الفهرس 
CHAPTER 1: INTRODUCTION 1Only 14 pages are availabe for public view
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Abstract
The elastic lateral-torsional buckling (LTB) behavior of steel beams has been extensively reported. Most of such reports have been limited to study the beams with doubly symmetric cross-sections besides a few other standard geometries. On the other hand, not so much research have been done on monosymmetric steel beams. LTB for this type of beams is one of the most critical design aspects. The critical moment (M_cr) is an essential parameter in the lateral-torsional buckling design of beams. It can be calculated using analytical methods or, more commonly, by using structural design software. However, the complex nature of the lateral-torsional buckling phenomenon makes it hard to embrace all the affecting factors and assumptions. Different programs use different methods to calculate〖 M〗_cr which leads to deviations of the results. Codes of practice allow designers to relate 〖 M〗_cr due to any in-plane loading to the standard uniform moment and member using the moment-gradient factor.
The present thesis aimed to study the effect of number of parameters on the elastic lateral torsional buckling behavior of beams with monosymmetric cross-sections. These parameters are loading type, compression flange to the tension flange width ratio, load location along the section’s vertical axis, and the beam end lateral conditions. The finite element analysis was adopted to accomplish this study, and numerical models of such beams were constructed using one of the commercial finite element method software programs (ABAQUS). Moreover, a modification factor for the moment-gradient factor C_b was developed incorporating these parameters.
At first, a verification study was conducted to evaluate the accuracy of the finite element models through a comparison of such models and the experimental tests carried out by in previous research. This evaluation was made to examine the accuracy of the FEA for the element type, mesh density, boundary conditions, material models, and solution algorithm. The results of finite element models were verified by comparing numerical results, such as load-displacement curves and failure modes, with the experimental results. The good agreement between the finite element models and the experiments opened the way to use the finite element analysis to achieve the above-mentioned parametric study.
Secondly, a parametric study was made on 3D-FE models of steel I-beams to determine the elastic critical moment. The investigated parameters included the height of the beam cross-section (300, 400, 500, and 600 mm), the span length of the beam (4000, 6000, 8000, and 12000 mm), the upper and lower flange thickness (10, 13, 16, and 19 mm). In addition, these models were categorized into five groups (A, B, C, D, and E) according to the value of the upper-to-lower flange width ratio, β_f. All groups contain monosymmetric section geometry except group C, which has a doubly symmetric section geometry.
Finally, based on the results of the parametric study, it is found that the moment-gradient factor C_(b )depends not only on loading conditions but also on the modified slenderness of the beams. In general, the value of C_b is small for short span lengths and reaches the code-specified values for longer spans. Instead of using the span length, the slenderness of the beams, λ_LT, was used to for models in the parametric study are converted to λ_LT which gave a more precise idea for the general variation of the moment-gradient factor. Empirical formulae were deduced to express the moment-gradient factor in terms of slenderness. These formulae could help the designer to perform the LTB checks for the steel beams with either doubly symmetric and monosymmetric cross-sections under a wide variety of loading and support conditions.