Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 20 It is routine in design to use continuous chord members and to pin the truss members. In fact, transforming pinned connections into rigid nodes hardly leads to any modification to the axial forces in the members, because the shear transmitted by the members has little influence on the equilibrium equation of nodal forces and, on the other hand, bending of the member due to secondary bending moments only causes a slight variation in the distance between the ends of this member compared to the difference in length due to axial force. Nevertheless, it is essential that the triangulated structures be designed properly so that the members are adequately arranged to withstand bending stresses, but not too slender so as to avoid buckling. Note that the greater the stiffness of the chords (which are usually continuous), compared to the global stiffness of the truss beam, the bigger the moments developed in the chords. For instance, for a wind girder in a roof, the stiffness of the chords is relatively small and the secondary moments remain small as well. For a stocky truss, i.e. when the flexural stiffness of the individual chords is not significantly lower than the global stiffness of the truss, it can be necessary to take into account the secondary moments. Then the members and the connections must be designed accordingly. This phenomenon can be illustrated in the worked example by arranging the IPE 330 sections as ‘standing up’ chord members, instead of being flat in the initial design (Figure 3.5). The chords therefore bend in the vertical plane of the truss member, mobilising their strong inertia. The calculation results demonstrate well a significant increase in the secondary moments. Figure 3.5 Options for the orientation of the chords In the upper chord in a standing up IPE 300 section near the half-span, the bending moment under gravity loads (ULS) is 28,5 kNm, compared to 2,7 kNm for the flat IPE 330 section. Similarly, in the lower chord, the bending moment is 23,4 kNm, compared to 1,7 kNm. The multiplier of the bending moments is 11 for the upper chord, and 14 for the lower chord. This is comparable with the ratio of the inertia in an IPE 330 section (about 15). 3.5.2 Assumption of rigid connections In another evaluation of the effect of member stiffness on the value of the secondary moments, the truss in the example was recalculated by making all