Hammersmith Bridge | Motts alternative stabilisation plan approved after slashing millions off proje
Hammersmith & Fulham Council has approved Mott MacDonald’s alternative proposal to stabilise Hammersmith Bridge’s cast iron pedestals.
The proposal is earmarked to cost just £6M, some £24M less than the original stabilisation plan drawn up by Pell Frischmann.
It will also be completed in under a year, some 12 weeks quicker than Pell Frischmann’s proposal.
Hammersmith Bridge reopened to pedestrians and cyclists earlier this summer, some 10 months after it shut due to fears that it could suffer a catastrophic collapse. It has been closed to motorised traffic for more than two years after cracks appeared in its cast-iron structure.
The council opted for Motts design after commissioning WSP head of civil, bridge and ground engineering Steve Denton to carry out an examination of both proposals.
He concluded that the Motts proposal was “technically superior”, could be implemented more rapidly and was “more cost efficient” than the £30M scheme tabled by Pell Frischmann.
Speaking to NCE, Denton stressed that the Motts proposal benefitted from being drawn up after the bridge’s investigation was complete, adding that Pell Frischmann’s original proposal made sense at the time it was developed.
As well as saving £24M and reducing the works programme to 46 weeks, the new plan - which has also been favourably reviewed by Heritage England - is expected to reduce the need for temporary closures. Engineers believe it will also avoid the need to divert the gas mains on the bridge for the stabilisation work.
The Motts solution requires the removal of the end panels to fit a temporary frame on to the bridge. The deviation saddles will then be jacked up from the pedestals using a system of oblong flat jacks to facilitate replacement of the existing steel rollers with laminated elastomeric bearings.
Denton said that the “really challenging part” of Motts proposal will be replacing the bearings while managing any differential force between the structure’s chains.
He added: “The investigation of the structure has shown that the existing roller bearings have become seized.
“This means that any differential force between the chains either side of the pedestals is transferred into the pedestals rather than being equilibrated by movement of the saddle and rollers.
“The management of this differential force is the reason for the temperature control system currently installed on the bridge
“The approach to replacing the bearing needs to account for this potential locked in force and enable it to be released in a controlled way.”
In order to replace the bearings while controlling the differential force, Mott’s design uses hydraulic cylinders to hold the saddle in place and enable this restraint to be gradually released (see diagram, below).
Denton explained that Motts proposal will also require a “lighter touch” in terms of its impact on the existing Victorian structure.
Whereas the Pell Frischmann plan suggested the driving of piles through historic fabric of the bridge, Denton said “the Mott MacDonald proposal has lower impact on historic features” and has been favourably received by Heritage England.
Denton’s report adds: “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.”
Denton added the Motts proposal benefits significantly from the insight gained from the refined pedestal analysis 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.”
In order to begin the full stabilisation works, Hammersmith & Fulham Council leader Stephen Cowan has agreed and signed an Urgency Report.
Cowan said: “We don’t want to lose a single day in delivering the full stabilisation of the bridge to ensure residents on both sides of the river no longer have to deal with closures or the threat of closures.
“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.”
Cowan said, in order to expedite the works at speed, the council will 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 9 June.
The council expects to keep the bridge open to pedestrians and cyclists for the vast majority of the duration of the works, but there may be short programmed periods of closure to allow some works to take place safely.
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.
Denton is now considering the two current options for the strengthening and restoration work - the existing TfL plan and the pioneering Fosters + Partners/ COWI proposal for a temporary double decker truss.