At the request of the Council of Forest Industries, a simplified (hand-calculation) design method has been proposed for estimating the ultimate shear capacity and load-deflection response of wood-framed and panel-sheathed diaphragms, by Forintek's Wood Engineering Department. In its current form, the simplified code design approach can predict ultimate shear capacity for a wide range of (blocked) diaphragm constructions, sheathed with panels ranging from 7.5 mm (3/8") to 18.5 mm (3/4") in thickness, with a 22% coefficient of variation. This is comparable to the 21% variability exhibited by the current Canadian diaphragm design method, and not alarmingly larger than the 16% shown by the more detailed APA design method. Indeed, for the three high-shear diaphragms sheathed by 18.5 mm (3/8") Douglas-fir plywood panels, the proposed simplified design method yielded shear prediction errors of only 0%, 1% and 6%. Furthermore, simplified functions were also able to provide good estimates of diaphragms failure mechanisms, and their load-deflection patterns, as measured in earlier verification tests. Predictions from the simplified model need to be experimentally verified for high shear capacity diaphragms sheathed in thick panels fastened to glulam frames by large diameter nails; before its possible introduction to the Canadian wood design code.
A literature survey on experimental data and analytical studies of the structural behaviour of wood framed shear walls and diaphragms has been carried out. The utility of various analytical methods for the study of internal forces in these structural elements due to external static or dynamic forces has been noted. It also has been concluded that the complexity of these analytical methods precludes their use as a tool for standard designs on a daily basis. For these standard designs currently established design methods will likely be continued to be used for some time. Establishment of design data by means of testing for panel thicknesses currently not included in the Canadian design code is recommended.
In order to maintain the competitive advantage in existing and new markets situated in seismic and high wind zones such as the Pacific Rim and the southeastern U.S., it is proposed to study deflections in walls, floor and roof assemblies. The proposed project will also be very useful in: a) setting deflection criteria as will be demanded by performance-based codes, and b) responding to the inevitable transition to displacement-based seismic design.
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.
The objective is to broaden the market for sheathing panels by establishing shear strength values for the design of floor and roof diaphragms using 18.5mm and 20.5mm thick plywood panels. As a result of an emergency no work was done on this project.