In this paper the required number of artificial accelerograms is determined from the dynamic analyses of two test bridges in order to get the reliable behaviour factors, where the allowable deformation of the structure is taken into account. The dynamic analyses are performed with "Dynamic Plastic Hinge Method". For the determination of the behaviour factor the procedure of Ballio /Perotti is adopted. As target functions for the simulation of artificial accelerograms the elastic response spectra provided in Eurocode 8 are used.