Design aid for moment strength of built-up crane runway girders

The evaluation of elastic moment for built-up crane runway girders is not an easy task as the difficulty involves the calculation of torsional properties including warping constant, the coefficient of monosymmetry, and torsion constant. To date, all AISC specifications offer no specific formulas for determining the elastic critical moment for WC (W-shape with Cap channel) or SC (S-shape with Cap Channel) girders. They provide only the approximate formulas which were derived from singly symmetric I-sections. Based on this study, the AISC approximate formulas would underestimate the elastic critical moment of WC/SC girders by 6-25% depending upon the unbraced length. This paper first summarizes the theoretical and AISC formulas for WC/SC girders. Theoretical torsional properties and relevant design parameters for WC/SC girders are computed and arranged in tabular form for design purposes. With these design values made available, engineers can perform more rational evaluation of WC/SC girders more conveniently. Lastly, a simplified but rational design approach is proposed for all available WC and SC girders listed in the AISC design manuals.