“…The disadvantage of this principle consists in the fact that its application requires a rather restrictive assumption on the parameters fc, rai, 7712, 61, 62, E, I,T and the conditions obtained are too restrictive and are not satisfied by the real values of the bridge parameters. P. Drabek, H. Leinfelder and G. Tajcova [4] have established the existence of a unique time-periodic solution near stationary equilibrium under rather general assumptions on the above mentioned parameters, provided the external time-periodic forcing terms are small in a certain sense.…”