Researchers at MIT have developed an approach for generating more buildable structures, bridging the gap between optimised design and real-world construction.
A technique called topology optimisation can design structures that reduce the amount of material used, in some cases by as much as 90 per cent, which would represent a multi-gigatonne reduction in construction-related carbon emissions. However, topology optimisation is mostly used by researchers for applications such as 3D printing rather than by engineers designing at the scale of buildings and bridges because topology optimisation doesn’t create structures that can easily be built on time and on budget, which is what builders really care about.
Now MIT researchers have created a way to make topology optimisation designs more buildable. Their framework allows users to apply constraints to algorithmically generated structures to limit their complexity. For instance, the approach allows users to limit how many components meet at each point of their design and how small they want their smallest parts. It also builds on previous work by designing structures with multiple materials and taking into account materials’ properties to distribute load and specify part connections.
‘There’s an interplay between the materials you’re using, the constructability of designs, and the optimisation of the structure,’ said Josephine Carstensen, MIT’s Gilbert W Winslow career development professor in civil engineering. ‘You need to be able to address all three at the same time. That’s what we tried to do here.’
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The researchers used their approach to design steel, wood and multi-material truss structures that support loads in buildings and bridges, showing that the carbon emissions associated with materials changed significantly when different constraints were applied. They hope their framework will move topology optimisation closer to being used in real-world construction.
‘In the literature, there’s sometimes been a disconnect between the carbon savings you can achieve on a computer and the realistic carbon savings you can achieve for built structures – especially when it comes to design technologies like topology optimisation,’ Carstensen said. ‘The problem lies in the lack of constructability of designs. These designs have been perceived as too difficult to make with conventional methods, so they are never even attempted. That’s what is exciting about our approach –we can add constraints so that you will never be in a situation where the design that comes out is too hard to make.’
Computer-based topology optimisation has been around for decades. It uses computer programs to optimally distribute material in a given space, for instance creating the strongest possible structures at the lowest weight. The resulting designs are often complex, spider web-like structures that would be a challenge for even the most capable engineers to build.
‘A big question Josephine and I were asking is why isn’t industry using it?’ said civil and environmental engineering PhD student Zane Schemmer. ‘What are the obstacles that prevent industry from designing things more efficiently, and how can we fill the gaps between research and real life?’
In recent years, several researchers have developed ways to make topology optimisation easier to use. For their study, Schemmer and Carstensen wanted to bring those approaches together and add new capabilities, such as creating designs that use multiple materials, which has been another challenge in the field.
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‘A big aspect of sustainability going forward will be not only using less material, but also implementing materials efficiently based on considerations like where you are in the world, your access to materials and each of their associated carbon costs,’ Schemmer said.
To build their framework, they used a class of equations called mixed integer algorithms that help make binary decisions about things such as materials and connections. ‘You can’t have a part that’s 72 per cent timber and 28 per cent steel,’ Schemmer says. ‘Instead, it says, “This truss or cable is going to be made out of this,” and then based on that decision, how do we make sure all of these connections meet their strength standards?’
The system’s decisions also take into account material properties. For instance, steel struts can withstand compressive loads, but steel cables can’t. The model also has more realistic modelling of how parts connect than previous approaches. ‘In 3D printing, the way things come together is easy,’ Carstensen said. ‘In construction, that’s not the case. If you’re building with timber, there’s a certain rule set, versus steel has a different rule set.’
Users can also decide how complex they want their design to be by specifying the maximum number of connections at each joint and the minimum angle between connected components. The model also creates minimum size limits for parts, further improving its constructability. ‘It’s tough to give a contractor these complex, intricate designs because it’s going to be super difficult to build,’ Schemmer said. ‘A lot of times contractors won’t pick up a project like that to begin with.’
The researchers compared structures designed with their approach to structures designed with conventional topology optimisation, showing dramatic differences in final designs that transformed how the structures would be built. Using the Lockport ‘Upside-Down Bridge’ near Buffalo, New York, as an example, they applied individual constraints, such as a minimum angle on part connections or minimum part sizes, to the bridge’s truss design, to better understand how each constraint affected the final designs.
Finally, they made truss designs that used wood only, steel only and combined wood and steel, showing how different projects offered trade-offs with respect to environmental impact and constructability. ‘We saw how the system knew that you could design a bridge of pure steel, but that might not be best from a carbon standpoint,’ Schemmer said. ‘Or you could design a bridge out of purely timber, but that might not be the strongest. But these materials can work together, so you use timber for the carbon savings and steel where you need extra strength, and there’s a balance you can find in these structures.’
The research has been published in Automation in Construction.