Wood or steel? Study Helps Builders Reduce Carbon Footprint of Truss Structures | MIT News


Buildings are a major contributor to global warming, not only in their ongoing operations, but also in the materials used in their construction. Truss structures – those crossed arrays of diagonal struts used throughout modern construction, from antenna towers to support beams for large buildings – are typically made of steel or wood or a combination of both. But little quantitative research has been done on how to choose the right materials to minimize the contribution of these structures to global warming.

The “embodied carbon” in a building material includes the fuel used in the production of the material (for mining and smelting steel, for example, or for felling and processing trees) and in transportation. materials to a site. It also includes the equipment used for the construction itself.

Now, MIT researchers have performed a detailed analysis and created a set of computational tools to enable architects and engineers to design lattice structures in a way that minimizes their embodied carbon while retaining all of the properties necessary for a given construction application. While in general wood produces a much smaller carbon footprint, using steel in places where its properties can provide maximum benefit can provide an optimized result, they say.

The analysis is described in an article published today in the journal Engineering structures, by graduate student Ernest Ching and MIT Assistant Professor of Civil and Environmental Engineering Josephine Carstensen.

“Construction is a huge emitter of greenhouse gases that has sort of gone unnoticed over the past few decades,” says Carstensen. But in recent years, building designers “have started to focus more on how to reduce not only the operating energy associated with the use of the building, but also the significant carbon associated with the structure itself.” And that’s where this new analysis comes in.

The two main options for reducing carbon emissions associated with truss structures, she says, are replacing materials or modifying the structure. However, there has been “surprisingly little work” on tools to help designers find emission reduction strategies for a given situation, she says.

The new system uses a technique called topology optimization, which allows the entry 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 optimized for different characteristics, such as than the weight, the cost or, in this case, the impact on global warming.

Wood performs very well under compressive forces, but not as well as steel when it comes to tension, i.e. a tendency to pull the structure apart. Carstensen says that in general wood is much better than steel in terms of incorporated carbon, so “especially if you have a structure that has no tension then you should definitely only use wood” to minimize shows. One compromise is that “the weight of the structure is going to be more than it would be with steel,” she says.

The tools they developed, which served as the basis for Ching’s master’s thesis, can be applied at different stages, either in the initial planning phase of a structure or later in the final stages of a structure. design.

As an exercise, the team developed a proposal for the reengineering of several farms using these optimization tools and demonstrated that a significant saving in incorporated greenhouse gas emissions could be achieved without loss. performance. Although they have shown that improvements of at least 10 percent can be achieved, she says these estimates are “not exactly apples for apples” and that the likely savings could in fact be two to three times greater. .

“It’s about choosing materials smarter,” she says, for the specifics of a given application. Often times, in existing buildings, “you will have wood where there is compression, and where it makes sense, and then there will be very thin steel elements, in tension, where it makes sense. And this is also what we see in our suggested design solutions, but maybe we can see it even more clearly. The tools aren’t ready for commercial use, however, she says, as they haven’t added a user interface yet.

Carstensen sees a trend in the increasing use of wood in large constructions, which represents significant potential to reduce overall carbon emissions in the world. “There is great interest in the construction industry for solid wood structures, and that speaks directly to this area. So the hope is that it would make inroads into the construction industry and actually reduce this very large contribution to greenhouse gas emissions. “


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