AbstractVibration-based damage detection techniques usually focus on the changes in the modal properties as a result of any structural damage. For the last three decades, varying levels of success have been achieved in identifying damage using modal frequencies as well as mode shapes. The present study aims at using the damage-induced changes in the fundamental mode shape for evaluating the severity of damage in structures. For this purpose, the concept of the spectral element method was applied to derive the expression for mode shapes in the frequency domain. The difference between the fundamental mode shape of the damaged and undamaged structures was formulated and mathematical correlation established to derive the measure of damage quantity. A numerical analysis involving a 14-story shear building was performed to show the effectiveness of the proposed approach in damage quantification. In order to check the efficiency under real measurement conditions, the approach was also verified experimentally on a three-dimensional miniature model of a 6-story shear building in the laboratory environment. Both simulation and experimental investigations show the competency of the fundamental mode shape in structural damage quantification. Also, the use of fundamental mode shape makes the proposed methodology easy to implement on real structures.