New Civil Engineer Civil engineering and construction news and jobs from New Civil Engineer
A double-decker temporary crossing running above the existing deck of Hammersmith Bridge would allow pedestrians and motorists to cross the Thames while permanent repairs are carried out to the ageing structure.
According to its backers, it would also aid the permanent repair programme, shave time and cost off the overall project (around £40M less than the current £141M restoration plan), create a safer environment for contractors working on the repairs as well as being more environmentally friendly than current proposals.
If given the go-ahead, the double-decker crossing could open to pedestrians early next summer, before opening to motorists around two months later.
It is the brainchild of Foster + Partners head of structural engineering Roger Ridsdill Smith, who has developed a comprehensive feasibility study alongside Cowi senior technical director David MacKenzie.
Hammersmith & Fulham Council has backed the plan and a construction plan is now with the Department for Transport’s (DfT) Hammersmith Bridge Taskforce to rule on.
It is the culmination of years of neglect and underfunding.
A ferry service run by TfL and sponsored by Uber is due to be in place by the end of this summer, but the Taskforce has previously said that it is open to the prospect of a temporary crossing.
Hammersmith Bridge, temporary solution
The double-decker solution involves building a new raised truss structure above the existing road deck. The structure will consist of two sections launched from abutments at either end of the bridge.
They will then be pushed through Hammersmith Bridge’s existing tower opening towards the midspan where the two sections will meet in the middle.
A vehicle ramp is then installed at either end to allow access to the upper deck of the temporary structure.
At first pedestrians and cyclists would use the upper level and the lower level would be used as a construction site; giving workers access to the Victorian structure’s deck and hangers.
This would allow contractors to remove Hammersmith Bridge’s existing deck in sections and drop them on to barges in the Thames to be taken offsite for refurbishment, lessening the load on the structure which in turn allows for motorised traffic to use the temporary crossing.
Once Hammersmith Bridge’s deck has been successfully removed the lower level of the temporary crossing will then be used by pedestrians and cyclists, while the upper level would allow motorists to cross the Thames.
Ridsdill Smith explained that the existing bridge’s constraints meant that it made more sense for motorists to use the top level of the double-decker crossing.
“That is something that we looked at in detail,” Ridsdill Smith said. “If we were to have the vehicles on the bottom then we would have to ensure the height of the upper deck was that much higher to give vehicles clearance. That in itself would involve building longer ramps at either end of the bridge.
“The second reasons is that our trusses need to be inside the width of the pier openings. If you put the vehicles on the bottom then you confine the width of the vehicle way as the access is pinched around the pedestals and piers but that is not the case if you go above.”
Hammersmith Bridge: Closed since August 2020
Unlike other temporary crossings suggested for the site, the double-decker solution does not require any additional road works either side of the Thames as it utilises the existing approach roads for Hammersmith Bridge itself.
Ridsdill Smith added: “One of the benefits of this proposal is that we don’t need to put new work into the river; because we don’t have a new route we also don’t need to connect in to roads that are not use to having significant amounts of traffic – that in turn means you avoid having to obtain all the relevant rights and permits of building a new route.”
But the double-decker solution will not just benefit those using it. MacKenzie explained that the structure will also aid contractors carrying out main repairs Hammersmith Bridge.
As well as aiding with the removal of the deck and hangers, MacKenzie said the temporary bridge will also give better access to bearings at the top of each tower which need replacing.
“In terms of site access, it’s a huge benefit,” MacKenzie said. “At first the lower level gives a safer environment to remove the deck for workers to work in.
“It then offers a platform for replacing the tower bearings as well. It is a complete solution, not just for getting people across the river but also for improving the refurbishment of Hammersmith Bridge.”
Ridsdill Smith added that while the solution is somewhat “unique”, each element of the design is something that has been done before and proven to work.
“Double decker structures have been done before. I can think of plenty, Brooklyn Bridge as a starter," Ridsdill Smith said.
“Aspects of this solution have been done before. We are not trying to make it something that has never been done before; we are assembling known bridge and engineering solutions for a particular challenge.
“Double-decker bridges exist, launching bridges is a known technology, the work that Cowi has done on existing bridges has been instrumental in evolving the construction sequence as has my experience working on the Millennium Bridge in terms of working in the Thames.
“All together, in my opinion, it makes for the best solution for Hammersmith Bridge.”
Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here.
CORRECTION: The ferry service is sponsored by Uber, not operated by them.
Height and width are restricted at the point where the temporary bridge passes through the existing arched towers. When can we see a section at this point drawn to scale showing two buses passing one another?