Laying down the future

The brainchild of Royal HaskoningDHV, CEAD and DSM, a new walkway has been made of the latter’s Arnite material, a high strength combination of (recyclable) thermoplastic PET and chopped glass fibres. Interestingly, CEAD’s large-format ‘robo-arm’ printer is only using a nozzle 6mm diameter, but it still took just 18 hours to create.

It certainly meant “pushing the envelope” of both the bridge design and printer tool path, said Maurice Kardas of Royal HaskoningDHV. Since the concept replaces heavy, rigid beams and uprights with something that is only 10% solid, Kardas described a load-spreading internal structure running between the top and bottom flanges “like a series of webs”.

“But normal 3D printing software “doesn’t normally consider structural engineering elements,” he remarked, leaving a time-critical 3D puzzle to solve.

The issue is that as soon as the Arnite is extruded it takes less than a minute to set: “If a connection is made too soon, the plastic is still too warm and soft, so the structure falls in…. Too long and the material is too cool to adhere properly,” explained Kardas. This means there’s a window of just a few seconds to knit-in each of the nodes and as a result tool-path configuration “took quite a lot of effort”, he admitted.

Interestingly, the footbridge also incorporates sensors and a digital twin; this allows for predictive maintenance, said Kardas: “For instance, if we know how many people or bicycles have crossed the bridge in a certain timeframe, we’re able to determine the traffic loads and determine stress levels.” Further, there are potential environmental gains: future bridge designs may have embedded air quality sensors and so on: a further tweak could be to use plastic from recycled bottles.

However, one of the main reasons for the sensors is to validate the design and production process. Kardas pointed out that “because this technology is so new, there are currently no national or international standards for printed bridges”, so approvals are far harder to win than for more traditional projects. These sensors will help but he added that a challenge lies ahead: “We must prove that bridges designed and printed in this way can fulfil the demands of our client when compared to more traditional methods. We will need to be able to prove the integrity of the structures we are printing – particularly the fact that all layers in a printed bridge have the same consistency, adhesion levels and so on.”

There’s a lot to play for, far more than this single walkway. Together with DSM, Royal HaskoningDHV is currently in discussions about scaling-up for longer spans and is looking at developments for gangways, heavier, traffic bridges and even deck replacements where Kardas explained it may be possible to “give a bridge a longer life” by reducing the load on the supports.

It also looks as if this technology could eventually be in demand for vessel gangways, both ship-to-shore and onboard, as the strength-to-weight ratio is far higher than for traditional steel: “The material has a density of 1800kg/m3, a Young’s Modulus of 18.000MPa and a characteristic tensile strength of 170MPa,” he added.

However, the future could be rather different to anything we’ve seen before, not just because this process gives architects room to play rather imaginatively with ideas, but also because the material used on these bridges can be reformed. “We can install a footbridge in one location but easily recycle and rebuild the bridge to serve another purpose …in another location,” said Kardas.

He concluded: “That’s the future, and where we see things moving – essentially towards a ‘bridges-as-a-service’ kind of model.”

By Stevie Knight