Influences of site conditions and ground-motion frequency characteristics on structural fragility

Site conditions and ground-motion frequency characteristics play a significant role in structural response and seismic damage distribution. However, the current structure fragility analysis techniques have hardly considered such effects. Taking the ratio of peak displacement to peak acceleration (i.e., R_d2a) as an index, the ground-motion records collected are subdivided into four groups. Meanwhile, based on the site average shear-wave velocity (V_s), the site conditions are classified into four categories which are bedrock, hard site, medium-hard site, and soft site. Using the OpenSees software, a simplified model for a six-story reinforced concrete (RC) frame structure on a single soil-layer site is built. The effects of ground-motion characteristics and site conditions on the fragility curves of structures for different structure damage stages controlled by the maximum structural inter-story drift angles are investigated using the incremental dynamic analysis methods. The analytical results indicate that: (1) The frequency characteristics of ground-motion greatly impact the fragility curves of structures, that is, the probability of structural damage under ground-motion loading of large R_d2a values stays higher than that of small R_d2a values. (2) The site conditions, nevertheless, impose definite influences on the structural fragility curves. Such influences correlate with the structural damage states, e.g., when the structure is seriously damaged, the possibility of structure damage tends to increase with the increasing stiffness of sites. (3) The site conditions maintain relatively smaller influences on the structure fragility curves than the ground-motion frequency characteristics. (4) The influences of the ground-motion characteristics and site conditions should be necessarily considered in the structure fragility analysis, especially during selecting input ground-motion. The investigation of the influences of the site conditions and ground motion characteristics on structural fragility herein by a simplified model provides a reference for the structure fragility analysis and seismic damage prediction techniques.