The Operational Modal Analysis [1] is a kind of test in which the input forces are derived from the environmental conditions. The dynamic behavior of a structure, on the other hand, is a kind of “fingerprint” that depends only by intrinsic characteristics (mass, stiffness, damping, boundaries, degree of freedom, etc.) and not by the applied loads. The structural response remains unchanged in absence of modification (structural damage, etc…), otherwise a variation of frequencies and vibration modes occurs. The present paper illustrates the dynamic identification of an existing building supporting a cell tower. An innovative technology has been employed to measure the forces transmitted by the tower to the building and an accurate FE modelling has been performed to assess the model. The comparison between the experimental evidences with the results of FE models allows the dynamic identification of the structure. The element of novelty of this work is the attempt to use a hybrid procedure that allows to dynamically identify a structure by environmental excitation but at the same time to measure the induced actions and thus develop the identification calculations with compulsive notes, allowing a calibration of the model to obtain a more accurate solution.