Fragility Analysis Of A Frame-core Tube Hybrid Structure Under Mainshock-aftershock Sequences

In past earthquake events,it is common to observe multiple strong aftershocks following stong mainshocks.Moreover,strong aftershocks can further aggravate the structural damage and even lead to strucrual collapase.Earthquake damage in highrise buildings and super high-rise buildings may result in more loss of life and property,which is unfavorable to post-disaster relief and reconstruction.However,according to current Chinese code or sepecification of peer review for high-rise buildings beyond the code-specification,only single mainshock ground motions are adopted without regard for the adverse impact of aftershocks.This paper investigates the seismic performance of frame-core tube hybrid structures under mainshock-aftershock sequences based on a case study.The main achievenments of this paper are as follows:(1)A material-based numerical model is developed and then verified to simulate the seismic behavior of a practical frame-core tube hybrid structure.Having evaluated the performance level of structural components based on material damage,a method for quantification of earthquake damage quantification in framecore tube structures is proposed.The results show that the proposed structure-level damage index can effectively indicate the damage states of frame-core tube structures.(2)A set of eighteeen as-recorded mainshock-aftershock(ms-as)sequences are selected based on the target response spectrum,where only mainshock motions are considered for matching purpose.In addition,five ms-as sequences with pulse-like aftershocks are incorporated to account for the near-fault pulse characteristics.The ms-as sequences are input bidirectionally.The spectural acceleration at the first natural period of the structure is adopted as the ground motion intensity measure and is used for scaling purpose.Scaled mainshocks show a broad distribution of intensity and are high enough to evaluate the seismic performance of frame-core tube sturctures when subject to high-hazard level ground motions.(3)To study the damage distribution and damage mode in frame-core tube structures,structural time-history response and performance levels of components are examined based on the results of incremental dynamic analysis.In regard to aftershock-induced damage quantification,different damage indicies are adopted for comparison purpose.Ultimately the impact of aftershocks is evaluated based on seismic fragility curves from a probabilistic perspective.It is indicated that for mainshock-damaged structures,damage further develops among different kinds of components and among stories with varying degerees.Furthrermore,damage indices differ in their quantification effectiveness and are significantly subject to aftershock intensities.With the incorporation of aftershocks,structural fragility increases considering different limit states,possibly making frame-core tube structures fail to meet desired seismic performance.