A simplified analytical model and formulation are defined to describe the nonlinear seismic behaviour of multi-span bridges with dissipative piers and a continuous deck transversally restrained at the abutments. These structural systems have a “dual load path” behaviour and their failure may occur in the piers or in the deck, depending mainly on geometry and stiffness distribution. By using a variational approach and introducing a transverse displacement shape assumption, the properties of an elasto-plastic single-degree-of-freedom system equivalent to the bridges are derived, and analytical expressions are proposed for describing the post-elastic system behaviour, the global dissipative capacity, and the failure modalities. In particular, the system global ductility capacity is expressed as a function of the local piers ductility capacity and of characteristic non-dimensional parameters describing the ratio of the deck to pier stiffness and the piers distribution along the bridge. Furthermore, the geometric non-dimensional parameters controlling whether failure is due to piers rupture or due to deck yielding are posed in evidence. The proposed analytical formulation is applied to the analysis of a set of continuous multi-span steel-concrete composite bridges with different properties. Three values of the ratio between the piers height and diameter are considered, in order to cover different types of seismic responses and failure modalities. The accuracy of the simplified model is evaluated by comparison with the results of incremental dynamic analysis performed on refined nonlinear finite element bridge models. It is shown that the proposed model and formulation are effective in predicting with sufficient accuracy the properties of dual-load path bridges during their elastic response and at collapse. They are also useful in unveiling the characteristics parameters to be considered in the seismic assessment and preliminary design of the deck and piers.