Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 28 4 VERIFICATION OF MEMBERS As seen in the preceding section, which dealt with the global analysis, the members are mainly subjected to axial forces. It was also observed that, in many cases, members are also subject to stress by bending moments, i.e. secondary moments. 4.1 Verification of members under compression The resistance of a member to compression is evaluated by taking into account the different modes of instability:  Local buckling of the section is controlled using section classification, and when necessary, effective section properties (class 4)  Buckling of the member is controlled by applying a reduction coefficient in the calculation of resistance. For a compression member, several buckling modes must be considered. In most truss members, only flexural buckling of the compressed members in the plane of the truss structure and out of the plane of the truss structure need be evaluated. For each buckling mode, the buckling resistance is obtained from EN 1993-1-1 [3] by applying a reduction to the resistance of the cross-section. This reduction factor is obtained from the slenderness of the member, which depends on the elastic critical force. For the diagonals and the verticals stressed in uniform compression. the elastic critical force is determined from the buckling length of the member in accordance with EN 1993-1-1, 6.3.1.3. The following can be observed, according to Annex BB §BB.1 of EN 1993-1-1:  For buckling in the plane of the truss beam: the buckling length is taken equal to 90% of the system length (distance between nodes), when the truss member is connected at each end with at least two bolts, or by welding (EN 1993-1-1 §BB.1.1(4)). (An exception is made by Annex BB for angle truss members, for which a different evaluation is given; it is not specified in this annex if the particular rule also concerns members made up to two pairs of angles: by way of simplification, it is recommended that a buckling length of 0,9 times the length of the axis be retained.)  For buckling out of plane of the truss beam, the buckling length is taken equal to the system length. For buckling in the plane of the truss of the chord members in uniform compression, the buckling length may be taken as 90% of its system length (distance between nodes).