Plane grillages and finite element models are widely used for the design of bridges. In a grillage idealization the entire superstructure of the bridge is suitably represented by different sets of bar elements. On the other hand, the finite element method makes use of plane and solid elements for the three-dimensional representation of the structure. Although these models are generally accepted as sufficiently accurate, they are associated with some drawbacks. To overcome the difficulties of plane grillage and finite element analyses, a new way of modelling steel composite bridges is presented in this paper. The proposed model is based on the simulation of steel I-girders through the use of equivalent trusses. The concrete slab is represented by a set of bar elements and the bearings by the use of appropriate springs. Diaphragms and stiffeners may also be taken into account.
This new way of modelling composite bridges using a spatial system of beam-like structural elements allows deformations and internal forces to be predicted reliably, and can be used for dynamic and stability analysis. Worked examples are also provided to illus trate the setup procedure for 3D modelling and to compare the different analysis methods. The work presented in this paper is part of a research project investigating the modelling of steel and composite bridges which is being carried out at the National Technical University of Athens.