck which sits on the existing abutments to accommodate two-lane traffic, and a steel walkway is cradled alongside this to allow safe pedestrian crossing. The geometries of the new spans are informed by the steepness of the abutments – with the existing abutments too steep for disabled access, the walkway hangs below the level of the roadway to achieve compliance with DDA regulations.
The diagrid steel structure is lightweight in comparison to the pre-existing concrete deck, so eliminates the need to verify foundation piles, and reduces bearing requirements. A composite steel and polymer sandwich plate deck system provides a lightweight surface for vehicular and pedestrian traffic. The result is an eye-catching, economic and sustainable solution for Hackney, reinstating much-needed access to the marshes.
- RICS Awards 2013, highly commended, Jan 2013