Numerical Analysis on Lateral Torsional Buckling Strength Characteristics of Parallel Flange Steel Beams due to Geometric Imperfections

Lateral torsional buckling (LTB) in beams results from bending loads causing combined lateral displacement and twisting due to insufficient restraint, exacerbated by imperfections like bowing and warping, potentially leading to premature failure without adequate lateral support. In the present research work, a nonlinear finite element (FE) model has been developed utilizing the commercial finite element software ANSYS. The detailed parametric investigation includes 20 distinct hot-rolled narrow parallel flange beam sections conforming to the specifications of IS 12778:2004. These sections feature unsupported lengths of 5 m, 10 m and 15 m, with overall depths ranging from 447 mm to 770 mm and flange widths spanning from 190mm to 270mm. Initial imperfections (equivalent imperfection) as per EN 1993:2005 is introduced to study the nonlinear buckling behaviour of the beams. The results are correlated with the code provisions given in Indian codes. It is observed that the bending moment/compressive stress results obtained from finite element analysis for both linear (eigenvalue buckling) and nonlinear analysis are lower than the calculated theoretical values. This discrepancy suggests that the actual buckling condition may be more critical than what is predicted by the theoretical calculations. In our study, employing second-order analysis methods using FE analysis ensures an accurate portrayal of buckling behaviour. This approach accounts for variations in member stiffness due to loading and imperfections, a capability absent in conventional elastic analysis methods.