The Triple Bottom Line of Structural Materials: Cost, Speed and Life Cycle Assessment
Photo by Peter Saunders.
WSP, mcCallumSather and ArcelorMittal unveiled the results of a theoretical case study at The Buildings Show in December, which showed the costs and sustainability of using steel, concrete or timber are similar for a typical 12-storey, 287,000-sf, L-shaped residential condominium tower in the Greater Toronto Area (GTA).
A conference session titled ‘The Triple Bottom Line of Structural Materials: Cost, Speed and Life Cycle Assessment’ featured Brant Oldershaw, P.Eng., WSP’s director of structural, mechanical and electrical engineering for Southwestern Ontario; Willems Ransom, principal and architect for mcCallumSather; Matthew Winters, P.Eng., Steligence project manager for ArcelorMittal; and Mike Cortese, principal sustainability projects manager for ArcelorMittal.
As they explained, WSP and mcCallumSather joined steel producer ArcelorMittal’s Steligence program to model and compare the performance of different building materials for the same theoretical project. Consulting engineering firms RJC and MTE also contributed to the project’s structural details.
“We were not designing a building, but a case model,” Ransom explained. “It was normalized for the market, but optimized to no single solution. It was flexible, with a typical building envelope and massing.”
While they developed and tested the virtual models, they estimated the project’s structural weight, total cost, speed of construction and environmental impact.
“Originally, we only compared steel and concrete frame systems,” said Winter. “Now, we added a mass-timber scenario, where it’s more challenging to ensure functional equivalency.”
“Mass timber is more spatially intensive and lends itself to certain spans,” Willems explained, “but we maintained the same functional floor plan as much as possible across the various options.”
“We kept it as generic as we could,” said Oldershaw, “with commonalities like concrete foundations and cast-in-place concrete for stairwells.”
To assess environmental impact, they conducted life-cycle analysis (LCA) calculations at each stage, including data from third-party-verified Environmental Product Declarations (EPDs) and focusing on global warming potential (GWP).
“We used One Click LCA software to compare three functionally equivalent buildings,” said Cortese. “With the EPDs, we ‘sourced’ local products where possible. The number of EPDs available is growing exponentially, which makes this work easier to do and more accurate.”
“I thought there’d be a bigger gap on LCA,” said Oldenshaw.
Photo by Peter Saunders.
The three main structural materials considered were ArcelorMittal’s own XCarb recycled and renewably produced (RRP) steel; a ready-mix average with general use low-carbon (GUL) concrete, per Concrete Ontario standards; and a combination of British Columbia glue-laminated (glulam) timber and Ontario cross-laminated timber (CLT).
The resulting GWP was similar in all three scenarios; slightly higher for concrete, followed by mass-timber, followed by steel. In each case, however, construction proved achievable within the stipulations of the Toronto Green Standard (TGS).
“The close results surprised me,” said Cortese. “It came down to consistency in philosophy, design and material selection.”
“As we tried to reduce embodied carbon for this case study,” Winters said, “we realized when we’re designing well and using materials to their strengths, we can hit our sustainability targets with any material.”
The estimated costs were similar, too, with steel coming in at $3,107,500, concrete at $3,121,500 and timber at $3,011,000.
“We put timber into an unusual context with this project,” says Ransom from his perspective as architect, “but realized it’s becoming a more equitable option that can be comparable to steel and concrete.”
The main point of difference was the time needed for construction. While each project involved 40 days for the foundation and 70 days for cladding, from there, timber won the race at 113 days for the structure, followed by steel at 157 days and concrete at 190 days.
Looking back at the project as an experiment, Ransom pointed to both the benefits and disadvantages of the multi-party collaboration that was required.
Willems Ransom: LCAs are easier to produce and there’s better software, but that leads to much more difficult decision-making process, which is positive.
“We used to design by rules of thumb,” he said. “Define the critical factors of impact demands a different level of investigation in the design process, which is greatly enhanced through collaboration with industry partners. We have data to mine now, but it’s changing at a rapid pace. So, we need to start to define processes to streamline decision-making. Integrated Project Design (IPD) is an ideal scenario, but takes a lot of time and people!”
