Case study: Network Rail FLOW Bridge

The advantages of early contractor engagement and the challenges posed by working across interfaces were highlighted by Tom Osborne in his case study about the FLOW bridge concept that Knight Architects developed for Network Rail. His presentation in GIRI’s webinar on off-site construction and its impact on error described how working collaboratively minimised changes and helped the team resolve potential issues during the development of the prototype.

Watch the recording of the webinar

Knight Architects was tasked by Network Rail to develop an adaptable, modular bridge concept that could be rolled out across the network to provide safer crossings for pedestrians. The design proposed fibre-reinforced polymer (FRP) as an alternative to concrete and steel to reduce construction time. The use of composite materials also allowed the team to experiment with a new design and improve the user experience.

Tom explained that the way the team approached the design was critical. “Usually, when a project team sets up to use modular construction, they rush to think about the built product, but we were keen to start the design with an exploration of the user experience. Existing railway footbridges are often dark, with sharp corners, poor visibility of the onward route, and a relatively unappealing crossing.”

Knight Architects wanted the new bridge to transform the harsh 90° angle of traditional crossings, smoothing this into a curve, and retaining visibility by adding glazing on top of the composite. Achieving this curve without making the crossing longer required the development of a separate spine that allows the supports to remain orthogonal to the structure, while the deck curves away from them.

Collaborative working contributed to the success. “There was a very large team, led by Network Rail, and we were working together almost from the outset,” said Tom. “Often, a lot of design input happens early in a project with relatively little overlap with the contractor or the fabricators. Their input happens later with relatively low designer input – and things can get lost in those gaps.”

GIRI’s research into the causes of error found that late design changes are a key root cause and that greater investment in design in the early stages is critical to avoiding error. This is what happened on the modular bridge project, said Tom. “The contractor invested more design time in the early stages so that less had to be done later. And when the design was happening, we were all doing it together. This meant that the design could be established quickly then iterated slowly and in an informed way by the whole team rather than being chopped up into packages of work and delivered by separate people.”

A key objective was a design that could be installed quickly; and it was not just a single design, but an adaptable solution. “The design needed to accommodate different spans and different geometric arrangements. The prototype we worked on only has stairs, but lift solutions are on their way, and it can be adapted to suit the character of each site, for example with different colours and textures. But in terms of lessons learned, it was the process rather than the product that was useful.”

Interfaces were a key challenge, said Tom. GIRI research has identified interfaces as another common root cause of error and discussions during the webinar highlighted how off-site construction can introduce a host of new interfaces when myriad separate elements come together on site – this was the case on the bridge project.

“One of the main challenges was the interface with the ground, and the accuracy of the FRP process and the spine relative to the accuracy that could be achieved with the groundworks. FRP is quite well-known digitally, but its translation into physical works is less well understood and was therefore a variable. Another key interface was that between the glazing and the FRP.”

Working closely with engineers, including Network Rail’s in-house engineering team, helped the team overcome these hurdles, and Tom credited the collaborative process for the project going from concept to completion of a full-size prototype within just 10 months.

“In terms of getting it right, it is important to realise that you won’t straight away. You have to allow enough time for the process to evolve as lessons come to light.”

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