Setting structural safety requirement for controlling earthquake mortality risk

Structural engineers design buildings according to the earthquake action (demand) specified in code of practice, whilst the rationale behind such requirement is commonly untold. In fact, even if a structure is designed strictly in accordance to the best standard and practice in the world, there is still a (small) chance of failure or collapse in an extreme earthquake event, due to the uncertainties in material properties and actual ground motions characteristics. This is the residual risk, which is unavoidable, and should be taken as a governing parameter for determining the performance goals of seismic design. This study attempts to establish the required (target) collapse risk limits for designing different types of ordinary buildings based on a well-accepted tolerable level of mortality risk and estimates of fatality rates in buildings. The proposed limits are compared with the target risk of collapse stipulated in the 2012 edition of the International Building Code (IBC). The risk-based approach presented in this paper should also be applicable for setting performance objectives for structural design of buildings and infrastructure against other natural, human-caused and technological hazards.