Arup Thoughts: Material efficiency in building structures

Material efficiency has a big impact on cost, emissions, energy and resources.

06/30/2014


Construction view of cranes in Hong Kong, Courtesy: ArupBuildings often use much more material than is necessary in their structures and I think it’s up to us, as engineers, to address the issue.

Material efficiency has a big impact on cost, emissions, energy and resources. Research carried out by Arup on behalf of the Institution of Structural Engineers found that structural materials represent around 2% of the initial cost and around 50% of the embodied CO2 and energy of buildings. Research by WellMet showed that buildings use 42% of the world’s steel production.

So it’s worrying to hear that research conducted with the WellMet 2050 consortium showed the average utilization rate for structures, the percentage of the available strength or stiffness which is used to achieve the structural design constraints, is surprisingly low. It’s around 45%. Why is this?

It can happen when more material is being added to reduce deflections or vibrations. It can also be caused by architectural decisions, such as using deeper beams for consistency across a floor where smaller beams would be suitable. Or it can be down to speeding up the design process by applying a design for extreme conditions such as highest loading in all locations.

I’m sure a design team could justify each of these decisions on a project-by-project basis. However, these decisions mean we are often using twice as much material in our buildings than is absolutely necessary. So how can we know whether what we are designing is materially efficient?

Perhaps material efficiency metrics are the answer. These would provide a benchmark against which a structural design could be measured. The metric should be easy to calculate from information readily available to a structural engineer, so it could be used to develop the design.

There is a precedent for material efficiency metrics in green building rating schemes; the material efficiency metric in Green Mark is called the Concrete Usage Index (CUI). This measures the volume of concrete per constructed floor area (m3/m2). More points are awarded the lower the CUI is.

This appears suited to Singapore, perhaps because buildings tend to be similar, and predominantly concrete. But it wouldn’t be suitable for comparing buildings with different frame types, those that use materials in combination or buildings with different uses.

The alternative is to look at the strength utilization of the structural members. This has the advantage of being the same metric for all building types and all structural materials. And it’s something that’s already usually checked for individual elements during the design process. So it wouldn’t be too onerous to consider for a whole building.

There are still barriers to overcome, such as how to measure the utilization. Should we use the maximum utilization for a single beam or look across the whole length? And what would we consider efficient? If the current average utilization is 45% we have a long way to go before we are targeting 100%.

Whichever way we decide to measure it, designing building structures to make more efficient use of materials is likely to take time. And currently it seems time is more precious than the materials we are using.

Andrea Charlson is a senior engineer in the Advanced Technology & Research team at Arup specializing in materials’ sustainability. This article originally appeared on Arup Thoughts. Edited by Brittany Merchut, Project Manager, CFE Media, bmerchut(at)cfemedia.com



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