Modular footbridge set to revolutionise railway crossing design

The bald statistics for railway footbridges make sobering reading. It can take a year on site to build one. Only a third of the £3M to £4M it costs to build each bridge goes on the materials and the creation of the structure itself – the rest is contractor and client costs.

What is more, the fall back footbridge design has not changed for years. Put kindly, it is visually uninspiring, and the notorious unreliability of standard-issue station lifts make it far from user-friendly. Despite the length of time this design has been in use, there has been no attempt to rationalise and streamline how things are done – in some ways, the process has become increasingly entrenched.

Ripe for disruption

If any part of the infrastructure sector was ripe for disruption, this was it. Network Rail technical head of buildings and architecture Anthony Dewar recognised these shortcomings two decades ago and, frustrated by the unwieldy process he witnessed, made it his mission to shake things up. 

Over the last few years his team has been making inroads into the way rail footbridges are designed and procured – Network Rail’s 2018 footbridge design competition being the first high-profile intervention – but the imminent arrival of the Ava bridge demonstrator project on site in Nottinghamshire signals an even more fundamental shift.

The Ava footbridge is a radical modular design being developed by a consortium of companies, including Network Rail, Expedition Engineering, Walker Construction, X-Treme Systems, MTC, Hawkins/Brown, SCX Special Projects, Norman Foster Foundation and Atelier Ten. The £5.4M initiative is funded jointly by Innovate UK and Network Rail. 

Network Rail programme manager for research and development Janine Fountain agrees there are issues with cost and timescale on conventional projects, a challenge the Ava project aims to overcome. 

The project triggered an innovative lift design

“The main costs are the planning and execution of railway possessions, the crane hire, bridge transport, getting planning permission and so on. When time on site is extended in the way it is for footbridge design, the cost of establishing and maintaining a compound close to the railway is drawn out and expensive.” 

A lot of the early discussion concerned the cost and disruption of possessions, Fountain says. “Our objective is to do it in just a single 52 hour possession – possibly even trimmed to 48 hours, which is less than a weekend – and we think this is possible.”

The full-size demonstrator will be manufactured and erected over a “live” railway test track at Widmerpool in Nottinghamshire later this year to show proof of concept. The structure will subsequently be deconstructed and reassembled for permanent use on the national network. Dewar is reassured that the demonstrator has sparked something of a bidding war from regions keen to have a railway first.

Our objective is to do it in just a single 52 hour possession – possibly even trimmed to 48 hours

The term “manufactured” is crucial in relation to the creation of the structure and central to the innovation process. 

Rather than being built in a steel fabrication yard from standard sections, then shipped elsewhere to be painted before being transported to site as a full span length, the Ava bridge is designed to be assembled in 1.2m long modules using structural elements cut from flat sheets of stainless steel and bolted together. 

The truss modules can be configured to suit the destination site, and fitted out with cladding, canopy, lighting and other mechanical and electrical (M&E) services before being erected as close to finished as possible.

X-Treme Systems is the manufacturing partner and, as managing director Noel Lovatt explains, the company comes to the project free of preconceptions, having never worked with Network Rail before, and having never built a bridge. 

Manufacturing environment

Lovatt’s first task was to introduce the other team members to the manufacturing environment, the opportunities in the automation process and the way X-Treme exploits digital technology, and relevant constraints such as sheet size. 

All the pieces needed to form the truss and other elements of the modules will be laser cut from 3m by 1.5m stainless steel sheets, then folded and bolted together. The manufacturing process is good because it is repeatable, says Lovatt, “and much more accurate than conventional fabrication, with tolerances as low as 0.2mm”.

The footbridge can be assembled quickly from premanufactured parts

The aim of the project is not just to slash capital cost by a third and construction time by half, but to reduce whole-life carbon, cut maintenance demands and hence costs, and improve reliability. With a design road-tested and signed off by Network Rail, the planning and approval stages should be massively accelerated. 

