Lateral buckling of plate girders with flexible restraints

Since publication of BS 5400: part 3 in 2000, the design check of paired plate girders during erection has become more onerous in the UK. This change has been brought about by the addition of two main features in BS 5400: part 3—a change to the mode of buckling considered in deriving girder slenderness and the addition of more conservative buckling curves when the effective length for buckling differs from the half-wavelength of buckling. BS EN 1993-1-1 requires no such reduction in resistance. The work presented in this paper was prompted by a proposal to modify the rules of BS EN 1993-2 for bridges in the UK's national annex to be more like those in BS 5400: part 3. The authors believed this to be unnecessary. This paper investigates buckling cases where the effective length for buckling is shorter than the half-wavelength of buckling and demonstrates that the series of correction curves used in BS 5400: part 3 are unnecessary and that the BS EN 1993-1-1 method is satisfactory and slightly conservative. The paper also outlines the design process to BS EN 1993 using both elastic critical buckling analysis and non-linear analysis. The case studies considered are a simple pin-ended strut with intermediate restraints, a pair of braced girders prior to hardening of the deck slab and a half-through deck with discrete U-frame restraints. For the latter two cases, the results predicted by BS 5400: part 3 and BS EN 1993-1-1 are compared with the results of non-linear finite-element analyses.