Deterioration in the stiffness of a bridge reduces its natural frequency. One explicit parameter to quantify deterioration is the depth of flexural cracks in the longitudinal girders. The paper proposes a simple macro-model for modelling cracks in reinforced concrete (RC) beams. The crack is modelled as a rotational spring whose rotational stiffness depends on the depth of the crack. The uncracked portion may be modelled by frame elements as usual. This model can be conveniently included in the popular direct stiffness approach. The proposed model for rotational spring stiffness have been applied to different cases of cracked RC beams and encouraging results have been obtained.