Keywords: beams, unbraced length, lateral-torsional buckling, finite element analysis, initial imperfections, codes.

The Egyptian Code of Practice - Allowable Stress Design (ECP-ASD) specifies two limits for the unbraced lengths which, if exceeded, lateral-torsional buckling must be taken into consideration. The allowable flexural stress calculated using this code predicts the maximum allowable strength of 0.58 of the yield stress, well beyond the specified limits for the unsupported length. This paper reviews the theoretical background for the allowable flexural strength of laterally unsupported beams according to both the ECP-ASD and the Egyptian Code of Practice - Load and Resistance Factor Design (ECP-LRFD). A nonlinear finite element parametric analysis is conducted to investigate the requirements of the formulation of the permissible unsupported length for laterally unbraced beams. Twelve beams are modelled varying the laterally unsupported span of the beams from 1 m to 12 m at 1 m intervals. A lateral initial imperfection of 1/500, 1/1000, and 1/2000 of the beam's span is introduced at the midsection of the beam to initiate lateral-torsional buckling. Load-deflection curves are plotted from the finite element analysis. The ultimate loads determined from the finite element analysis are compared with those required by the ECP-ASD and ECP-LRFD and are found to correlate relatively well with the latter. The factor of safety or utilization ratio from each code is found to have a variable value.

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