Abstract

Seismic design using maps based on “risk-targeting” would lead to an annual probability of attaining or exceeding a certain damage state that is uniform over an entire territory. These maps are based on convolving seismic hazard curves from a standard probabilistic analysis with the derivative of fragility curves expressing the chance for a code-designed structure to attain or exceed a certain damage state given a level of input motion, e.g. peak ground acceleration (PGA). There are few published fragility curves for structures respecting the Eurocodes (ECs, principally EC8 for seismic design) that can be used for the development of risk-targeted design maps for Europe. In this article a set of fragility curves for a regular three-storey reinforced-concrete building designed using EC2 and EC8 for medium ductility and increasing levels of design acceleration (ag) is developed. These curves show that structures designed using EC8 against PGAs up to about 1 m/s 2 have similar fragilities to those that respect only EC2 (although this conclusion may not hold for irregular buildings, other geometries or materials). From these curves, the probability of yielding for a structure subjected to a PGA equal to ag varies between 0.14 ( ag=0.7 m/s 2) and 0.85 ( ag=3 m/s 2) whereas the probability of collapse for a structure subjected to a PGA equal to ag varies between 1.7 ×۱۰−۷ ( ag=0.7 m/s 2) and 1.0 ×۱۰−۵ ( ag=3 m/s 2) .

Introduction

In the past decade the philosophy often known as risk-targeting has started to be employed to develop maps of design accelerations for use with seismic building codes (Luco 2007, 2009). The aim of this method is to estimate seismic accelerations that, when used for design purposes, lead to a chosen level of risk (e.g. annual probability of collapse of 10−۵) that is uniform over an entire territory. This procedure relies on convolving hazard curves from a standard probabilistic seismic hazard assessment with fragility curves that are a function of the design acceleration (e.g. buildings designed against a higher acceleration have fragility curves shifted to the right). This approach has recently been tested for mainland France by Douglas et al. (2013) with respect to the recent French seismic zonation to be used in conjunction with Eurocode 8 (EC8). Amongst other conclusions, they emphasized the lack of studies on how fragility curves change as a function of the design acceleration. This deficiency of knowledge is a current block on the adoption of risk-targeting for the next generation of seismic design codes.