Abstarct: This thesis evaluated methods of structure assessment and damage detection in structures baxsed on modal data processing methods. For this purpose, the modal test and the factors obtained from it were introduced. After surveying these factors, the scientific applications of modal test for concrete structures were introduced and some of conducted studies on vibration behavior of reinforced concrete beams were discussed; and it was concluded that the results of modal test can be useful for identification of damage pattern in reinforced concrete beams. Moreover, modal data processing methods for damage assessment in structures were reviewed and the major aims of their applications were discussed. These discussions were provided along with categorization of the methods and presentation of advantages and disadvantages of each. Damage detection methods using modal data are divided into two major categories: (1) the methods suggested baxsed on changes of primary modal factors (frequencies, attenuation and mode shapes) that provide an overview of hardness changes in the structure, and (2) the methods in which the structure is assessed by modal computational quantities including strains, strain energy and softness. In this study, a new algorithm baxsed on local energy of modal strains was proposed for simultaneous identification of location and severity of damages in beam structures. The proposed solution was able to detect single and multiple artificial damages in the numerical model. The results of numerical analysis showed that the proposed method is able to accurately detect location and severity of damages using a limited number of vibration modes. The damages occurred in the first stages can be detected by applying the proposed algorithm in modal strains’ space; and then the decisions can be made for prevention of sudden failure of the structure.