Two reinforced concrete arch ribs, which were in service for 28 a, were removed to the laboratory. Modal experiments were performed during its loading process. Based on the experimental results and theoretical analysis, the modal changes were researched in failure process. A supposition that the structural damage can be divided into consistent damage mode and complex damage mode was given, the reason why the modal frequency increases possibly during its early loading period was explained. Then, the effect of location of plastic hinge on modal frequency was analyzed. Finally, the function between the first modal frequency and loading level in failure process was studied with the curve fitting method. The results show that the function is approximately quadratic function. This provides some suggestions for evaluation of ultimate load of the existing concrete arch rib with the application of modal frequency.%对一服役28 a旧桥中的2根钢筋混凝土拱肋进行加载过程中的模态试验.基于试验结果和理论分析研究钢筋混凝土拱肋破坏过程中的模态变化规律.通过对损伤进行一致损伤模式与复合损伤模式的划分,解释结构模态频率在受荷初期可能会增大的原因,分析塑性铰位置对拱肋模态频率的影响；采用曲线拟合方法研究拱肋破坏过程中一阶模态频率与所受荷载之间的函数关系.结果表明:该函数近似为二次函数.这为应用模态频率值来预测钢筋混凝土拱肋的极限荷载提供了参考.
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