FE analysis of weld stresses in composite bridges : A case study of the influence of the cross bracings

Sammanfattning: Composite bridges in Sweden are commonly designed as a twin I-girder section. To stablizie the bridge during launching and concreting horizontal cross bracings are used. These are normally connected to a transverse web stiffener that is welded with a throat size of 5 mm regardless of the dimensions of the bridge. During an inspection of Vårby Bridge south of Stockholm in 2006 cracks were found in this weld between the transverse web stiffener and the top flange. After discovering of these problems the Swedish Transport Administration (Trafikverket) wants the designer to present calculations for this weld. The aim of this thesis was to investigate and compare the stresses in the weld between the top flange and the transverse stiffener for different cross bracings. A finite element model of a typical composite bridge, with five different types of cross bracings, was investigated using the FE program Brigade Plus. A selection of loads including temperature-, traffic- and fatigue loads were considered. The thesis clearly shows that the design of cross bracings will affect the stress levels in the weld.