timber skyscraper exterior

Optimising truss structures could lower construction’s carbon footprint

A new analysis could help identify optimal materials for the criss-crossing struts that bolster bridges, towers and buildings.

Buildings are a major contributor to global warming, not just in terms of their ongoing operation but also in the materials used in their construction. Truss structures — the criss-cross arrays of diagonal struts used throughout modern construction, in everything from antenna towers to support beams for large buildings — are typically made of steel or wood, or a combination of both. Little quantitative research has been done on how to select the right materials to minimise the contribution to global warming of these structures.

The 'embodied carbon' in a construction material includes the fuel used in the material’s production (for mining and smelting steel, for example, or for felling and processing trees) and in transporting the materials to a site. It also includes the equipment used for the construction itself.

Researchers at MIT have performed a detailed analysis and created a set of computational tools to enable architects and engineers to design truss structures in a way that can minimise their embodied carbon, whilst still maintaining all necessary properties for a given building application. Although in general wood produces a much lower carbon footprint, using steel in places where its properties can provide maximum benefit can provide an optimised result, the researchers say.

The analysis is described in a paper - Truss topology optimisation of timber-steel structures for reduced embodied carbon design - published in the journal Engineering Structures, by graduate student Ernest Ching and Josephine Carstensen, MIT assistant professor of civil and environmental engineering.

“Construction is a huge greenhouse gas emitter that has kind of been flying under the radar for the past decades,” says Carstensen. In recent years, building designers “are starting to be more focused on how to not just reduce the operating energy associated with building use, but also the important carbon associated with the structure itself.” This is where the new analysis comes in.

The two main options in reducing the carbon emissions associated with truss structures, she says, are substituting materials or changing the structure. However, there has been “surprisingly little work” on tools to help designers figure out emission-minimising strategies for a given situation, Carstensen said.

The new system makes use of a technique called topology optimisation, which allows for the input of basic parameters such as the amount of load to be supported and the dimensions of the structure, and can be used to produce designs optimised for different characteristics, such as weight, cost and, in this case, global warming impact.

MIT computational tools for architects and engineers

Image credit: Ernest Ching, Josephine Carstensen et. al

Wood performs very well under forces of compression, but not as well as steel when it comes to tension — that is, a tendency to pull the structure apart. Carstensen says that in general, wood is far better than steel in terms of embedded carbon, so “especially if you have a structure that doesn't have any tension, then you should definitely only use timber,” in order to minimise emissions. One tradeoff is that “the weight of the structure is going to be bigger than it would be with steel,” she said.

The tools developed by the team, which were the basis for Ching’s master’s thesis, can be applied at different stages, either in the early planning phase of a structure or later on in the final stages of a design.

As an exercise, the team developed a proposal for reengineering several trusses using these optimisation tools and demonstrated that a significant saving in embodied greenhouse gas emissions could be achieved with no loss of performance. While they have shown that improvements of at least 10 per cent can be achieved, Carstensen said those estimates are “not exactly apples to apples” and the real savings could actually be two to three times greater.

“It’s about choosing materials more smartly,” she said, for the specifics of a given application. Often in existing buildings “you will have timber where there’s compression, and where that makes sense, and then it will have really skinny steel members, in tension, where that makes sense. That’s also what we see in our design solutions that are suggested, but perhaps we can see it even more clearly.” The tools are not yet ready for commercial use because a user interface is still to be added.

Carstensen sees a trend to increasing use of timber in large construction, which represents an important potential for reducing the world’s overall carbon emissions. “There’s a big interest in the construction industry in mass timber structures and this speaks right into that area. The hope is that this would make inroads into the construction business and actually make a dent in that very large contribution to greenhouse gas emissions.”

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