Engineer explains sustainable structures to architects

What should architects expect from sustainable structural engineering? Mike Moffatt, P.Eng., a structural engineer with Read Jones Christoffersen, had the enviable task of answering that question to a roomful of architects and others at Construct Canada in Toronto on December 1.

He began by saying that the integration of structure with the other building elements will become more important as the drive for sustainability advances. In integrated and holistic designs, the systems and components have to be synchronized, otherwise they may work against each other. Therefore the structural engineers have to be at the table with the architects and others from the start of a project.

“The impact of structure can’t be underestimated,” Moffatt said, adding that a building’s structure constitutes 50% of its weight.

Moffatt noted that with the idea of reducing materials typically structural engineers are asked “to do more with less.” With sustainable designs, structural engineers are asked to do with “even less.” But, he said, “The challenge is to be aware of opportunities where allowing “more” will result in a more sustainable structure.

For example, he suggested, we should be designing buildings for a 100-year lifespan, but this approach means also designing for larger live loads than the current standards. You have to think of possible future uses, which could mean that the structure will have to support heavy equipment.

“Currently the National Building Code requires office floors above second floor only to be designed to a 50 psf [pounds per square foot] live load, which is limiting for the future adaptability of the space,” Moffatt pointed out. “We should always have the discussion with our clients about designing for live loads of 80 psf to 100 psf.” He said that for a CANMET building that RJC has designed in Montreal, a conscious decision was made to design it for a 150 psf live load so that the structure will be able to support lab and material testing equipment over the lifespan of the building.

Efficient layouts also help. Moffatt urged architects to consider more open grids and columns to ensure the building can be adapted for new uses in the future. “Shear walls are not easy to remove,” he pointed out, whereas columns “give maximum flexibility.”

Moffatt discussed different structural materials, citing the use of Portland limestone cement which allows up to 15% limestone clinker in place of cement, thus reducing the greenhouse gases produced in cement’s manufacture. Steel is recyclable and has recycled content, giving it sustainable characteristics. However, “you need to pay attention to the source” of the steel, he said. Wood and engineered wood products are sustainable because wood absorbs greenhouse gases, but it is not right for every application.

The efficient and sparing use of materials is a key to sustainable design, so structure can be integrated into the architecture, such as working as a ceiling or floor finish. “The most sustainable element is the one you don’t need,” said Moffatt.

If a structure is kept warm and dry it will generally last 50 to 100 years, he said.

As for the building envelope, Moffatt explained that RJC is doing thermal modeling of walls in precise detail and feeding this information into the building energy models.

He also advised the audience to be careful about the impact of adding exterior architectural elements such as canopies and balconies. These can act as thermal bridges and work against the careful energy saving features of the design.

Mike Moffatt speaking at Construct Canada on December 1.  Canadian Consulting Engineer sponsored the event