Elastic Lateral Torsional Buckling of I-Beam with Lateral Support

Abstract

In AISC Specification for Structural Steel Buildings, the parameters that are needed to compute lateral torsional buckling critical moment of a beam is mechanical properties of material of the beam, section properties, and unbraced length of the beam. Unbraced length is the distance between two consecutive laterally braced points. The AISC formula for computing critical moments does not consider the end conditions of the part of the beam being considered. It is assumed that the ends conditions is simply supported. Actually, if the beam is laterally supported, the ends of the part of the beam is not simply supported, that is the end of the part of the beam is not free to warp. This paper presents the result of a study about the critical moments if the beam is considered as a whole. The end of the beam simply supported. There are two lateral support between the ends. There are three load case. First is uniform moments, second is concentrated load at the middle of the beam and third is uniformly distributed load. The material of the beams is assumed to be elastic, linear, homogen and isotropic. The differensial equation of lateral torsional buckling of beam is derived first and then, the solution is sought using finite diference method. The middle part of the beam is being considered. The parameter is the distance of the lateral support. The result is that the critical moments obtained is significantly higher than the critical moments from AISC formula. The concept of effective length for the beam is proposed.