Structural health monitoring (SHM) for evaluating and maintaining structural integrity of a building is a very important research field. This paper proposes the use of squared modal frequencies, to detect damage to individual parts of the structures as well their extent by using a limited number of sensors, as proposed by Mita and Hagiwara1. This damage assessment method evaluated in numerical simulations of a five-story shear structure and in shake-table tests of a five-story steel model. The damage to the structure was simulated by reducing the stiffness of each floor. The study showed that it is possible to identify, localize, and evaluate the magnitude of the real damage in a multi-story structure from shifts in its natural frequencies.