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In this paper, the basic beam strength curves are investigated numerically by the finite element technique. At first, an imperfection sensitivity function for beam strength is introduced and various coefficients in the function are numerically identified by a finite displacement elasto-plastic analysis with both geometrical and material imperfections. And using a number of imperfection sets generated by Monte Carlo simulation from measured imperfection data, the strength distributions of steel beams with rolled and welded I-section are evaluated from a probabilistic view. These numerical results are compared with the experimental buckling strength database. Secondly the effects of bending moment distribution on beam strength are examined, and an equivalent slenderness formula applicable to inelastic range is developed. Finally, the mean strength curves and the resistance factors for rolled and welded beams are proposed.

Inelastic lateral-torsional buckling strength of steel beams depends markedly on the location and extent of high moment regions causing the reductions in sectional regidities.Rolled I-sections with two different sizes are tested carefully under a concentrated load at the central point and two equal concentrated loads at the one third points of simply supported beams, respectively. The test results of load-deformation curves and strength variations are compared with both loading conditions. Appropriate design strength formulas for the various load patterns of beams are also discussed by a review of the available experimental data on the ultimate strength of stocky beams.