Overall Stability Research Of Steel Frame Beams

The laterally unsupported steel frame beam may fail by lateral-torsional bucklingif the lateral deflection of the critical flange is not effectively prevented. Thechecking calculations for the overall stability of steel beams in the current code arebased on the analysis of simply supported beams subjected to concentrated loads,uniform loads or end moment alone. As a result of the complex end restrains and thesimultaneous action of end negative moments and transverse loads, the overallstability of the steel frame beams is quite different from that of simply supportedbeams, which makes the checking calculations beyond the scope of the currant code.In accordance with the characteristics of the overall stability, the steel framebeams were divided into two categories: hinged ones and rigid and semi-rigid ones.The partial torsional restraints provided by the end supports of some hinged steelframe beams have adverse effects on the overall stability. Meanwhile, both the endnegative moments and the lateral bending restraints provided by the end supports ofrigid and semi-rigid steel frame beams have advantageous effects. All of the aboveshould be taken into account in the accurate evaluation of the elastic critical momentof steel frame beams. Due to the complexity of the restrains provided by the supports,this thesis proposed a simplified model for each category. Only the torsional restraintwas taken as elastic restraint in the hinged steel frame beam model while only thelateral bending restraint was taken as elastic restraint in the rigid and semi-rigid steelframe beam model. Based on energy approach, this thesis derived the elastic criticalmoment general formula of steel frame beams by employing the effective lengthfactors to allow for the effects of the end restrains. By the use of the general formula,the least torsional stiffness to assume full torsional restraint of hinged steel framebeams end supports and the elastic critical moment calculation formula of rigid andsemi-rigid steel frame beam were obtained.Using finite element software ANSYS,603beams with different end restrainsand loads were analyzed. For the hinged steel frame beams, the least torsionalstiffness calculation formula was in good agreement with the finite element results.For the rigid and semi-rigid steel frame beams with transverse loads acting on theshear center, the finite analysis agreed with the theoretical calculation well. Whiletransverse loads acted on upper flange, the finite analysis was close to the theoretical calculation on condition that the end moments were not great. The error between thefinite analysis and the theoretical calculation was a little large for the rigid andsemi-rigid steel frame beams under the simultaneous action of large end moments andtransverse loads, and the error analysis was performed. This thesis proposed acorrection approach which was carefully verified by the finite element analysis withappropriate parameters and the finite analysis agreed with the revised theoreticalcalculation well. This thesis would provide references for the further improvements ofthe code and the engineering applications in the overall statbility of steel framebeams.