Effective lengths of laterally continuous, laterally unsupported steel beams

Abstract

Laterally unsupported steel beams of sufficient length may fail by elastic or inelastic lateral–torsional buckling. The fundamental equations governing elastic lateral–torsional buckling, taking into account such parameters as the shape of the bending moment diagram, the level of application of the load, and the effect of end restraints to lateral movement and to twist, are reviewed. Provisions in the current CSA Standard CAN3-S16.1-M84 are discussed. The methods currently available for dealing with the interactive lateral–torsional buckling of laterally continuous beams are evaluated statistically. Two new methods for considering this interaction, called the iterated effective length method and the equivalent beam method, are presented. A statistical evaluation of these methods shows that they are in reasonable agreement with available test data. Resistance factors for use in limit-states design are developed for the existing methods discussed as well as for the new methods. Key words: beam, bending moment, buckling, effective lengths, elastic, inelastic, lateral–torsional buckling, laterally continuous, steel.