Structural health monitoring is defined as the structural behavior surveillance during a predetermined time or throughout the structure's lifetime, in order to detect materials or structural degradation. Periodical measurements can detect changes in the natural frequencies of the structure, and indicate the structural damage or deterioration. In this work, we experimentally demonstrated the use of a fiber Bragg grating?based biaxial accelerometer in dynamic measurements on an elevated reinforced concrete water reservoir. This elevated water reservoir structure was suitable to be used as a demonstration of the application of optical accelerometers in the structure natural frequencies measurement, under environmental excitation. The obtained experimental values for first natural frequency in the transverse and longitudinal directions were 0.805 Hz and 0.903 Hz, respectively. These values present a relative error smaller than 0.5% when compared with that obtained by the structure numerical simulation.