Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 29 For buckling out of plane of the truss, it can be more difficult to determine the elastic critical force for the following reasons:  There is not necessarily a lateral support at each node of the truss  The lateral support points are not necessarily effectively rigid. When there is no lateral support at each node along the chord, the segment located between support points is subject to variable compression between bays. In these circumstances:  A conservative approach would be to use the normal compression force at its maximum value and to take the buckling length as the distance between supports but this can lead to an under-estimate of the chord resistance.  Refined methods can be adopted by investigating an equivalent buckling length under constant compression. In the worked example, where the truss supports a roof, with purlins at the level of the upper chord of the truss:  All the purlins connected to a roof bracing can be considered as lateral rigid support points.  Intermediate purlins can also be considered as a rigid point of support. Insofar as a diaphragm role has been attributed to the roof (class 2 construction according to EN 1993-1-3).  With regard to the lower chord, these lateral support points are provided by additional vertical bracing elements between trusses (see the braces under the truss purlins in Figure 2.2). Another point to note, which is very common, concerning determination of the compression resistance, is the case of pairs of members. It is quite common, as was stated, to make up members from a truss structure using two angles, or two channels (UPE). In order to ensure that such built-up members will behave as sole members in the flexural buckling mode, the two components are connected by small battens (Figure 4.1). Since the role of these members is to prevent relative slip of one component compared with the other, they must be connected without slack. The gap between the angles, and the thickness of the battens, should be the same as the thickness of the gusset to which the built-up member is connected.