A bridge designed for manufacture

The new type of footbridge will enable the UK’s rail infrastructure operator to make cost efficiencies and cut lead times. Making use of modern manufacturing techniques requires creating a superstructure by bolting together many small stainless steel components like a large-scale Meccano kit. This is a sharp contrast to traditional bridge fabrication, which is typically based on elements made from carbon steel and welded together.  

Network Rail is leading the development of the bridge in a consortium with contractor Walker Construction, manufacturing and construction partner McNealy Brown, as well as input from the Manufacturing Technology Centre (MTC). Architect Hawkins\Brown and structural engineering consultancy Expedition Engineering developed the design, while ARX developed and manufactured the lift. Stainless steel company Outokumpu joined the team in 2021. Together, the team has created a bridge superstructure that makes the most of modern manufacturing techniques.  

“The beauty of modern manufacturing is that we can use automation and digital control to increase the speed and scale of production. It’s possible to produce a standard set of components in kit form to build bridges faster and cheaper,” says Phil Webb, Managing Director of Walker Construction.  

One advantage is optimized use of raw materials by maximizing the number of components cut from a single sheet – and because workers in the factory are protected from cold and hazardous conditions, the approach can deliver high-quality and long-lasting structures that minimize the need for on-site construction work. 

Replicating I-beams with laser-cut plate 

Creating a bridge from many small components posed two major challenges for structural engineering consultancy Expedition: designing for manufacture and sourcing stainless steel pre-loaded bolts.  

“When working on a traditional steel bridge, an engineer typically selects from a range of standard sections or specifies bespoke girders fabricated from plate,” says Hazel Needham, Associate Structural Engineer at Expedition. Standard I-beams are optimized for strength and stiffness with a thin web and thick flanges. This creates a form that is strong and stiff but also that minimize use of material.  

“However, with the AVA Bridge, due to manufacturing constraints and a desire to limit welding we were restricted to designing sections that could be folded from a single sheet of stainless with a maximum thickness of 12mm. While we could use thicker plates for components that didn’t require folding, for example the column base plates, we had to think of a creative solution to design an efficient girder.” 

Each element on the bridge is laser cut from sheets of stainless steel and folded into C profiles and angles. For the primary girders the individual components are spliced together to form a long continuous girder. The team duplicated the performance of I-beams by layering multiple plates in the beam flanges to increase the strength and stiffness.  

The primary girders run along the outside of staircases and span. As well as being the primary longitudinal support system for the bridge they form the solid parapet required to meet the Network Rail safety standards for station footbridges. The c-shaped cross-section and careful detailing of connections means that no additional cladding is required on the outer face of the structure, saving cost and carbon.

Optimizing the design 

As the supplier of the stainless steel sheet and plate, Outokumpu provided technical support and advice that proved invaluable, according to Hazel: “Designing a structure from stainless steel is unusual but the overall process follows many of the same principles used in carbon steel structures. One area where Outokumpu’s advice was particularly helpful was in determining the minimum bend radius of the plate.” 

With Forta LDX2101 it was possible to use a bend radius of 2.0 times the plate thickness, which is a tighter radius than had been originally anticipated. This helped to reduce the width of flanges as the laser cut bolt holes could not be located too close to the bend. 

The resulting saving is marginal for an individual plate; however, the savings soon add up when it is applied to every bridge element and provides an overall reduction in quantity of material, cost of the bridge and carbon.  

Non-standard bolts

Another challenge was the requirement for pre-loaded bolts, which Network Rail specified as safety-critical as they do not loosen over time. Pre-loaded bolts work by creating friction between the plates they’re holding together, rather than by withstanding shear forces. It’s essential that fasteners are made from materials that are at least as corrosion resistant as the underlying structure – but stainless steel pre-loaded bolts are not a standardized product.  
“We only know of two projects that have used pre-loaded stainless steel bolts: the Stonecutters Bridge in Hong Kong and the Spire in Dublin,” says Hazel. “Because they have not been used extensively, they are not available as standard products so we had to have them specially produced and tested.” 

Expedition oversaw this process using Bumax, the specialist provider of stainless steel fasteners. Bumax manufactured the bolts and developed a four-step installation method. This required hand tightening; pre-loading; tightening to the full load; and then returning 48 hours later to reapply the full load.  
The bolts underwent laboratory tested at Duisberg Essen University in Germany. The university has expert knowledge of stainless steel pre-loaded bolts from its involvement in the EU’s SIROCO research project, which evaluated the performance of pre-loaded bolts made from a range of different stainless steel grades.  

Configuration can be adapted to suit

Network Rail’s goal for the AVA Bridge is that the structure should be adaptable to fit a wide range of station configurations. The basic design links two platforms with two sets of stairs and two lifts but it is modular for maximum flexibility.  

There are two basic configurations: wide and narrow. The wide version places the lifts in parallel to the stairways to provide an accessibility advantage for passengers in the form of through-lifts. These have two doors so that passengers can pass straight through and don’t need to turn round. In contrast, the narrow version has a minimal span and takes less land as its lifts are in line with the stairways. The lifts themselves are based on a new design for improved accessibility and reliability, which were developed by mechanical handling specialist company ARX. 

The design can be used flexibly to link multiple platforms, with staircases aligned in different directions or being wide on one side and narrow on the other.  

This latter arrangement is the case with a prototype and demonstrator bridge, which was installed in Sittingbourne, Kent. The prototype enables evaluation of both design configurations, performance of the bridge and its components, and whether it reduces the need for site work through off-site construction. 

Minimizing possessions of the line 

The AVA Bridge design has been developed with series production in mind.  

“The objective was to use the latest digital manufacturing technology to develop a suite of digital 3D plans that can be adapted for almost any given site,” says Phil Webb. “When a scheme is approved, the digital design files will be uploaded it into a manufacturer’s Enterprise Resource Planning (ERP) system. This ties together material orders, production schedules and planning to define production lead times and costs.  

“The beauty of automated manufacturing is that it minimizes variability of bridge components by producing them to tight mechanical tolerances of around 0.2 mm. This makes it straightforward to build sub-assemblies and then major sub-assemblies for delivery to site.”  

When a bridge is ready for delivery to site, the design minimizes costs. It can be transported by standard trucks as separate modules: the span, staircases and lifts. These arrive ready to install on concrete pads that fit together snugly, having been fully factory assembled and tested, with services integrated. There is no need for site welding.  

This plug and play approach compares with traditional bridge fabrication, where a construction company needs to carry out construction, installation and fabrication over multiple track possessions. These are periods when Network Rail pays train operating companies a penalty fee as it has to close the line to traffic.  

It’s likely that the primary structure of the AVA Bridge and its lift system can be installed in a single  possession of as little as 27 hours, ready for final fit out and commissioning.  

Seeding a new bridge manufacturing industry 

The AVA Bridge concept takes a cost and time saving approach to bridge manufacture. Using stainless steel, the consortium has estimated that the design can reduce CAPEX, reduce overall project time and lower carbon footprint compared with a standard bridge.   

AVA is the latest addition to the Network Rail’s Footbridges & Subways Design Manual, a document that sets out the design principles and considerations for delivering long-lasting and successful footbridges at the UK’s railway stations. By including AVA in the manual, Network Rail has made the bridge a go-to design for its network.  

In the longer term, Network Rail’s plan is to release the AVA Bridge design as an open source design. The operator is hoping that the project will seed the development of a new bridge manufacturing supply chain. As a partner in the project consortium, Expedition has the advantage of being involved in the initial project – but future AVA Bridges can be delivered by structural engineers and manufacturing workshops anywhere.