Structural Assessment of the Källösund Bridge using Finite Element Analysis - Evaluation of the load carrying capacity for ULS

Report, 2004

In this study, it has been demonstrated and documented how higher load carrying capacity with respect to combined shear and torsion can be shown for road bridges through non-linear finite element (FE) analyses. In this report, the evaluation of a prestressed box girder bridge, the Källösund Bridge, is documented as a case study. The Källösund bridge is a 45 years old, cast-in-situ concrete bridge with four spans, built with the free cantilevering method on the west coast of Sweden. A previously performed initial structural assessment, performed according to current regulations and common engineering practice, showed insufficient load carrying capacity, both with respect to combined shear and torsion in large parts of the bridge, and with respect to bending moment in the outer spans. Since the capability to simulate and demonstrate higher capacity with respect to bending, using FE analyses, have been showed in several cases previously, this study focused on combined shear and torsion. The critical load combination, including vehicle types and traffic positions, was evaluated through linear analyses for combined shear and torsion in the most critical cross-section according to the initial assessment. The possibility to simulate a shear type failure was demonstrated through local analyses of critical parts of the bridge. Through non-linear analyses of the complete bridge, a load carrying capacity was shown, corresponding to a bogie load of at least B = 250 kN (i.e. 1.3 times the required design value), together with the other design loads according to Vägverket (1998). The analysis stopped due to convergence problems, before a shear failure was fully developed. With improvements to the model, and the modelling of the load, it is likely that a higher capacity can be shown. However, the analysis needs to be complemented for other possible load cases, before an increase of the bridge capacity can be considered as proven. Furthermore, non-linear FE analyses of concrete shear failures for this kind of concrete structures are quite rare, and should be verified against tests. How further evaluation of the bridge should be performed , by means of FE analyses, have been suggested. Also, based on the experience from the project, areas that are in need of further research and development are presented. The study was mainly financed by the Swedish road Administration (Vägverket), and is a part of a larger project, carried out in cooperation with the Lund University of Technology and Luleå University of Technology.