Common menu bar links
CONSTRUCTION INNOVATION, March 2010
[ Table of contents | Subscribe ]
Ensuring that buildings can adequately resist lateral loads from natural hazards is a key consideration of the National Building Code of Canada (NBC), particularly in British Columbia where the threat of a strong earthquake is high. Damage from earthquakes in California and Japan over past decades indicates that light wood-frame buildings must have walls that are properly constructed and connected in order to provide an acceptable minimum performance under earthquake loading. This, coupled with significant changes in design and construction of light wood-frame buildings since the 1960s, has led to a review of the simple prescriptive structural requirements in Part 9 of the 2005 NBC, with respect to both seismic and wind loads.
Three years ago, a task group established by the Canadian Commission on Building and Fire Codes (CCBFC) examined the requirements. The group used available information from Canada and the United States, as well as findings from past earthquake surveys and a suite of full-scale earthquake simulation tests that were performed on houses in Canada, the United States, China and Japan. The task group's proposed changes, which went to public review in fall 2008, were accepted by the CCBFC and will be incorporated into the 2010 NBC.
The recommendations include introducing a risk-based approach with three risk levels (low-to-moderate, high, and extreme) for exposure to wind and seismic forces, using environmental load data in the 2010 NBC Appendix C. For areas where the risk is low-to-moderate, no new requirements have been added. In a limited number of regions of extreme risk, engineering design according to NBC Part 4 is required.
The bulk of the new requirements, however, apply to areas of high risk, mainly the Pacific coast of British Columbia. For these areas, prescriptive requirements have been added to NBC Part 9 so that builders can incorporate adequate lateral load resistance without the need for structural engineering design. These include constructing walls using braced wall panels in braced wall bands that are continuous horizontally and vertically throughout the building and extend from the top of the supporting foundation, slab or sub-floor, to the roof framing above.
Limits and requirements are also in place for materials, spacing and dimensions for braced wall bands and braced wall panels, as well as fastening and framing details. In high wind areas, there are additional requirements for attaching roof sheathing to framing and roof framing to walls.
For more information, visit the national codes website at www.nationalcodes.ca or contact Frank Lohmann at 613-993-9599 or frank.lohmann@nrc-cnrc.gc.ca.
