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Dutch plan to 3D print a stainless-steel bridge over a canal, but not all has gone to plan

Steel News - Published on Mon, 05 Feb 2018

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Eureka Magazine reported that building bridges has been a preoccupation of mankind for centuries. Over the years, designs have been as varied as the materials used to build them and while you may think engineers have got it all figured out when it comes to bridge design and construction, a project to build a bridge across an Amsterdam canal is challenging conventional wisdom.

Amsterdam-based start-up MX3D is 3D printing a 12 metre long stainless-steel pedestrian bridge to be installed across the busy Oudezijds Achterburgwal canal in the old city centre later this year. The concept is relatively simple: put a welder on a robot arm, place one on either side of a waterway and begin ‘printing’ until they meet in the middle. Easy, right?

Chief technology officer from MX3D, Tim Geurtjens, explained that “When we started we thought let’s get an old robot arm the kind you see on car production lines and put a simple EUR 1000 welder on the end. We thought, ‘this is going to be easy’. But things turned out to be much more complicated.”

Broadly speaking, the technology is about adding layers of material. However, MX3D is not using modern additive techniques like selective laser melting or even direct metal laser sintering. Instead they’ve chosen a more traditional process… welding.

Mr Geurtjens said that “It is a MIG welding technique. It is melting the base material and adding a metal wire and that is fusing together. We are basically building with molten metal. One of the most complicated things was to get control of the welding process. You can study welding for your entire life and still not know everything about it. It is a very synergic process if you change one value, everything changes as well we really underestimated that.”

While the team initially considered printing the bridge in steel, they quickly moved towards stainless steel to overcome problems of corrosion. Using steel would have meant that a coating would be needed, which would completely cover the bridge’s surface, hiding the fact it was made of steel and more importantly the fact it had been 3D printed.

Mr Geurtjens said that “Changing material to stainless steel was a very expensive decision. Stainless-steel is not cheap but this is maybe the only 3D printed bridge we are ever going to make, so let’s do it right.”

A key part was the control of the process, which was primarily driven by bespoke software that the team developed themselves to optimise all the various parameters. There was a lot of trial and error involved and while some simulation was used, the practicality of the process meant that a significant amount of practice was needed to prove out the process and properties of the printed material.

Mr Geurtjens explained that “We are developing software, parameters and printing strategies for the different kinds of 3D printable ‘lines’. For instance, vertical, horizontal or spiralling lines require different settings, such as pulse time, pause time, layer height or tool orientation. All of that is incorporated in the software.”

The control is now so good that the team can print intricate structures, far more like the organic structures often printed in smaller ‘in-box’ additive machines. The process also does away with support structures due to the inherent strength of the material. This creates an ‘out of box’ 3D printing method that makes it possible to create 3D objects in almost any size.

While the concept of 3D printing a bridge in situ is an exciting and intriguing one, the practicality comes with a host of challenges, least of which is actually getting the technology to work. In short, it’s a health and safety, and operational nightmare.

As with most welding processes, sparks are emitted from the welding head which are bright enough to damage eyesight if looked at directly. In addition, getting the necessary permits to build the bridge would be a long winded, if not impossible, undertaking. The other difficulty is that access to the site would be limited due to the narrow streets.

Mr Geurtjens said that “From the beginning, we knew we wanted to have the bridge there. But we walked around and thought this is never going to work. We’d need to set up shop there, so have an office, power supplies, shield everything off, shield off the robot and welder from the weather and weatherproof the equipment. The logistics and cost would make it very difficult.”

The team therefore decided to create a lab in which to develop the process and begin fabrication of the bridge as intended. Once built the bridge could be tested and put through the necessary rigour required for a public foot bridge, before it could be transported, assembled in place and commissioned. To date, the team has fabricated half the bridge using the technique and expect to complete fabrication within the next few months.

Given the pioneering nature of the build, the process has numerous permutations as well as the possibility of voids produced during printing or perhaps the inclusion of oxides on the material. It means it’s difficult to know how the structure will hold up over time. To answer this question, the team plans to lace the structure with sensors to measure, monitor and analyse the performance of the bridge which, upon completion, will be the world’s largest 3D printed metal structure.

The sensors will collect data on structural measurements such as strain, displacement and vibration and measure environmental factors such as air quality and temperature, enabling engineers to measure the bridge’s ‘health’ in real-time and monitor how it changes over its life.

Mr Geurtjens explained that “We will make the bridge a smart bridge by fitting it with all kinds of sensors and accelerometers as well as measuring temperature and humidity basically any sensor we can lay our hands on we’ll put on the bridge and start measuring. First of all though, we will measure the structure’s integrity.

He added that “Obviously, to get a permit for the bridge is quite challenging as it is very hard to prove that a new material and process like this is actually strong enough to be used. So, we will do some full load tests, equivalent to 300 people on top of it to measure deformation and measure the health of the bridge. But we won’t stop there. What we will do is measure structural integrity and see what the bridge does. Then afterwards, we’ll leave the sensors there, so when we put the bridge on location we will keep measuring and generating data from it.”

Autodesk is supplying the cloud services that will power the bridge’s data collection and processing. MX3D is also working with researchers from The Alan Turing Institute to develop machine learning algorithms that will enable the bridge to interpret its environment. This data will also allow MX3D to ‘teach’ the bridge to understand what is happening on it, such as how many people are crossing it and how quickly.

The data from the sensors will also be input into a ‘digital twin’ of the bridge, a living computer model that will reflect the physical bridge through its life, with growing accuracy in real-time as the data comes in. The performance and behaviour of the physical bridge can be tested against its digital twin, which will provide valuable insights into designs for future 3D printed metallic structures. It will also enable the current 3D bridge to be modified to suit any required changes in use, ensuring it is safe and secure for pedestrians under all conditions.

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Posted By : Nanda Koijam on Mon, 05 Feb 2018
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