The design of an important viaduct in Treviso (a town in Venetian area) is here described. This struc- ture, called «Silea» Viaduct, was designed following the philosophy of the new prestational codes. The viaduct, built in 2007, has medium span and composite structure. In this paper the different safety level has been evaluated on the basis of National and European Codes. The structure is provided with viscous dampers and mechanical dampers to bidirectional earthquake excitation and the main parameters defining the seismic behaviour of seismically isolated bridges are investigated.