In the study of the rotor dynamic performances of a rotating machine, it is possible to divide the plant in four main components: rotor, stator, bearings and supporting structure. The current trend for rotating machinery considers the reduction of weight an important problem. As a consequence of the complex behavior the last generation plants, characterized by low weights and flexible support structures, the separation margin between rotor critical speeds and operative conditions is quite difficult to be reached. The presence of flexible supports can generate dangerous coupling effects among the different bearings. Therefore the model of a complete rotor dynamical system must consider all the phenomena coming from the coupling of its mechanical components. The aim of this work is to demonstrate the importance of taking into account the support structure since the preliminary project phases. To this end, this study investigates a problem of enhanced vibration level on a five stage MCL rotor, part of a compression system made up of a gas turbine driver with two centrifugal compressors and one gearbox. Each rotating machine was installed on a separate baseplate steel structure and the individual rotors were connected by elastic coupling. This research work has been developed cooperating with Nuovo Pignone General Electric S.p.a., that has provided the technical and experimental data required for the model validation.