Wood vs steel: How can you combine the two to provide strength and a lighter carbon footprint?
One challenge we have when it comes to making the construction industry more environmentally friendly, is the materials we use. When you need a material that’s strong enough to hold a structure up, often the fabrication of that material results in high CO2 emissions.
Therefore, it makes sense to look at alternative materials. Truss structures can be made of steel or wood – or a combination of both. But with steel production accounting for 7% of the global CO2 emissions total – more than the emissions from all road freight – it makes sense to take a serious look at how we can use less steel in truss structures, while maintaining the strength required.
For any non-technical readers, truss structures are those crisscross, diagonal struts that you often see making up the support beams above your head in an exhibition centre or other large venue.
Researchers at MIT have analysed the optimum methods for combining wood and steel, and have created a set of computational tools so that architects and engineers can design truss structures that minimise embodied carbon. The goal is to use steel where its properties are essential, and wood in places where steel is not necessary.
This analysis was described in a paper in the journal, Engineering Structures, late last year by graduate student Ernest Ching and the MIT Assistant Professor of Civil and Environmental Engineering, Josephine Carstensen.
Carstensen says there are two main ways to reduce the carbon emissions associated with a truss structure. One, you can substitute materials, or two, you can change the structure. However, little work has been done to develop tools that allow designers to minimise emissions.
With the system developed at MIT, basic parameters can be inputted – including the structure’s dimensions and the amount of load to be supported. The parameters are used to produce designs optimised for a choice of characteristics including weight, cost or impact on global warming.
Wood generally has a much lower carbon footprint than steel, but it does weigh more, which needs to be considered. Wood performs well under forces of compression, but steel has greater tensile strength.
The MIT team conducted an exercise where they proposed reengineering several trusses using their own system for optimisation. This demonstrated that significantly reduced emissions could be achieved with zero performance issues. Savings on emissions are likely to be in the region of 10% to 30%.
While the tools are not yet ready for commercial use, they are hopefully a precursor of what’s to come. We’ve seen that there’s a growing trend for using timber in construction projects, including in mass timber structures. Done in the right way, using timber will significantly reduce industry CO2 emissions.
I think many of us in the industry are already thinking this way, but (once they’re available) the tools will make it easier for all of us to adopt this approach. We always look at which materials are most appropriate in the projects we work on. With a roof truss in a standard house this is already wood, so there’s not much scope for improvement. For bigger or more unusual buildings you need steel for its strength to weight ratio – wood can often be too heavy.
And, of course, with wood’s anisotropic properties (it’s stronger in one direction than the other) you’ll need steel in the connections, even when you’re using timber. Commercial factors are always a consideration and we fit in with our clients’ needs – whether they’re focused on the most economical solution or the solution with the lightest carbon footprint, or something in between.
If you need assistance with the structural elements of an upcoming project – whatever your priorities – please do get in touch.