The paper aims to select a simple and effective damage index for estimating the extent of damage of rectangular concrete-filled steel tube (RCFT) structures subjected to ground motions. Two experimental databases of cyclic tests conducted on RCFT columns and frames are compiled. Test results from the database are then used to evaluate six different damage indices, including the ductility ratio ( μ ), drift ratio, initial-to-secant stiffness ratio ( D_(KJ) ), modified initial-to-secant stiffness ratio ( D_(ms) ), energy coefficient ( E ), and the combined damage index ( D_(PA) ) as a benchmark indicator. Selection criteria including correlation, efficiency, and proficiency are utilized in the selection process. The optimal alternative for D_(PA) is identified on the basis of a comprehensive evaluation. The evaluations indicate that D_(ms) previously proposed by some of the authors is the most appropriate substitution of D_(PA) , followed by the drift ratio. For the case of the slenderness ratio less than or equal to 30, the same grades of relation between the investigated damage indices and the benchmark are observed. However, in the case of the slenderness ratio larger than 30, the drift ratio tends to be the optimal alternative. In most cases, μ is proved to be an inadequate replacement of D_(PA) .
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