Cost-efficient Earthquake Resistance
Canadian Consulting Engineer | Jan/Feb 2024
Leon Plett, P.Eng., MIStructE, Struct.Eng., LEED® AP | Managing Principal
Leon Plett is managing principal of RJC Engineers’ structural engineering team in Victoria.
Their provincial level of designation differs from elsewhere, due to the complex engineering required to address the region’s risk of earthquakes. And as seismic codes continue to evolve to ensure new buildings are more resilient, they face the challenge of keeping costs reasonable, during a housing crisis.
Why are seismic codes changing?
As we gain more information about the geological record in this region, we find out there have been more frequent earthquakes over the course of history than previously thought, which increases their probability today. Newly discovered faults further increase risk. Modern building designs are based on these updated seismic hazards.
In addition to the Cascadia fault that runs all the way down the west coast, the more recently discovered Devil’s Mountain Leech River fault extends from southwestern Vancouver Island into the Strait of Georgia over to northwest Washington State. It runs right through the most populated areas of southern Vancouver Island, including Victoria.
Those hazards feed into the response spectrum, which measures the risk of ground shaking to our buildings.
How does that affect your work?
It changes the magnitude of what we need to do for new buildings. We resist seismic forces primarily through the use of shear walls or bracing. These structural elements must become larger and more numerous to resist more force, which in turn requires a larger foundation, all of which increases the building’s structural cost.
In a residential tower with a concrete core for stairs and elevators, that core’s walls must now be 30% to 70% thicker, depending on site class (i.e. based on soil and other geological conditions). A 1-m thick wall in the old code could become a 1.5-m thick wall in the new code, which adds to the cost and reduces occupiable floor area.