The common disadvantage existing in evaluation for durability is of higher cost with lower efficiency since structural members are evaluated one by one in terms of chosen tested indices. Based on the three-level-multi-index evaluating model for durability of existing concrete bridges structures, the application of random layered sampling theory to durability evaluation is studied in this paper. The relationship of evaluating cost, evaluating precision and evaluating sample size is discussed and the calculating formula for sample size and its optimum distribution among different layers is presented. At the same time, the credible interval for class of membership of bridges structures is given out. Further, the chart-flow for evaluating durability is summarised. The result of application to actual projects shows the layered random sampling method on the basis of optimum distribution among different layers can achieve coherence between evaluating cost and evaluating precision so well as to meet different needs from bridge-owner.
Keywords: existing concrete bridges, durability evaluation, layered random sampling, optimum distribution, credible interval, concrete bridge structures, structural engineering