Strength Variation of Laterally Unsupported Beams

Nominally identical 25 rolled beams for each group of three different lengths are tested carefully under a concentrated load applied vertically at the midspan of the compressive flange of span center. The variation of the geometrical and material imperfections are measured and the influences of the various parameters on the scattered test points of the lateral buckling are reviewed. The main parameter which highly influences the variation of the ultimate strength is the variable value of actual plastic moment. The obtained test results are also compared with the existing test data and design formulas. The coefficients of variation of the strength increase with beam length and these values are considerably low as compared with the past scattered data. Finally, a design formula is proposed to ensure empirically the lower bound for the scattered test values.