FM Conway began work to stabilise the Grade II* listed Hammersmith Bridge, which crosses the River Thames in west London. The chain suspension bridge, designed by Sir Joseph Bazalgette and built in 1887, has been closed to motor vehicles since 2019 when cracks were discovered in the four cast iron pedestals at either end of the bridge.
The pedestals support saddles, which in turn carry the bridge’s suspension cables just before they dive down into their anchors in the river’s bank. Roller bearings between the pedestals and saddles should allow for movement due to traffic loading and thermal contraction and expansion, but the bearings had seized, causing stresses to transfer to the cast iron and cracks to form.
One of the first tasks was to remove the cast iron panels around the pedestals – around 60 for each pedestal, each weighing two-three tonnes. They have been lifted out one-by-one and carefully labelled and stored for restoration. Blast cleaning and painting of the remaining cast iron and steelwork followed, with concrete then poured into the hollow pedestals.
“Using extensive concrete trials to achieve the bespoke specification has been the biggest technical challenge on the project,” says FM Conway contracts manager Craig Mason. “Since there is no access to install bars within the pedestals, steel fibres, as well as rebar, were both chosen to reinforce the concrete and ensure the necessary strength was achieved.”
To get the mix right, a 1:1 scale mock-up of the pedestal was built to trial and test different concrete mixes over the first six months of the contract. Samples from the trial have been tested, with the distribution and orientation of the fibres in the hardened concrete receiving special attention.
Bespoke oval hydraulic jacks have been designed to raise to saddle off the pedestals so that new elastomeric bearings can be installed. Temporary steel frames, through which horizonal restraint forces will be transferred, will be installed around each pedestal using cranes positioned on the riverbanks.
One of the first tasks was to remove the cast iron panels around the pedestals – around 60 for each pedestal, each weighing two-three tonnes.
A significant hurdle was the 2022 summer, which brought with it unprecedented temperatures. When the cracks were discovered and analysed by works’ designer Mott MacDonald, one of the mitigation elements put in place was the cooling of the chains inside their sleeves. When the temperature rises above 18˚C, a safety assessment takes place.
“When the heatwave hit, we had to produce additional chillers and generators,” says Craig. “We have to assess whether it’s safe to keep the bridge open whenever the temperature rises above 18˚C. We managed to keep everything cooled and safe, even at 40˚C.”
Another key consideration was the historical nature of the bridge, which brought with it unique challenges. The Grade II* listing meant that stringent planning permission was required at every stage, from the design itself through to the types of materials used, how the casings were to be removed and the salvage of elements such as nuts, bolts and screws.