The secret to decarbonising the construction industry lies in replacing concrete with CLT slabs and reducing the size of building foundations to support lighter-mass timber structures. That is according to a new study, Comparison of Embodied Carbon of a Mass Timber Building Structure with a Steel Equivalent, just published in Buildings Journal.
The study also confirms that mass timber’s only major limitation is connected to transportation emissions, given that the majority of mass timber is still imported from overseas. It also found that buildings that used CLT floors, glulam beams and columns were 35% lighter than traditional steel-and-concrete buildings, emitted 40% less carbon – and used substantially less concrete in the foundations.
It comes as the US government is in the process of releasing new guidelines that, for the first time, will quantify harvested wood inventory and emissions. This game-changer could allow developers to capitalise on displacement benefits, the amount of carbon emissions avoided by using timber over a higher-emitting material.
Led by Mahboobeh Hemmati from the University of Arkansas, researchers from the Fay Jones School of Architecture and Design and the Forest Products Laboratory at the USDA Forest Service modelled the embodied carbon footprint of Adohi Hall, once America’s largest mass timber building and compared it to a traditional steel structure.
Opened in 2019, the four-story building was the first large CLT-based residential complex built by a US university. It is driving a new wave of mass timber projects in the American South, which is now set to surpass the Northwest as the region with the fastest-growing mass timber market.
“Adohi Hall’s structure predominately comprises CLT slabs, supported by glulam columns and beams,” the researchers said, “with the basement and ground floors constructed with reinforced concrete slabs, as well as a combination of steel and concrete columns and beams.”
As a result, “Adohi Hall presents an ideal examine the level of efficiency (of mass timber) compared to steel when hypothetically constructed,” according to the researchers, using Revit and 3D modelling to conduct a Life Cycle Analysis (or LCA) on both construction systems.
According to the findings, the mass timber building has a total mass of 10,306 tonnes (including the foundations), while the comparable steel structure has an estimated weight of 15,694 tonnes—with far less concrete (6,688 tons) used in the mass timber build over the steel.
“This comparison demonstrates that the mass timber structure is significantly lighter than its steel counterpart,” with the reduction achieved “by replacing concrete slabs with CLT panels and by reducing the size of the foundations, which is made possible by the lightweight nature of the mass timber structure.”
At the product stage, the mass timber structure LCA for the mass timber structure is 2,853 tonnes of CO2, whilst the steel-based alternative generated 4,478 tonnes of CO2 – a 36% reduction, “attributed to the inherent properties of mass timber as a building material,” the researchers said, adding that “steel has far higher energy consumption in production.”
The research found that concrete was responsible for more than 70% of emissions across both structures. “While there would be 1892 tonnes of CO2 with the mass timber structure, there would be 3026 tonnes of CO2 if the mass timber structure was substituted for steel.”
The findings, part of a new series of research articles tracking the LCA of mass timber buildings over traditional buildings, follow a study last month reporting that 12-to 18-storey high-rise construction was the “new sweet spot” for mass timber construction systems.
Produced by PCL Construction, one of North America’s largest construction contractors, DCI Engineers, the engineers behind the world’s first post-and-plate mass timber high-rise, and design firm Weber Thompson, it found that mass timber hybrid construction was cost-competitive with concrete-based systems.
“While slightly more expensive (4%)… it’s a competitive price that may influence developers to think twice about the materials they use in their next project, especially to differentiate their produce in a competitive leasing market,” the study said.
Amongst the key findings, it found that hybrid timber buildings could store up to 3,350 metric tones of CO2 across its useable life, which is “the equivalent to the carbon sequestered by 4,000 acres of US forest in one year, or eliminating 745 gasoline-powered passenger vehicles from the road.”
- To find out how America is now turning to mass timber construction systems to build its next generation of public buildings, visit Wood Central’s special feature.
