Ultimate strength of singly symmetrical I-section steel beams with variable flange ratio

摘要

Long steel beams in entrances of hotel lobbies and halls often support a masonry wall and are laterally unsupported. These beams do not reach their in-plane flexural capacity due to the occurrence of elastic or inelastic lateral-torsional buckling (LTB). Designers usually select doubly-symmetrical sections for these beams. In this paper the ratio of the compression flange size to the tension flange size is increasingly varied to obtain a higher resistance to lateral buckling of the compression flange. However, increasing the size of the compression flange and decreasing the size of the tension flange leads to the limiting case of a T-section which is weak in in-plane bending. Code provisions usually refer to generalized cases and special cases are always embedded in these and are not directly addressed. One of these special cases is the singly symmetric I-section. Special direct solutions for such problems are becoming more popular based on accurate inelastic ultimate load analysis. This paper develops a three dimensional finite element model, using ANSYS for the elastic and inelastic flexural-torsional buckling of I-beams, which is used to investigate the effect of varying the ratio of the flange sizes on the optimum performance of the beam. Slender webs are proposed and a study is made on the material of this web and the performance of its local and global buckling and its correlation to the total ultimate load of the beam system. The flange material consumption is kept constant and only the shares to the tension and compression flanges are varied to achieve the most economical system for design. The main goal of this research is to present a direct solution system that is simplified for the use of inexperienced designers, which provides the most economic system selection for design.