This study explores how to evaluate structures and detect damage using modal data processing methods. It introduces modal testing and the factors derived from it. The practical applications of modal tests for concrete structures are examined, along with the advantages and disadvantages of research on the vibration behavior of reinforced concrete beams. The study concludes that the results of modal testing can be used to identify damage patterns in reinforced concrete beams. Additionally, the main goals and categories of modal data processing methods for damage assessment in structures are reviewed, along with their advantages and disadvantages. Damage detection methods using modal data are divided into two categories based on changes in primary modal factors or modal calculation quantities. The study proposes a new algorithm based on the local energy of modal strains to identify the location and severity of damages in beam structures. The proposed solution can reveal single and multiple fictitious damages in the numerical model. Numerical analysis results show that the proposed method can determine the damage location and severity with a limited number of vibration modes. Applying the proposed algorithm to the modal strains can help identify the damage early and prevent the sudden failure of the structure.