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The torsional stiffness of hollow section chord members of welded truss girders reduces the effective length for out-of-plane buckling of the diagonals. On the other hand local deformations have to be taken into account at the connection of the diagonal to the chord.Springs connecting the diagonals to the chord members are used in a beam element model of the truss girder to present both of these effects. A finite-element analysis using a shell element model for the K-joint gives the spring constants. A buckling analysis of the beam element model gives the effective length for out-of-plane buckling. The investigation of a variety of truss girders used in scaffolding shows that the effective length factor 0 75 given in ENV 1993 is very much on the safe side in many cases.The presented method takes account of these interactions between buckling of the chord and buckling of the diagonal. Finally a model is proposed to calculate the effective length of diagonal members where their buckling load is not influenced by buckling of the chord member. This is done by using a single beam representing the diagonal with springs at both ends representing the cross section deformation of the chord members where they support the diagonal.