Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 19 4000 101 kN 158 202 202 202 158 101 7100 7200 8500 8600 7100 7100 Truss (combination n°1), including self-weight 461 (616) 303 (405) 101 (135) -101 (-135) -303 (-405) -461 (-616) 562 -562 Diagram of the global shear force V (kN) In parentheses: values of N d = V /cos  3273 (818) 5455 (1364) 6320 (1580) 5455 (1364) 3273 (818) Diagram of the global bending moment M (kNm) In parentheses: values of N ch = M / h Figure 3.4 Worked example – Approximate calculation The values of the axial forces in the chords obtained by the simplified approach, M global / h , are shown in Figure 3.4. The values are very close to the values obtained using structural analysis software (see Figure 3.2), for the sections close to the applied loads. The small difference comes from the slope (3%) of the chords of the truss in the worked example, not taken into account in the hand calculation. The values of the axial forces in the diagonals obtained by the simplified approach, V global /cos θ , are also very close to the values obtained using software. 3.5 Secondary forces 3.5.1 Influence of chord rigidity Chord members in trusses which are used in construction are rarely pinned at the nodes and are more often rigidly connected; this means that members connected to the same node have to keep their respective angles. During deformation of the structure under load, the ends of the members all rotate at the same angle around the node. In these conditions, bending loads (bending moments and shear forces) called secondary forces are added to the axial loads in the members calculated assuming the nodes are pinned (primary forces).