In the new 2005 edition of the National Building Code of Canada, the permissible deflections under earthquake conditions will be much more restrictive and could potentially become the governing factor for the design of wood frame construction.
To proactively respond to the code changes, this three-year project was to develop design procedures for determining the stiffness or deflection of shearwalls and diaphragms under these extreme seismic and wind load events. While investigating the formulae for predicting deflections, issues related to the overall strength or load carrying capacity of shearwalls were also addressed.
Using a “mechanics-based” approach, deflection formulae were developed for unblocked shearwalls, two-sided shearwalls (gypsum wallboard on one side and wood-based panels on the other side), and shearwalls without hold-downs. In collaboration with staff from the Canadian Wood Council, these deflection formulae will be submitted for implementation in the next edition of the Canadian Standard for Engineering Design in Wood (CSA O86), which in turn forms the basis for acceptance under the National Building Code.
The work has also helped to address the following issues in the CSA O86:
· Height limitations for unblocked shearwalls (currently capped at 2.44m)
· The use of diagonal lumber as sheathing for walls and diaphragms. This information, which is particularly important in upgrading wood buildings to new code requirements, addresses concerns raised by designers.
Technical information generated from this project was disseminated in the wood engineering community and CSA O86 committee meetings. Two papers entitled “Lateral Resistance of Tall Unblocked Shearwalls” and “Deflections of Nailed Shearwalls and Diaphragms” were presented at the 8th World Conference on Timber Engineering in Lahti, Finland in June 2004. An article entitled “Racking Performance of Tall Unblocked Shearwalls” has been submitted to the ASCE Journal of Structural Engineering for publication. Another article entitled “Performance of Shearwalls with Diagonally Sheathed Lumber” is being prepared and will be soon submitted to the ASCE Journal of Structural Engineering.
By proactively responding to future design code changes, this project will allow the construction industry to take advantage of various shearwall options in their designs of wood frame buildings, and will assist the wood products industry to maintain its competitive advantage in existing and new markets situated in seismic and high wind zones such as those around the Pacific Rim and in the South Eastern United States.