In this research program, studies were carried out to assess the wind and seismic requirements for conventional wood-frame construction. This report contains information on seismic research. Information on wind research is provided in a separate report entitled ‘Wind and Seismic Design Provisions for Small Wood Buildings - Part B: Wind’.
For the seismic research, it consists of four main study areas: shake table tests on small houses, targeted braced wall tests, evaluation of code bracing requirements for conventional wood-frame buildings, and recommendations for improvement.
Results of shake table tests of two two-storey full-size wood frame buildings, funded under the Canada Wood/FII China Codes and Standards project, have been used to study the performance of small wood buildings with different braced wall lengths. The results showed that the two-story building specimen could withstand successive application of three different seismic ground motions in the order of 0.55 g Peak Ground Acceleration (PGA). Test results are in general agreement with the results observed in actual earthquakes in California, New Zealand and Japan, that wood-frame buildings without major structural deficiencies withstood seismic shaking in the order of 0.5 to 0.6 g PGA without collapse (Rainer & Karacabeyli, 2000).
A series of supplementary full-size braced walls were tested to quantify the effect of floor / upper wall and corner wall on the lateral load capacities of braced walls. Test results showed that floor or / and upper wall could greatly increase the lateral load capacities of braced walls. For a 1.22 m braced wall with continuous top plate extended over braced wall panels, test results indicated that the lateral load capacity of the wall was approximately 50% of that of the walls that are fully restrained. Braced walls with 1.22 m corner walls had similar performance as walls that are fully restrained. The results indicate that the mechanics-based method implemented in CSA O86-1 is very conservative for determining the lateral load capacities of braced walls.
The adequacy of the bracing requirements for conventional wood-frame construction in the 2004 CWC Guide and the proposed Part 9 of 2010 NBCC were assessed. Two buildings, 15 m × 15 m and 4.8 m × 15 m in floor dimension, were studied. The lengths and locations of the braced wall panels in the buildings were chosen to represent as much as possible the most unfavourable case for lateral load resistance. The results showed the imbalance between the required lengths of braced walls in short and long directions of the rectangular building. While the lateral load capacity in the long direction of the building is adequate for the two- and three-storey building and is in fact overly conservative for the one-storey buildings, the lateral load capacity in the short direction of the building is not sufficient to resist the base shear forces. In most of the studied cases, neither the 2004 CWC Guide nor the proposals for the Part 9 of 2010 NBCC meets the seismic requirements of the Part 4 of NBCC 2005 for the high seismic zones.
A new method was proposed to address deficiencies of bracing requirements for conventional wood-frame construction in 2004 CWC Guide and Part 9 of 2010 NBCC proposals. Instead of specifying the minimum length of braced wall panels as a constant percentage of the length of a building parallel to the direction of loading considered, the new method specifies the minimum length of braced wall panels as a function of floor area of the building. Using the new method, it was concluded that the required length of braced wall panels should be a percentage of the building length perpendicular to the direction of loading considered. This will address the imbalance between the required lengths of braced walls in short and long directions of a rectangular building.