Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 36 Then, the factor k zz can be calculated: 0,854 4357 1 1477 0,899 0,628     zz k Second interaction criterion (eq. 6.62) 0,944 1 34,97 0,854 2,86 1690 1477     OK Note on secondary trusses The presence of secondary trusses in the central part of the truss (see diagram 2.3) permitted the reduction by half of the buckling length of the upper chord in the plane of the truss. The secondary truss is sized in order to support a buckling restraint load whose value depends on the compression force in the supported chord and on its slenderness ratio (see EN 1993-3-1 on subject of design of pylons in annex H4). 4.1.2 Lower chord in compression With respect to the complete design of the structure, it is also of course essential to check the lower chord, subject to the lower compression force, but without support from a secondary truss. Verification of the lower chord in compression is similar to that described for the upper chord in compression, in 4.1.1. Lateral restraint of the lower chord is provided at each purlin (Figure 2.2). The only specific point which would be interesting to develop is an analysis of the buckling out of plane of the truss. Buckling of the lower chord is to be considered similarly to that of the upper chord, for a length equal to the distance between truss panels, thanks to the presence of sub-panel braces (See Figure 2.3). The difference is that the axial force in the lower chord varies along the buckling length, in two panels, whereas the force was constant along the buckling length for the upper chord. It should also be noted here that, for the chord member with the greatest bending moment, the variation in axial force is very small; in a real design, the small reduction in buckling length due to variation of normal axial force can safely be ignored.