This paper presents the use of Frequency Response Functions (FRFs) to determine damage index and crack damage in reinforced concrete (RC) beam structures using vibration signals. Three methods consisting of Static Residual Strength Index (SRSI), Intermediate Load Damage Index (ILDI), and Modified Flexure Damage Index (MFDI) were adopted in this research. Impact tests on simply supported RC beams were conducted to measure vibration signals on the beams by recording the curvature mode shapes during the experimental testing. The ICATS software was carried out to capture the Frequency Response Functions (FRFs) data at each load step. Cracks occurred on the beam due to the applied load, reducing its natural frequency, indicating an initial stage of the damage having occurred. The midspan vertical deflection of the beam results in its mode shape changes and curvatures increased. The mode shape curvature square difference was used to determine the extent of damage and location of damage indicating the beam residual strength. A numerical algorithm of the finite difference method was performed using the FRFs data to calculate the different FRFs for undamaged and damaged beam conditions based on the mode shape curvature square (MSCS) method. The damage index and crack detection based on the numerical computation were determined by subtracting the MSCS between undamaged to damaged beams. The resulting accuracy of the damage index used to define the level of damage and damage location was absolutely achieved by comparing the numerical and observed experimental results.