Influence of secondary torsion on curved steel girder bridges with box and I-girder cross-sections

Steel curved girder bridges are largely used today in motorways and railways. They are often composed of thin-walled crosssections, entirely made of steel or with an upper concrete slab. The deck may have I-girders or box cross-sections: in any case curved girders are subjected to twisting moment, associated with bending, even for dead loads. Moreover, in thin-walled sections the influence of non-uniform torsion becomes sizable with respect to Saint Venant torsion, modifying the state of tangential stresses in the section and introducing axial stresses due to warping being prevented. Open sections of I-girder bridges are especially subject to these phenomena and warping can be significant not only for curved bridges but also for eccentrically applied traffic loads in straight ones. In this paper a method for the evaluation of the effects of non-uniform torsion, based on an energetic approach, is proposed; the method is simple and fast, with a reduced computational burden with respect to finite elements. The solution of curved girder bridges is performed by the Hamiltonian Structural Analysis method, which is implemented for straight and curved girders with thin-walled cross-sections. A parametric analysis is proposed for single and multi-span bridges, with variation in type of loads applied, crosssection parameters, geometric curvature of bridge deck and stiffness ratios of the cross-section. The parametric study is presented through dimensionless diagrams of internal forces for the evaluation of the global behaviour of curved girder bridges together with indications on the stress state of the cross-section. The interpretation of the results of the analyses performed can be useful to designers for the conceptual design stages and for optimization of the geometric and mechanical parameters of I-girder and box girder cross-sections.