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Summary

Seismic isolation is a method to decouple the structure from the damaging effects of horizontal ground motion in the event of an earthquake. This is achieved through the lengthening of the natural period of the structure, and adding damping, thus reducing the demand as determined from the typical 5% damped design response spectra. As seismically isolated buildings have very small inter-storey drift, and much lower floor accelerations in comparison to fixed base structures, seismic isolation is often used in buildings that are expected to be “operational” or “fully operational” after a major earthquake. Since the 1970’s the use of seismic isolation has continued to increase worldwide, especially in countries after they experienced a damaging earthquake; for example this was evident in Japan following the 1995 Kobe earthquake, in Italy following the 2009 L’Aquila earthquake, and in Chile following their 2010 earthquake, where in all cases the use of seismic isolation increased significantly after such events. In Canada, implementation of seismic isolation in buildings has been limited so far. This is due to the lack of provisions in the National Building Code of Canada (NBCC), provincial building codes or municipal by-laws. The 2015 edition of NBCC is the first one to include provisions for seismic isolation in the body of the code. It is expected that more base isolated buildings will be used once provinces and municipalities adopt the NBCC 2015 into their codes in the very near future, including the provisions for seismic isolation. Although there has been a steady growth in the application of seismic isolation in concrete and steel structures (outside of Canada) that are designed to have low or no damage under major earthquake events, the application in wood structures is essentially stagnant in North America. There are a number of impediments to the implementation of base isolation in wood structures. It is perceived that a base isolated building may cost much more than a conventional wood building. As wood-frame construction has generally performed well during major earthquakes by just meeting the building code design criteria, it could be challenge to convince the developer and/or the building owner of the value of the seismic isolation system relative to its cost. In addition, the installation of a seismic isolation system requires the introduction of a stiff base slab at the first floor of the wood-frame superstructure, so that loads from the wood-frame building can be transferred to the isolation system and structural integrity of the wood-frame superstructure is maintained. While this can be an additional requirement for wood buildings, it can be accommodated in podium buildings where a wood super-structures is supported by a concrete or steel sub-structure. Also, isolators are usually not suitable for resisting large tensile loads, which usually occurs at the edges of shear walls or the columns in braced or moment-resisting frames due to the light seismic mass of wood structures. The effects of uplift must be carefully examined and evaluated. The added mass that is necessitated to improve the acoustical performance does increase the mass of the wood structures, hemce partiallt alleviate the issues of large tensile and overturning forces. Market study and cost analysis show that the use of base isolation on average will increase initial construction costs up to 6% and possibly vary according to economies of scale. However, when construction costs are compared between wood-based buildings with base isolation (adding the maximum premium of 6%) versus conventional concrete-based buildings, wood buildings mostly maintain a competitive price advantage per square feet of floor area. Therefore, there seems to be a potential for base isolated timber structures to be economically viable options for building construction. Further studies and research, however, are needed to address the identified impediments in order to allow wide-spread adoption of seismic isolation on wood buildings. Carrying out the studies and research now would help to prepare for the future if and when the seismic design philosophy in the building code moves towards the stricter criterion of low and no damage for all buildings and not just the essential “post-disaster” buildings. Given the generally light weights of wood structures and the need for a rigid base for wood-frame superstructures, one main focus of future research would be on development of new isolation systems that could accommodate these properties.

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