Boosting accessibility is the fundamental goal and with the cost of the average footbridge cut from £3.6M to £2.6M, the number of footbridges being built can be boosted without any budget increase. Not only that, the demonstrator will showcase a new, custom-designed lift pod, intended to sweep away the poor reputation of station lifts once and for all.

The Ava bridge is designed to be assembled in 1.2m long modules

Expedition has been appointed as the engineering consultant for the project and associate Eva MacNamara explains that the team is currently deep in the detailed design, due for completion as this issue of NCE went to press. “This is the ‘crunch point’ at which the different experiences and knowledge of the team members will come into play,” she says. 

The ambitious success criteria have meant that the team had to completely rethink how footbridges are designed and constructed and come up with a credible alternative. Saving weight and reducing maintenance were fundamental to this, and the move from standard steel fabrication to a manufacturing process using stainless steel sheet addresses both of these.

Expedition knew that a modular solution would allow the design to be adapted to any location and make it easier to bring to site for assembly. Using structural elements made of folded steel sheet cut the weight of the structure dramatically, enabling foundations and crane capacity to be reduced. It is also believed to be the first use of stainless steel preloaded bolts on the railway, MacNamara reveals.

The design has already been through several iterations with a prototype module produced before the initial design was finalised. While this might seem like a reversal of the traditional project timeline, it is standard for manufacturing processes where efficiency and optimisation are driving forces. 

The 1.2m long modules are configurable to suit the location, the number of tracks that need to be crossed and so on

The type selection process proposed two options for the final form – however, these had to subsequently be revised and made “less climbable” in response to concerns from Network Rail’s regional managers who are responsible for procuring footbridges.

One of the project’s biggest technical innovations was the creation of a totally new lift design – something that was not even in the brief to start with, Dewar reveals. 

Station footbridges usually use standard lift technology that is more suited to high-rise buildings. They require a deep lift pit to house the equipment and effectively have to be assembled on site over several weeks. What is more, they can be unreliable, with spare parts that have to be ordered and high maintenance costs. 

The Ava team worked with SCX Special Projects – another newcomer to the rail sector – to develop a “plug and play” lift pod from scratch. It will be tested and commissioned offsite, requires a very shallow 0.7m deep lift pit, and has 100% redundancy. With standard off-the-shelf components and a belt-driven system, it can be easily repaired.

Walker Construction managing director Phil Webb says that the initiative has been a refreshing change from the normal process he is familiar with after more than 30 years in the sector. SMEs are normally “at the bottom of the food chain” with no opportunity to influence the development of a project, he says. “Here we had the chance to influence the design, cost and buildability of the structure.”

Eliminating wet trades

In the trackside environment the foundations and substructure are the most complex parts of the project, so a design that eliminates wet trades is a major step forward. A lighter structure that reduces the lifting capacity needed is also a big plus. 

Constructing the steel screw pile foundations at Widmerpool is set to begin in September with a total four month site programme anticipated. Webb is confident of cutting this to three months once the learning curve has been overcome. 

The 1.2m long modules are configurable to suit the location, the number of tracks that need to be crossed and so on, and MTC is developing a configurator tool that clients can use to visualise how the footbridge will fit each site.

“We want the process of designing, manufacturing and installing to be as close to industry 4.0 as possible,” Dewar says. 

He acknowledges that this represents a major leap for the railway footbridge sector, which he estimates is lagging at closer to 1.5. Until very recently, only PDFs of “standard catalogue” footbridges were provided for the supply chain – no CAD drawings and certainly no digital models. Now, Dewar says, information for each of the new suite of footbridges is provided in digital format.

The Ava Bridge consortium must complete its work programme by March 2022 in line with the Innovate UK funding terms. Webb reveals that the team has already had interest from other infrastructure clients, not just those in the rail industry. With the economies of scale that digital manufacturing can offer, the best value will be gained if they can be commissioned in bulk.

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