Fragility-based assessment of seismic performance margin for RC frame structures

Performance margin ratio (PMR) was presented to assess structural performance under earthquake. The PMR was calculated by the ratio of the median of fragility function to the earthquake intensities corresponding to frequent earthquake (FE), design based earthquake (DBE) and rare earthquake (RE). Four kinds of 5-story and 4-bay RC plane frames were selected as study cases, which were designed for different seismic fortification levels. Then the PMRs of the reference structures were calculated by fragility analysis. The results show that increasing seismic fortification levels leads to the increase of both fragility medians and the seismic actions with respect to FE, DBE and RE. Nevertheless, the resultant increment of fragility medians is less than that of seismic actions, thus a decreasing trend of PMR is found according to the increased seismic fortification levels. Under DBE and RE, the structures show sufficient performance margins to satisfy the performance requirements of 'no unrepairable damages under DBE' and 'no collapse under RE', respectively. The increase of seismic fortification level may probably cause the structure cannot satisfy the performance requirement of 'no damage under FE'.