Evolving conventions

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The outcomes from a multi-sector collaborative project to develop lightweight, durable, high-strength fibre-reinforced polymer (FRP) composite trusses could result in tomorrow’s railway footbridges being rather different from those currently built using conventional materials.

Over a century ago, masonry and cast iron were superseded by wrought iron, before steel became the favoured material of bridge engineers. More recently, several FRP structures have been placed across the UK’s railways in low risk and relatively short-span applications such as footbridges and for farm access. One such was installed at Standen Hey near Clitheroe in December 2007. That deck, spanning 11 metres, was constructed of interlocking trapezoidal units that formed a voided slab. To achieve longer spans, forms of structure incorporating trusses as the primary load-carrying elements are usually more economical.

Early in 2007, Tony Gee and Partners – consulting engineers and designers of FRP structures – were contacted by Network Rail. It transpired that an opportunity had arisen for both parties to join a collaborative project to be part-funded by the UK’s Technology Strategy Board. A year later and a £1.8 million investment to develop Advanced Composite Truss Structures (ACTS) had started. At its heart was the creation of a node for joining truss elements, manufactured from 3D woven carbon fibres.

Node design

Trusses are used in a wide range of structures including aircraft wings and motor vehicle frames, so the potential benefits were of common interest across different markets. Consequently, aircraft manufacturer Airbus UK, Bentley Motors and protective vehicle supplier NP Aerospace also joined the project. Further partners drawn from across the defence, civil transportation and FRP composites industries, as well as academia, provided design, manufacturing and testing expertise to project manager Materials in Transport.

The design of the node is deliberately generic but incorporates features that will be familiar to structural engineers, such as gusset-like stiffeners between the sockets into which the truss elements fit. Remarkably, the 3D carbon fibre preform is woven in just one piece and folds outwards to form the shape of the node. Only one of the gusset stiffeners is required to be stitched to the preform. Producing a component design that complied with strength and manufacturing criteria involved close collaboration between partners – modelling analysts at Nottingham University, weaving specialist Sigmatex and mould tool designer Composite Integration. With numerous design iterations, each requiring 3D modelling of the interwoven fibres and complex stress analysis of the component, it took a year to finalise the geometry and materials.

A vital part of the manufacturing process is the infusion of the resin into the carbon fibre preform while within the mould. The locations of injection points and outlets were developed with the use of computer modelling software that provided real-time simulation of the infusion process and identified potential air pockets.

Tony Gee carbon fibre
A carbon fibre preform in an open mould

Manufacture and testing

The complexity of weaving a carbon fibre fabric in three dimensions, providing through thickness strength, is such that few possess the necessary technology. Fortunately the project benefited from the expertise of Runcorn-based Sigmatex. While the UK is no-longer a leading manufacturer of textiles for clothing, it still has the technical know-how to produce advanced materials such as carbon fibre preforms. The vision and techniques required to create a complex 3D node from a fabric woven on a flat loom are akin to those required by exponents of origami.

The preform is positioned within an aluminium tool, designed to the shape of the node. Comprising ten parts, the tool is designed to compact the fibres, deliver resin under vacuum conditions and provide heat to cure the resin. The completed component is removed from the mould and ready for joining together the chords and diagonal elements of a truss, an exercise to be trialled by FRP fabricator Pipex Structural Composites.

Components of an FRP truss can either be bonded into the node within the moulding tool, co-bonding or, by applying resin between the component parts post-manufacture, adhesive bonding. A further partner, the Joining Technology Research Centre at Oxford Brookes University, is assessing and testing possible materials for these types of joints. Non-destructive assessment of material sample panels and actual nodes is being undertaken by QinetiQ.

The nodes are being subjected to a series of load tests by NP Aerospace. A specially designed testing rig applies combinations of axial, flexural and torsional loading effects. The aim of these is to ensure that the nodes will not be damaged by impacts – accidental or deliberate – in addition to the normal in-service loads expected from a bridge structure.

Tony Gee mould
The assembled mould

The next steps

From a railway engineering perspective, the project will be successful if it can be shown that it is possible to design and manufacture affordable FRP bridges with longer spans than have been feasible to date. While the lightweight, durable characteristics of fibre-reinforced composite materials are expected to offer significant advantages during installation and in-service, demonstrating long-term affordability will be essential for market acceptance. With this goal in mind, and using an existing footbridge in Surrey as a case study,

Tony Gee and the other project partners are examining the cost components of an FRP truss bridge in comparison to those for a similar steel truss structure.

With the manufacturing processes having now been demonstrated but not yet perfected, the focus of the project in its final year is the testing work package. The ACTS project will be completed early in September 2011 and Tony Gee is keen to explore possible applications. A low risk site outside the railway environment would be an ideal place to demonstrate the technology but ultimately it would be rewarding to see FRP footbridges adorning our stations.

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