In April of 2009 new provisions in the BC Building Code came into effect allowing the maximum height of residential wood-framed construction to increase from four storeys to six. This change was brought on by a successful test of a 6 storey wood-framed structure that was placed on a shake table in Japan. The shake table is the largest in the world and accounts for the three dimensional forces that are present during an earthquake. Testing took place on a building constructed of primarily of wood and was fully furnished at the time of the test. Surprisingly the structure withstood a full minute of a magnitude 7.3 earthquake. Once the test was completed the engineers deemed the building had suffered no structural damage and was safe to be occupied. You may wonder how just a simple wood structure can stand up to the forces of an earthquake but it all comes down to the overall design of a shear wall and type of bracing system that can be used.
The shear wall is basically a structural member that is designed to withstand the lateral loads associated with wind and earthquakes. Shear walls are usually constructed of typical framing members, sheathing and fasteners. The framing members are usually 2” x 4” or 2” x 6” wood studs and plates. In most cases the studs are spaced either 16” or 24” on centre. Because the frame must support the vertical loads, two or three studs are typically nailed together at the perimeter of the wall to provide the extra support to keep the frame rigid. Sometimes diagonal braces are used within the frame to increase the overall stiffness of the frame. This stiffness is small compared to the stiffness of of what the sheathing offers. Sheathing for load bearing walls is usually 4’ x 8’ sheets of ply wood or OSB (oriented strand board). The interior walls may be sheathed with GWB (gypsum wall board). The plywood or OSB sheathing is connected to the framing members with nails and sometimes staples.
When the lateral loads of seismic activity is applied to the wall, the fasteners work with the rigid sheathing panels and the flexible frame of the shear wall to carry the load. The fasteners are the most important components in determining shear wall performance, influencing both stiffness and strength. Creating as solid hold down system is no easy task. Since buildings are always moving, shear walls tend to shrink over time causing causing the wall to loose its strength. However using the proper methods and equipment, shrinkage can be combated. Hold down systems are typically anchored in a concrete slab or right onto the foundation wall creating a rigid frame. Then the right fasteners are used to keep the shear wall, one continuous structural member. The placement of the fasteners are crucial to keep movement and shrinkage to a minimum.
Since 2009 wood-frame construction has been applied in buildings of up to 6 storeys in height and of irregular shape. Since it is very expensive to test large scale models of buildings to determine the seismic response of them, the investigation of how a shear wall is to perform is an important analytical method. Therefore it is of great importance to develop the proper design and study the performance of shear walls in mid-rise wood-frame construction to ensure continued safety during seismic events.
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