The devastating earthquake that rocked Japan on March 11, 2011 generated numerous inquiries to the NRC Institute for Research in Construction Canadian Codes Centre with regard to the magnitude of earthquake that Canadian buildings are designed to withstand. There is no simple answer to this question, as the effect on a particular building depends on an earthquake’s magnitude, its distance from the building, and the building’s characteristics.
The recently published National Building Code of Canada 2010 (NBC) incorporates several new technical requirements to ensure that Canadian buildings are protected. They include prescriptive requirements in Part 9 (dealing with housing and some small building types) that provide for adequate lateral load resistance through the use of braced wall panels, as well as fastening and framing based on local seismic conditions that apply primarily to high risk areas, mainly the Pacific coast of British Columbia.
Revisions made to requirements in the NBC for Part 3 buildings (dealing with all other types and sizes of buildings) address building site properties, irregularities, steel structures, static and dynamic procedures, and diaphragms. A quadratic equation was used to determine spectral acceleration values for Canada’s different zones to improve the fit of seismic data.
The following examples provide some guidance regarding the degree of protection for buildings in Canada’s three largest cities. In Montreal, where the probability of an earthquake greater than magnitude 7.5 is small, high-rise structures are designed to withstand magnitude 7.0 earthquakes that are at least 30 km away. In Toronto, highrises are designed to withstand magnitude 7.0 events no closer than 50 km, while short structures are designed to withstand 6.0 magnitude earthquakes at least 30 km distant.
In Vancouver, although the expected shaking from three sources of earthquakes (continental North American plate, subducting Juan de Fuca plate, and the Cascadia subduction zone) must be taken into account, it is only Cascadia subduction zone earthquakes that result in magnitude 9 events such as the one that struck Japan. These occur in the vicinity of Vancouver Island, roughly 140 km from Vancouver, and their effect is diminished by distance. Consequently, structures in Vancouver are only required to withstand 7.0 magnitude events that are at least 50 km distant.
Contact Cathy Taraschuk at 613-993-0049.