Consulting engineer, Mott MacDonald, has devised a £6m stabilisation plan as an alternative to the previous £30m scheme devised by Pell Frischmann.
H&F commissioned Dr Steve Denton, head of civil, bridge and ground engineering at consulting engineer WSP, to compare the two options to stabilise the bridge’s cast iron pedestals. He concluded that the Mott MacDonald proposal was technically superior, could be implemented more rapidly and was more cost efficient than the Pell Frischmann scheme.
As well as saving £24m and reducing the works programme to 46 weeks, the Mott MacDonald plan – which according to the council has been favourably reviewed by Heritage England – is expected to reduce the need for temporary closures and avoid the need to divert the gas mains on the bridge for the stabilisation work.
The new proposal was presented to engineers and officials from the Department for Transport (DfT) and Transport for London (TfL) at a meeting with Dr Steve Denton and H&F engineers held on 9th August 2021.
The bridge was reopened to pedestrians, cyclists and river traffic on 17th July but has been closed to traffic since Aprtil 2019 when cracks were found in the pedestals to which the suspension chains are fixed at each end of the bridge,
H&F council leader Stephen Cowan said that the council would go ahead and fund the £6m package in anticipation that the DfT and TfL will subsequently reimburse the council with their one-third shares as outlined in the government’s TfL funding announcement of 1st June 2021.
He said: “Whilst putting the safety of the public first, we believe that the importance of maintaining pace and progress, the real savings achieved by the deployment of the preferred stabilisation works option and the current vulnerability of Hammersmith Bridge demands rapid action.”
Dr Denton’s report explains the engineering. It states: “The design concept for the Pell Frischmann proposal is to construct an external frame that provides an independent load path, eliminating reliance on the pedestals until they are strengthened and new bearings are installed. Given the uncertainties about the pedestals at the time the concept was first developed, it is entirely understandable why such an approach was initially taken.”
By contrast, the Mott MacDonald solution involves the use of elastomeric bearings, which allow any pressure to be applied equally to all four corners while protecting the vulnerable 134-year-old cast iron structure.
Dr Denton said that the Mott MacDonald proposal benefits from the insight gained from the refined pedestal analysis that engineers have done over the last year.
He said: “As a result, jacking to enable bearing replacement is undertaken using the pedestal itself, with the robustness of the pedestal having been enhanced prior to jacking.”
He concluded: “The Mott MacDonald proposal is significantly more cost effective.”
Whereas the Pell Frischmann plan suggested the driving of piles through historic fabric, Dr Denton said “the Mott MacDonald proposal has lower impact on historic features”.
Councillor Cowan said “Having reopened the bridge to pedestrians, cyclists and river traffic earlier this month, we are keen to press on with the next phase of work. I am grateful for the advice from Professor Denton and look forward to reaching agreement with the Government and TfL on the funding of the stabilisation and full repair works.”
The pedestrian stabilisation plan is the first phase of works on the bridge. The second phase will involve extensive strengthening and full restoration and will allow the bridge to reopen eventually to vehicles.
Dr Denton is now considering the two current options for the strengthening and restoration work – the existing TfL plan and the Fosters & Partners/Cowi proposal for a temporary double decker truss.