A plan to thread a new truss between the iron suspension towers of the 134-year-old Hammersmith Bridge over the River Thames could shorten its closure by years and allow repairs to be done offsite. Frustrated London commuters would otherwise have to wait at least six years for the heavily used bridge to reopen after it closed in 2019 for essential repairs.
Because of cracks in the iron structure, the London Borough of Hammersmith and Fulham (LBHF) shut its bridge to motor vehicles in April 2019, affecting 22,000 daily crossings. When cracking worsened, the bridge last summer closed also to pedestrians and cyclists.
Cracks have been found in the cast-iron pedestals that support suspension cable saddles at the abutments. Engineers suspect that roller bearings between the saddles and pedestals seized up at least 30 years ago.
The resulting thermally induced imbalanced cable forces on either side of the saddles cracked their cast-iron pedestals. Comprising stiffened plate girders with sloping tops, the pedestals are roughly 1.8 m tall at mid-height, 3.7 m long at their bases and 1.5 m wide.
The suspension “cables” that the pedestals support are chains of roughly 5-m-long wrought iron plates pinned end to end. From the abutments, the chains reach back through sloping tunnels in the river banks to anchorages in underground chambers.
LBHF and the city’s transit authority, Transport for London (TfL), supported by the design firm Pell Frischmann Consultants Ltd., came up with an estimated $200-million rehabilitation program that could take more than six years to implement after funding has been secured.
However, Roger Ridsill Smith devised a plan that would allow traffic to flow much sooner than previously expected. Ridsill Smith, a senior partner and head of structural engineering at the architect Foster and Partners, was asked to find a solution to the bridge closure by a real estate developer with interests in the area.
The plan involves rolling a new steelwork truss bridge within the existing structure, seated on new bearings at the towers and abutments. With depths ranging from about 6 m to 3.5 m, the truss would support an upper vehicle deck and a lower level for pedestrian, cyclists and maintenance access.
Each half of the truss would be fabricated on either riverbank, then rolled toward each other to meet at midspan with their leading edges cantilevering to avoid loading the old structure. At 65 m, the cantilevers would be long, “but we do launch structures much longer than that,” says David MacKenzie, executive director for bridges at COWI Consult U.K. Ltd. “There are no technical showstoppers,” he adds.
Ridsill Smith says he wanted COWI on board because of the design firm’s practical experience in bridges. Now the team is undertaking a feasibility study for LBHF, due for completion next month. The council expresses considerable interest and optimism for the proposal, says a spokesperson.
The temporary truss bridge would allow traffic to flow sooner than originally anticipated, and also provide a platform to dismantle the old deck for refurbishment, says Ridsill Smith. The deck would be lowered in roughly 20-m sections and taken off site. “The intention would be to repair as much as possible of the existing structure…rather than replacing it,” he adds.
Rehabilitation work would include replacing the edge stiffening trusses, installed in the 1970s, and placing a stiffened steel-plate orthotropic deck in place of the existing timbers, adds MacKenzie.
In the meantime, engineers are trying to establish the extent of cracking. Workers have removed all the pedestals’ decorative iron castings and will grit-blast paint off to allow engineers from Mott MacDonald Group to complete surveying the ironwork late next month.
To prevent the seized bearings from causing more damage, LBHF is spending around $600,000 a year on controlling temperatures in the chain tunnels. By adjusting thermal conditions in the tunnels, the system limits differential temperatures and forces across of saddles.
The effectiveness of temperature controls “over a full range of ambient temperatures” has yet to be fully explored,” notes the council spokesman. But so far, an investigation of one of the pedestals by Mott MacDonald found the system provides “some mitigation”.
While there is no shortage of engineering solutions to the faulty bridge, funding remains elusive. LHBHF has no spare cash, and any hopes it had of support from TfL were dashed last year when the transit authority was nearly bankrupted by the pandemic.
Since the government declined to fund the work, the council is drawing up financing options, including charging the long-suffering commuters for crossing the bridge. But the council still insists that the government or TfL should care for what is a “strategic transport asset” and an “industrial, architectural and engineering heritage” artifact.