29.09.2021 / Finn Graham
The Futura footbridge, with its elegant design and use of innovative materials, has the potential to provide the UK rail network with sustainable, accessible and high quality footbridges.
The Futura solution is a concept composite footbridge, initially developed for a design ideas competition in 2018. The competition, run by The Royal Institute of British Architects (RIBA) on behalf of UK rail operator Network Rail, challenged entrants to reimagine the future of railway stations across the UK. Together with Marks Barfield architects, COWI in the UK designed the Futura Bridge with a vision of how we see the progression of footbridges in station environments, and tackled the challenges set out in the UK Government's ‘Access for All’ initiative, which aims to improve rail network accessibility nationwide. Following the competition, Network Rail were excited about the submission and commissioned COWI to develop the idea further. The National Composites Centre were brought on board to provide expert advice and manufacturing capabilities for this specialist material. The Futura project was born.
The current Futura solution is an innovative design made from fibre polymer composites, also known as fibre reinforced polymer (FRP). FRP has a wealth of attributes and while it is currently considered to have relatively low maturity in the civil engineering industry, in the coming years it has the potential to challenge the dominance of concrete and steel worldwide. Futura’s sustainability credentials are designed to help the UK achieve net zero status by 2050.
FRP is not new to the civil engineering industry. It has been used a handful of times for railway footbridges in the past, but has never been developed into a scalable solution beyond building one single bridge at a time. Following a few previous successful projects using FRP in COWI, we have been eager to use this material to progress sustainable bridge solutions, and this was the perfect opportunity.
The Futura Bridge is being designed to have a 120-year design life. This is a critical aspect of the design criteria and the validity of this can often be scrutinised. It is difficult to guarantee the design life of a new material for 120 years given it has not been around long enough to face the test of time, however if this idea is not challenged, we would never develop new materials. Of course, there are tests we can run that induce a rapid number of cycles on a section, be it temperature, moisture or fatigue, to replicate a long period of time, but to replicate 120 years would still take years to do. One of the oldest uses of this material is in Royal Navy Minesweeper ships, using FRP in the hull and main body, which are exposed to incredibly harsh marine environments. This material is incredibly strong and relatively maintenance free, and these ships can be 50 years old, out at sea for 20 years at a time, before coming back in for some minor touch ups.
FRP is stronger than traditional materials like steel and concrete due to the microstructure of the material. Steel consists of lots of molecules stuck together, and the process it’s made with causes extremely small defects in the microstructure. Likewise, if you load a concrete beam enough, cracks will develop. As these defects grow the element begins to fail and the rest of the material becomes less effective at resisting further load. FRP on the other hand is made up of millions of tiny fibres, and if one of those fibres is defective the load redistributes to the millions of others. In the inherent microstructure, there is a much better distribution of load, improving the reliability of the material.
A key difference between FRP and steel is that steel is not considered an additive manufacturing process. Generally, a steel plate or section is cut down and welded together to form a product. The process to make the steel plate or section is independent of the final use. Concrete is arguably more of an additive manufacturing process than steel but is still rife with constraints surrounding reinforcement placement, concrete curing, formwork, etc. FRP on the other hand is truly an additive manufacturing process. To form the final material you add one very thin mat of fibre and then another and another until you get to your desired thickness. Resin is then added and cured to achieve the final product. If you require a certain thickness in one part, but a different thickness in another, you can simply add a given number of sheets in one location and less in the other, saving on material. This is in stark contrast to isotropic steel, where common thicknesses are often specified for maximum load effects even though this may only be required in a very specific location.
The fibre in FRP can refer to several different materials. One commonly used FRP material is carbon fibre, while other types, such as drawn fibres from glass, flax fibres, basalt fibres, or a range of synthetic fibres, all of which have different material properties and sustainability credentials. The field of sustainable fibre products is evolving rapidly with the focus on sustainable solutions, and the Futura project is aiming to utilise bio-materials where possible.
