05.14: Lateral torsional buckling of steel beams: Effect of discrete flexible intermediate supports at the upper flange

Abstract

ABSTRACTTo avoid lateral torsional buckling in steel constructions, beams are often stiffened with discrete flexible intermediate supports (e.g. purlins in roof structures with bracings), which reduce lateral deformations of the flanges out of plane. Very often, these intermediate lateral supports can't be assumed as fully rigid and only restrain one of the flanges (i.e. no torsional restraint available), leading to a design situation that is only insufficiently reflected in EN 1993‐1‐1. Neither commonly used interaction formulae, derived on the basis of free single span members with fork end conditions and for members supported laterally and torsionally by intermediate rigid restraints, nor the simplified assessment method for beams with restraints in buildings (clause 6.3.2.4), based on a lateral buckling check of the compression flange, lead to an appropriate solution.In this paper, the results of an extensive numerical parametric study on beams with intermediate flexible lateral supports at the upper flange are presented.In the first part of the paper the focus is on the appropriate selection of the relevant buckling mode for the geometric imperfections, to obtain the minimum load carrying capacity within a numerical nonlinear calculation. As expected, a decrease of the spring stiffness always leads to a reduced elastic critical buckling load. Nevertheless, when assuming geometric imperfections based on the 1st buckling mode, this may lead to a higher ultimate load than for the system with rigid intermediate supports, when identical imperfection amplitudes are used.In the second part of the paper a first design proposal is presented, valid for different moment distributions and number and stiffness of intermediate supports.