| Resumo: | This thesis is a research work aiming the increasing of knowledge of the behaviour of steel plate girders subjected to shear buckling in fire situation. The main objective of this thesis is to overcome the lack of rules for the design of steel structural elements subjected to shear buckling at high temperatures. For this purpose, a numerical model was developed in the finite element software SAFIR to simulate the behaviour of steel plate girders under shear loading at elevated temperatures. These numerical analyses fall into the methodology commonly referred as GMNIA – geometrically non-linear materially imperfect analysis. After validation of the numerical model with experimental tests from the literature, the influence of the geometric imperfections and residual stresses on the bearing capacity of the girders, at both normal and elevated temperatures, was evaluated. Eurocode 3 states that the shear buckling resistance of steel I girders is given by the sum of two components, the web resistance and the contribution from the flanges. Firstly it was assessed the contribution from flanges and it was found that the results obtained with the Eurocode 3 expressions could be improved. Thus, it was proposed the application of a corrective factor in order to improve the predictions of Eurocode 3 for the contribution from the flanges to the shear buckling resistance. The main part of the shear buckling resistance comes from the web. The expressions of Eurocode 3 for determining the web resistance to shear buckling were evaluated. This analysis demonstrated that some of the results are not on the safe side and the accuracy of these expressions could be improved. So, changes to the expressions applied for the design at normal temperature were proposed. Furthermore, new expressions for fire design of such structural elements were also proposed. The expression of Eurocode 3 used for the safety calculation of steel structural elements under interaction between shear and bending was also evaluated. It was verified that the application of the proposals for modification of the expressions used to determine the shear buckling resistance introduces improvements on the results provided by this expression, mainly at elevated temperatures. Finally, an analysis of the influence of different parameters on the ultimate shear strength of steel plate girders subjected to shear buckling, such as the web thickness, the web depth, the flange thickness and the steel yield strength, is presented.
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