Sustainability is arguably the most important and challenging problem of our generation. Early next year we will be building the Futura bridge as a one-to-one scale prototype, putting the material and bridge form to the ultimate test. Material selection is ongoing and detailed Design for Manufacture is progressing. We’re considering the most sustainable and environmentally friendly materials available to us, while balancing the constraints of cost, durability, design life and structural performance. The manufacturing method for the Futura bridge will likely use a permanent form work inside the structure, which in other industries is made from a foam that is detrimental to the environment. We’re looking to replace that foam with a product made 100% from recycled bottles to improve the overall sustainability of the solution.
An important aspect of FRP that needs careful consideration is recyclability. It is often claimed that FRP is not recyclable, and its reputation as a plastic comes with negative connotations. In fact, FRP is recyclable and there are numerous methods being developed to improve this. Some of these include incineration or co-incineration recycling where cement clinker is produced as a waste product, mechanical recycling which involves cutting down FRP to be used as chopped strand in other applications, or thermal recycling where the material is heated and the resin and fibre separated for re-use. The unfortunate truth is that FRP is not currently mass-recycled and hence some people generally consider it an unrecyclable material. Several industries are working on this subject tirelessly - the automotive, marine and aerospace industries in particular - primarily because the products that these industries produce have a relatively short lifespan. In the years to come, recycling of these materials will be commonplace, and the market for purchasing recycled FRP materials will be mature. For FRP materials to prosper, recyclability and the use of more sustainable materials, such as flax and bio-material fibres & resins, needs to be encouraged.
The sustainability of structures such as bridges need to be viewed in terms of whole life assessment. Initial embodied CO₂ estimations indicate that Futura will have a similar cradle to grave carbon footprint as an equivalent steel bridge. However, it is estimated to have a significantly lower equivalent embodied CO₂ life cycle due to its improved maintenance and durability, and its lightweight nature which reduces transportation and foundation CO₂ emissions. It also has no need for bearings and an inert material structure which does not corrode like steel.
At present our focus is on the design stage of Futura as well as creating and testing coupons and one-to-one scale parts of the bridge. We are developing the ‘Futura kit of parts’ concept that can be adapted to differing bridge orientations, span lengths, single-/multi-span, and covered/un-covered use cases. This will form the Futura catalogue that can be used as a simple early stage design tool to pick and choose the best Futura bridge kit for an individual railway station environment. This is an ambitious approach to the standardisation of quality design of a product across the network.
Early next year we will shift our efforts towards building and testing a single full-scale prototype as a proof of concept. The National Composites Centre will fabricate the bridge in-house, providing a unique opportunity to keep the whole design and construction team together from concept through to completion. They will install it on a test site before carrying out a suite of full scale tests. Furthermore, the one-to-one scale prototype will undergo rigorous stakeholder engagement, with Network Rail as well as accessibility groups, members of the public and local communities offering their thoughts on the design. All of the lessons learned and comments received will be integrated into future versions of the Futura bridge.
FRP-based technologies are established in a range of industries and with minor adaptations we could see plane and boat builders, marine turbine blade manufacturers, and even Formula One car manufacturers also producing Futura footbridges in the years to come. There is huge potential for mainstream acceptance of this material if we can focus on the areas that need improvement. Recyclability, sustainable biomaterials, education, and procurement strategies are all important topics to discuss and to continually improve upon.
The education aspect is a long-standing debate within the civil engineering industry. What education should we be giving our future leaders? We should at the very least be introducing students to these kinds of alternative construction materials, not just teaching them about steel and concrete. How can they challenge the norm if they have never heard of FRP? We should open their mind to alternative solutions. This is at the core of what we do in COWI. It's about knowledge sharing and continuously pushing the boundaries.
Beyond the Futura footbridge solution, there are other parts of the design and construction communities that are progressing FRP as a material of choice. We have an enormous job on our hands to meet the sustainability goals that we have set for ourselves, and the magnitude of the change that's required to meet our goals is colossal. It is unlikely that FRP will completely replace steel and concrete, as both are incredibly mature and established industries. But we believe that over the next 10 years we will start to see a sea change in these material choices. We at COWI are excited to be a part of this shift towards a brighter, more sustainable future.