Nelinearno ponašanje i granična nosivost limenih nosača opterećenih lokalizovanim opterećenjem
AuthorTurnić, Dragana T.
MentorMilovanović, Gradimir V.
Committee membersMarković, Nenad G.
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Abstract: The subject of the thesis is the analysis of steel girders behavior (welded steel I beam) under the influence of the localized load. The girders without longitudinal stiffeners were tested, as well as the girders with the longitudinal stiffeners in the vicinity of the loaded area. The influence of the length of uniform distributed load on the loaded flange, in the plane of the web, known as patch loading was analyzed as well as the behavior of the girder in the non-linear area, as well as the ultimate load which is manifested by buckling in the load application zone. During the bridge assembly procedures, when the bridge is being launched to its definite position over the temporary and permanent supports, the influences which occur may exceed the load of the structure in certain points due to the patch loading. Because of its extreme complexity, a complete theoretical solution for the problem has not been found yet. The solution procedure requires the combination of experimen...ts and numerical analysis. Because of the importance of the numerical simulation of the girders, in this thesis, a special attention was paid to the theoretical foundations and solutions which would contribute to the improvements of efficiency of software packages concerning the onset of singularity points during the simulation. The developed software packages are based primarily on the application of Finite Element Method (FEM) and Boundary Element Method (BEM). In these procedures, algebraic and/or logarithmic problems with singularities of often occur, and one of the goals was to give a contribution towards a more efficient solution of these problems. The research comprised application of numerical procedures where FEM was implemented using most up-to-date software packages; many improvements were made using the analytical methods referring to the singularity problems. Theoretical basis and solutions elaborated here can include an occurrence of singularities, so that it contributes to improving the efficiency of software packages, i.e., it gives a faster and more accurate solution of the problem. A special class of Gaussian quadrature formulas is V derived and it provides improved solutions of certain more complex problems with algebraic and/or logarithmic singularities. An alternative approach with Gaussian quadratures for Müntz systems, developed by Milovanović and Cvetković (SIAM J. Sci. Comput. (2005)) is also mentioned. In the existing literature up to now this area has not been paid due attention. In addition to these solutions and the general theoretical fundamentals, direct answers to the problems of patch loading are provided by presenting the compatible developed numerical model in the software package ANSYS Workbench 15, after the experimental research performed. A large number of the results of the experiments, performed during research, conducted at the Faculty of Civil Engineering in Belgrade under the direction of N. Marković , served as a basis for the research and creation of the compatible model in the computer simulation. The goal numerical simulation was to determine the ultimate load and monitor development of deformations and stresses. The actual models for numerical simulation of girder behavior allow inclusion of initial deformations-imperfections into the calculation on the basis of the actual girder model. In the numerical simulations of the model, the form, character and course of deformation process as well as the buckling were identical as in the experiments with actual models, for each concrete sample. Numerical simulations tested the girder model behavior, and the values of the ultimate load of the girder was determined for six material models, and the quantitative analysis of those results was performed. The behavior curve corresponding to the multi linear curve according to EN1993-1-5  gave a very good approximate value of the ultimate load. Because of the topicality of this field, a review of the state-of-affairs is provided, as well as the main course of research in this field. The existing experimental results and standard statistical method, common for these issues were used, for the purpose of result verification, as well as contemporary methods of numerical approximation. The results of the research are applied for the structural design, particularly for the design of steel bridges constructed by launching over the temporary or permanent supports, and in the new versions of the national codes for steel structures design. VI The impact of actual imperfections of the beam web plate was included, and a comparison with the beams without imperfections was performed within the numerical simulation, for the purpose of observing the stress development. The cases of research of impact of actual imperfections on the magnitude of the limit load are present in the literature to some small extent. This thesis contributes to a more realistic modeling of the girders, which provides opportunities for new behavior tests of such or different girder models using numerical simulation. The tests carried out in this way, facilitate beam analyses which would indirectly contribute to the adoption of correct approaches in obtaining the theoretical results.