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Nowadays trusses are often designed with I-sections only and no direct connection between the diagonal members and the chords. For the truss joints, gusset plates – welded to the chords – are used, which are slotted for the welding connection to the diagonal member. The main focus of the paper is the specific stability behaviour of such diagonal members in compression, leading to an interaction behaviour of member buckling out of plane and local buckling of the gusset plates. Within a comprehensive study a lot of different compression members and joint configurations were studied in detail, based on nonlinear numerical analyses. The results of four full scale tests on a lattice girder are presented in detail. In the four tests different I-sections with different gusset plate geometries were investigated. The most interesting fact is the comparison of the load carrying behaviour of the compressed diagonal members with increasing free length of the gusset plates. The member capacity of the tests is compared with the numerical results of a FE-model, considering the measured geometric imperfections and the measured material parameters. The overall load carrying behaviour of the truss in the test is analysed and compared with simplified models for global analyses, both for the load – deformation behaviour and for the relationship between load and member axial force. Finally a simple design model for the prediction of the ultimate capacity of the compression members is presented, which consists of a buckling verification based on the equivalent member with increased buckling length (section capacity near midspan is verified) and of an additional verification of the section capacity of the gusset plate. For the latter one the interaction of local and global buckling effect is important, leading to a simplified engineering model with all the relevant sources of geometric imperfections considered. It ends up with a check of the section capacity of the gusset plate, based on 2nd order internal forces (N&M). For slender gusset plates and also diagonal members with small slenderness, the section capacity of the gusset plates limits the load carrying capacity of the member and the buckling verification of the equivalent member may be on the unsafe side.

To 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.