The work addresses the effect of pre-stress on the dynamic behaviour of concrete structures. At this purpose peculiar attention is paid to the eigenfrequencies and the relevant modal shapes of beams that share the same geometrical and mechanical parameters but are acted upon by different levels of pre-stress force. From a theoretical point of view, an increasing compression is expected to diminish the value of the eigenfrequencies, with a stronger effect on the first ones. However, the non-linearities of the material and the non-uniform distributions of the cables within the beam sections do not allow an easy evaluation of the magnitude of this effect. This topic has crucial importance in the achievement of monitoring procedures that should be able to predict inefficiency or failure of pre-stress devices from the dynamic behaviour of the whole structure. Preliminary numerical analysis are performed on simply supported concrete beams with straight post-tensioned cables. The results obtained from non-linear finite element techniques are then presented and critically discussed, having the main aim of providing an affordable numerical framework to further experimental tests.