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Tue September 21 2021

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Rescuing Hammersmith Bridge

26 Jul All access to an historic London river crossing was suddenly lost last year when it was found to be close to collapse. Now a radical replacement proposal offers a solution to a problem fraught with structural woes and funding rows. Helena Russell reports

The sudden closure of Hammersmith Bridge to all traffic – both over it and beneath it – last August looked like the last nail in the coffin for London’s much-loved historic landmark. 

Vehicles had been banned from using it since April 2019, when cracks were found in the pedestals to which the suspension chains are fixed at each end of the bridge, and the news that some of these cracks were getting bigger at an alarming rate looked like it might sound the death knell for this crossing. 

That the complex and fragile bridge was in need of repair came as no surprise – it was already known to be in a poor condition. But the latest discovery made the issue far more urgent and, after years of wrangling over who should meet the cost, it seems the dramatic loss of access has finally focused minds on finding a solution. 

This article was first published in the July 2021 issue of The Construction Index magazine. Sign up online.

The historic 253m-long suspension bridge, which crosses the River Thames in west London, was designed by the celebrated engineer Sir Joseph Bazalgette and opened to traffic in 1887. As the bridge carries a local road, its ownership transferred to the London Borough of Hammersmith & Fulham (LBHF) in 1985 after the Greater London Council was abolished.

A structural integrity review in 2014 involved uncovering parts of the structure that had never been inspected since its construction and confirmed that it had been poorly maintained for decades. The review established a four-stage process for strengthening and refurbishing the bridge to enable it to once again carry 18-tonne double-decker buses. 

Transport for London (TfL) has been project managing the work to repair the bridge, but the council claims that so far its applications to the Department for Transport (DfT) for the funds to progress the work have fallen on deaf ears. 

The cost of simply making the bridge safe and usable again by pedestrians and cyclists is estimated at £46m; to fully restore it and reopen it to buses and other vehicles would be an eye-watering £163m. 

Meanwhile the council, LBHF, has to find £2.7m every year just to prevent further deterioration and stop it from falling down. The years of neglect have unsurprisingly left their mark on the bridge with knock-on impacts on its availability. 

In the mid-1970s the stiffening trusses of the deck were replaced along with some of the hangers, the tower roller bearings and the roadway deck. In 1996, the chain saddle rollers on top of one of the towers were found to have seized and it was immediately closed to general traffic, but a load test subsequently enabled single-deck buses to continue to use the bridge. 

Strengthening and refurbishment work in the late 1990s allowed the bridge to be reopened with a 7.5-tonne weight limit.

The most recent review revealed the extent of structural problems with the bridge; roller bearings on the pedestals had become seized, compromising its flexibility and causing fractures in the cast iron structure. Weekly safety inspections were implemented, with sensors installed on the bridge to monitor its behaviour. 

How any repairs and long-term maintenance will be funded is still to be fully resolved, but the creation last September of the government-led Hammersmith Bridge Taskforce, and the sudden emergence of a radical technical solution, could finally signal progress.

The existing bridge is Grade II* listed and was built for the Metropolitan Board of Works. But it was constructed over foundations built for an earlier crossing – the first suspension bridge built over the River Thames – and, ironically, it is this frugal decision at the time which its modern-day guardians have to blame for the structural issues they are currently trying to resolve.

When Bazalgette’s Victorian masterpiece was built, it was not thought necessary to remove the anchorages of the previous crossing. Consequently the suspension chains of the existing bridge are supported on substantial pedestals that sit directly above the disused anchorages, and deviate the chain system around the obstruction to the ‘new’ anchorages which are further behind the river bank. 

The cast iron pedestals, the saddles and the roller bearings on top of the saddles were enclosed with decorative castings when the bridge was built and it was only when these were removed as part of the structural integrity review that the troublesome cracks were revealed. 

Investigation and analysis work carried out by Mott MacDonald established that the roller bearings between the cable saddles and the cast iron pedestals had seized, leading to high stresses that the brittle material of the pedestals was not able to accommodate. These stresses were thought likely to have caused the cracks, which were most pronounced in the north-east pedestal. What’s more, the extent of the problem was only fully established once the decorative castings were removed and the pedestals were grit-blasted clean of paint. 

Eddy current techniques had been applied over the paint, but once the metal was cleaned, magnetic particle inspection could be used for a more accurate assessment. 

The pedestals themselves are cast iron elements in the form of hollow cellular boxes. Each has three longitudinal webs with intermediate diaphragms and full-height stiffeners. The upper plate has a machined top surface for the rollers and the bottom is fixed to the foundation stones by 12 vertical bolts. The elongated openings in each face enabled the sand mould to be removed after the pedestal was cast. They were complex castings for the time, and defects in the material bear witness to this.

Six cracks were initially noted in the north-east pedestal but after cleaning a further seven were revealed, ranging in length from 30mm to 205mm. Other pedestals were not so seriously impacted, with three cracks found on the north-west pedestal and none on the south east. Acoustic emission sensors were installed to monitor the cracks, and it was an alert from this system in August last year that sparked the sudden closure.

The notification of an acoustic ‘event’ prompted engineers to carry out an immediate inspection of the pedestals. A crack in the north-east pedestal which had been 160mm long was found to have jumped to 240mm – a dramatic change that demanded precautionary action.

With the bridge suddenly out of bounds even to pedestrians, pressure to find a permanent solution was ramped up. The council’s four-stage plan for the crossing had been rudely interrupted with only the first two stages complete, and a rethink was necessary.

The sudden loss of any connection across the river was the catalyst necessary to move things forward, and gave the council a strong case to take to the highest level. “I’ve written to the Prime Minister seeking the government’s urgent constructive engagement and financial support,” the leader of the council at London Borough of Hammersmith & Fulham, Stephen Cowan, said in his letter to residents a month after the closure. 

