Abstract
The article proposes an approach of six degrees dynamic model of a rigid-solid with some types of symmetries. These symmetries lead to simplified mathematical models, which are more easily to solve. If the rigid-solid is symmetrical beared by triorthogonal elastic links, the mathematical model becomes still simple and the vibrations are decoupled into four subsystems of movements: side slipping and rolling, forward motion and pitching, lifting motion, gyration. There are two case study of modal analysis: for a viaduct with five arches made from reinforced concrete „U” beam and for an arch (between two piers of the viaduct) made from four reinforced concrete „U” beam.
Keywords: modal calculus, structural symmetry, decoupled vibration, eigenvalues, reinforced concrete bridges vibrations