Widening of The Nockeby Bridge : Methods for strengthening the torsional resistance
Abstract: i Abstract The Nockeby Bridge, in the western part of Stockholm, is a prestressed concrete bridge with an openable swing span of steel. The bridge was built during 1970 and should now be widened with 0.5 meters on each side. The concrete bridge deck is supported by two main-beams and cross-beams are located at the position of all supports. Previous studies of the bridge show that the torsional resistance is too low and the bridge needs strengthening while widened. The aim of this master thesis was to study and compare different strengthening methods for The Nockeby Bridge. Eight different bridges in Sweden and China were reviewed to find possible strengthening methods for The Nockeby Bridge. External prestressing tendons and additional cross-beams between the two main-beams were seen to have good influence on the resistance. The effect from strengthening with carbon-fiber reinforced polymer was questioned during small loads and was not seen as a suitable strengthening method for The Nockeby Bridge. Four different FE-models were generated to be able to compare two strengthening methods. The compared strengthening methods were a method with additional cross-beams between the main-beams and a method with external prestressing tendons. All FE-models were built up by solid- and truss elements where the concrete was modelled with solid elements and the prestressed reinforcement was modelled with truss elements. Only a few load-cases were included to limit the scope of the study. The included load-cases were deadweight, prestressing forces and vehicle load from standard vehicle F, G, H and I. Two influence lines were created to be able to place the vehicle loads in an unfavorable way. From the FE-models, shear stresses were extracted along two lines, one on each side of the main-beam. The torsional part of the shear stresses was calculated from these two results and compared with the torsional resistance of the bridge. While calculating the torsional resistance, the normal force in the cross-section from prestress was extracted with the function “free body cut”. The results showed that none of the tested strengthening methods were enough to strengthen The Nockeby Bridge. However, the method with additional cross-beams was seen as a better method than external prestressing tendons. A combination of the two methods might be suitable but was not tested. Adding four cross-beams in each span might also increase the resistance enough, but this was neither tested. It was also seen that a reduction of the torsional stiffness had a large influence on the result. Such a reduction is allowed in some cases and should be utilized if possible. Furthermore, it was seen that solid-models were extremely time consuming and there is not a good alternative to design a bridge with only a solid model.
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