Torsion in ZIP bridge system
J.C. Walraven – Mentor
C. Van der Veen – Mentor
P.C.J. Hoogenboom – Mentor
C. Quartel – Mentor
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Abstract
In skew bridges torsion occurs. This leads to a substantial amount of reinforcement stirrups. Minalu already did research to torsion in bridge decks with different types of finite element models. The question when torsion cracks will really occur is still unanswered. This question is the main subject of this research. The focus of the research is on a skew bridge with a skew angle of 45 degrees. In that bridge the largest torsional moments will occur. Beside that also a straight bridge is analysed, the torsional moments in a straight bridge are always lower than in a skew one under the same loading. The loads of Eurocode 1991-2 are used. Two important load configurations governing for torsional moments and shear force are used: a configuration which is used in daily practice at Spanbeton and a configuration developed by Minalu. An attempt is made to model the whole bridge including physically non-linear behaviour with the program ATENA 3D to analyse the torsion effects. With the current state-of-the-art modelling technology that appeared to be impossible. For that reason a simplified model is developed to simulate the stress state and cracking in one ZIP girder. It was concluded that it is important to use more quadratic elements over the thickness of the web to obtain correct torsion shear stresses. From the simplified model it is concluded that, despite some shortcomings, clearly a substantial length at the ends of the girder is uncracked. To be sure that the computer model is correct a calculation of the principal stresses is carried out at the ultimate limit state. The stresses due to prestressing, own weight and the weight of the fresh poured concrete can be calculated by hand. The calculation of the force distribution of the loads on the deck can be carried out using finite element methods. Scia Engineer (orthotropic plate model) and ATENA 3D (volume elements) are used for this calculation. Especially the determination of the torsional moments from ATENA by using an analysis of the rotations is interesting. This calculation results in the torsional moments, bending moments en shear forces acting on the ends of the girder. The main conclusion of this research is that in ultimate limit state no cracking will occur in the end of the considered girder in the skew bridge. This means that only the minimal shear reinforcement must be applied and the full torsional stiffness can be used in finite element calculations. A practical method to check this for other bridges using ZIP-girders is proposed.