| At first, this thesis studied the position of shear center of tapered I-beam. If only the web is linearly varied along the member, and the other dimensions are kept constant, and it can be shown that the shear center axis of the tapered I-beam is still a straight line. Flexural-torsional buckling may be still studied using the shear center line as a reference axis to define the lateral displacement and the rotation.Secondly, the elastic lateral buckling of simple supported I-beams with doubly-symmetrical and mono-symmetrical cross-sections under moment gradient isinvestigated by FEA method, it is found that the formula for betab (equivalent momentcoefficient) currently used in Steel Structures Code is not accurate enough in some cases, e.g. when beams are bent in double curvature. An improved design formula is proposed on the basis of results of FEA. Furthermore, we thought the application tomono-symmetry section should be confined and -1≤k≤1 todoubly-symmetry section.At last, considering the effects of taper ratio, we derived critical load of tapered I-beam by energy method. Using the formulation thus obtained, but adjusting the coefficients in this formulation based on the results of FEA, we obtained a new formula for the Critical moment for lateral-torsional buckling of tapered I-beams, it has a similar formulation as those for beams with constant cross-sections,, with clear meaning and easy to application in practice. We found the precision of the new formula is better than CECS 102:2002. Moreover, we point out the formula in CECS 102:2002 not to be exact because it considered the monosymmetry twice.
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