In the present paper, an application of a dynamic identification process in the frequencies domain, is presented. A steel structure 90 meters long, 22 meters high and simply supported by four pair of concrete piles is considered. The structure has been dynamically loaded by using a mechanical shaker, giving sinusoidal excitation at prescribed frequency values. Experimental modal frequencies, deformations and damping ratios are evaluated starting from inertance function by using the MDOF Circle Fit Method. A finite element model of the structure has been developed and Modal Assurance Criterion (MAC) is, then, used to compare experimental and numerical vibration modes; good correlation is obtained both for global flexural modes, global torsional modes and longitudinal modes.