The immediate outcome was the Department for Transport’s (DfT) creation of a government-led taskforce, chaired by Baroness Vere, to ‘work towards the safe reopening of the bridge’. The remit of the taskforce is ostensibly to bring together the key stakeholders – LBHF, London Borough of Richmond upon Thames, TfL, the Greater London Authority and the Port of London Authority – on a regular basis to support and implement any decisions made by the council. 

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At the same time, the DfT agreed a £2.3m tranche of additional funding to cover the cost of the cleaning and more detailed examination of the pedestals, which is now completed and the results are being assessed. A temporary cross-river ferry service is also due to be implemented, subject to funds being approved, with river bus operator Uber Boat being announced as the preferred bidder earlier this year. 

The ferry is expected to have a minimum capacity of 800 passengers per hour at peak times and tickets will be the same as a standard bus fare. 

But the DfT has said no further funding will be forthcoming until the council has produced a detailed business case for remedial works and put forward its preferred option, including how it will be paid for. There could still be some way to go, with the official line being that government is only committing to contributing ‘up to one-third of the total funding for the project’, on the proviso that the council and TfL commit to cover the rest. 

This article was first published in the July 2021 issue of The Construction Index magazine. Sign up online.

It also seems likely that the various parties will continue to debate how the results of the investigations should be interpreted and how conservative, or otherwise, are the assumptions that have been made.

A radical proposal put forward by architect Foster & Partners and consulting engineer Cowi is now being given serious consideration. Despite only emerging a few months ago, it has been presented to both the council and the taskforce and, with its multiple potential benefits, is a credible option that will likely merit consideration in the council’s business case.

Meanwhile the bridge must remain in limbo until the funding arguments are resolved. It is not just highway traffic that will be affected indefinitely – boats need special permission to sail below it, and the Thames path is closed on both banks.

Is this the solution?

A proposal to create a temporary crossing on the footprint of the existing bridge, allowing the existing deck to be cut out in pieces and taken away for refurbishment while the bridge remains open to traffic, emerged late last year as an alternative to the conventional in-place renovation that was planned.

The design being put forward by architect Foster & Partners and consulting engineer Cowi draws on similar techniques applied to major bridges overseas; these were first seen on the 850m-long Lions Gate Bridge in Vancouver, Canada, in 2008. 

In this scheme, traffic continued to use the bridge during the day, with night-time closures being used to cut out a section of the deck and replace it with a new section. 

The idea for Hammersmith came about after council leader Stephen Cowan asked Sir John Ritblat, of property investment company Delancey, if he could assist. 

 Foster & Partners was already working with Ritblat on a separate project, so, as senior partner Roger Ridsdill Smith explains, they were in regular contact: “He [Ritblat] suggested we could also think about Hammersmith Bridge. A lot of perfectly good ideas had been put forward about temporary bridges alongside the existing one, but our idea was to put a temporary structure on the same alignment as the bridge,” Ridsdill Smith reveals. 

He called in long-term collaborator Cowi to work up the proposal. “They really helped us develop it – in particular the aspects of off-site removal and refurbishment of the bridge,” he says. 

The scheme proposes a temporary double-deck truss structure which would be assembled in two halves, one on each side of the river, and launched into place above the existing deck but not bearing directly onto it. Once in place, the two cantilevers would be connected at mid-span, and the launch bearings would be replaced by eight operational bearings – two at each tower and two at each abutment.

A ramped arrangement over the side spans would provide the elevation to get vehicles to the upper level, where there is more space to accommodate the proposed two-lane highway. Cyclists and pedestrians would approach along the side of the ramp and enter the lower truss deck close to the tower.

Ridsdill Smith says the proposal has three main aims: The first is to allow vehicles, as well as pedestrians and cyclists, to use the temporary crossing. This is one of the main advantages of this scheme over the alternatives that have so far been considered. Secondly, the structure will be able to support the existing deck, the load of which will be transferred from the existing cables, and thirdly it will stabilise the deck while it is cut into pieces for removal. 

Vehicles won’t be allowed onto the temporary crossing until the old deck has been removed – only after the removal of this dead load will the temporary structure have the capacity to accommodate the extra live load from traffic. 

Cowi executive director David MacKenzie explains that the unloading process will have to be carefully managed as suspension bridge cables are very sensitive to the removal of loads. “We will have a series of cross-heads to effectively lift up the load of the existing deck and transfer it from the suspension system onto the temporary truss. The truss is set high to begin with and as it takes the load of the existing deck it will move down.

“We will then cut the deck into sections - potentially up to seven sections in the main span and two in each of the side spans - and these will be lowered onto barges to be taken away for refurbishment and truss replacement,” he says. With the deck removed, elevating the chains away from the pedestals and repairing or replacing the seized bearings will be much simpler.

The proposal has many benefits, one of the strongest being that repair work can be carried out much more easily and safely when the deck units are in a factory setting, at ground level and not over water. Likewise it will eliminate a hazard risk for river users, and remove potentially polluting or noisy work to a controlled environment. 

This article was first published in the May 2021 issue of The Construction Index magazine. Sign up online.

Furthermore, any unexpected discoveries won’t affect the critical path; costs will be reduced, with no need to maintain a substantial site close to live traffic, and it will essentially de-risk many of the operations.  

Ridsdill Smith estimates that installation of the temporary crossing will take around 18 months, followed by another year for offsite renovation of the deck and its replacement. Initial figures suggest that it will cut £40m from the cost of full in-place refurbishment, with the majority of the savings coming from the reduction in risky on-site works.

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