How cost and carbon footprint were cut on new Thames Path bridge

Currently, when pedestrians on the Thames Path reach Barnes Bridge in Chiswick, they are required to leave the route and navigate their way over the railway tracks before re-joining the path. The creation of the Dukes Meadows Bridge will allow future pedestrians to walk under the Barnes Bridge and stay on the Thames Path, cutting a 500m detour from their route and remaining on the river bank.

Moxon Architects created the initial design of the bridge but engineering firm Cowi was brought in to take the design forward. Alongside construction partners Knights Brown, Cowi’s task was to improve buildability and reduce costs for the London Borough of Hounslow.

Not only was this achieved, but through use of a specialised in-house tool, Cowi has said that it was able to significantly reduce the bridge construction’s carbon footprint. The specialised carbon calculator had project data inserted into it and it accurately returned the amount of embodied carbon per square metre of useable bridge deck.

In the Dukes Meadows Bridge project, the figures were put in at various stages of the development process, and engineers were able to see how their value engineering decisions had affected the project’s carbon intensity. As desired, they found that it was reduced alongside the reduction of costs.

The firm said that the improvements are clearest on the bridge’s most notable feature, its half-through truss form with distinctive angled diagonal members that provide structural support as well as allowing users wide views up and down stream. This was originally specified as solid steel, but Cowi’s design team switched the diagonals to a hollow fabricated section – a move that slashed materials and associated carbon emissions by more than half, while retaining integrity and improving buildability.

Efforts are also being made to reduce the immediate environmental damage resulting from the construction. This includes updating and improving the bridge’s foundation design, with the team finding ways to incorporate temporary elements into the permanent works. In this way, the risk of concrete fines leeching into the Thames and damaging the ecosystem were minimised.

The final bridge will also have to withstand impact from marine vessels. The bridge’s resistance was improved by making the pier connections integral with the deck, so that the force of an impact is distributed across the entire structure. This innovation also significantly reduced the amount of material used in the bridge’s foundations.

Another innovation is the way in which the main span of the bridge will be put in place. Original plans had it being lifted into place by a dual crane lift, but this method would have required the rail lines possessions. Instead, Cowi and Knights Brown have suggested floating the structure in by pontoon at high tide and then, as the tide recedes, the span will seat itself on its foundations.

Once complete in spring next year, the Dukes Meadow Bridge will sweep beneath the Barnes Bridge – and will have been built without affecting any of the 2M annual passengers that ride on that railway.

The final bridge will cost £4M (excluding preliminary studies, design phases, etc), to be paid by the London Borough of Hounslow with some supporting funds.

Cowi UK managing director Andy Sloan said: “These innovations saved both cost and carbon, and were realised with an initial focus on reducing capital cost during the value engineering phase with compatible carbon savings confirmed upon completion of the phase.

"The reduction in embodied carbon realised in this phase were estimated to have saved at least 20% relative to the reference design before considering any changes to the material specification.

"If additional savings from the material specifications can be realised throughout construction, this is anticipated to save an additional 10% of embodied carbon. This is equivalent to between 80 to 120 tCO₂e (or 250 to 375 kgCO2e/m² of deck area) of savings for this footbridge.”

Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here.