VALUE CASE FOR THE USE OF SEISMICALLY ISOLATED LIGHT-FRAME WOOD BUILDINGS IN NEW ZEALAND

New Zealand residential buildings are typically made of light-frame wood construction with gypsum wallboards contributing to the structural strength and stiffness. Such buildings typically perform well in earthquakes with minimal casualties, as was demonstrated in Christchurch during the 2011 Canterbury earthquake sequence. However, damage was widespread and even minor cracking of gypsum wallboards required invasive repairs, which displaced large numbers of the population and cost $16 billion NZ dollars, as well as contributing negatively to the livelihood of those affected. High damage costs were similarly reported in the 1994 Northridge earthquake in California and the1995 Kobe earthquake in Japan. These high damage costs have prompted researchers to investigate methods for reducing seismic loss to these residential buildings through the development of low damage design systems, such as seismic isolation. Seismic isolation has been used very successfully to protect larger commercial and industrial buildings from earthquakes around the world. Research in Japan in the early 2000s and later in the United States and New Zealand showed that seismic isolation could also be used effectively on light-frame wood buildings with sliding type devices such as friction pendulums. This paper presents a value case for the use of seismic isolation in New Zealand residential buildings by comparing the expected annual loss for fixed-base and seismically isolated light-frame wood buildings. First, a wind load analysis is undertaken to determine the combination of seismic weight and isolation friction coefficient required to provide adequate seismic protection, while minimizing the risk of buildings moving under strong wind gusts. A vulnerability analysis is then undertaken to determine appropriate vulnerability functions for seismically isolated light-frame wood buildings. In the methodology presented in this paper, loss data generated by insurance claims information from the 2011 Christchurch Earthquake is used to determine vulnerability functions for the current housing stock. By further considering the loss attributed to drift and acceleration sensitive non-structural components and using a simplified single degree of freedom building model subjected to nonlinear analyses, vulnerability functions for seismically isolated buildings are developed. The vulnerability is shown to reduce significantly for isolated buildings compared to that of equivalent fixed-base buildings. The fixed-base and seismically isolated vulnerability functions are then used to compare expected annual losses. Results indicate that losses are significantly reduced for the seismically isolated building models. As such, if low-cost isolation systems can be developed there would appear to be a strong value case for such systems.