Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 15 3 GLOBAL ANALYSIS 3.1 General Section 1.1 describes the general behaviour of a truss. In reality, structures deviate from this theoretical behaviour and their global analysis involves consideration of the deviations. In particular, the deviations include the occurrence of bending in the members, in addition to the axial forces. These bending moments, known as “secondary moments”, can cause significant additional stresses in the members which make up the truss. The deviations in design take various forms:  All the members which make up the structure are not usually articulated at their original node and their end node. Truss chords, in particular, are usually fabricated in one length only, over several truss purlins: the continuous chord members are then connected rigidly to their original and end nodes. Rotation of the nodes, resulting from general deformation of the truss beam then causes bending moments in the rigidly connected members; the more rigid the chord members, the bigger the moments (see Section 3.4).  The members are not always strictly aligned on their original and end nodes. Bending moments which result from a misalignment of axes increase in proportion to the size of the eccentricity and the stiffness of the members. This phenomenon is illustrated in Section 3.6.  Loads are not always strictly applied to the nodes and, if care is not taken to introduce secondary members to triangulate the point of application of the loads between nodes, this results in bending moments. 3.2 Modelling Several questions arise in respect of the modelling of a truss. It is always convenient to work on restricted models. For example, for a standard building, it is common and usually justified to work with 2D models (portal, wind girder, vertical bracing) rather than a unique and global 3D model. A truss can even be modelled without its supporting columns when it is articulated to the columns. Nonetheless, it is important to note that:  If separate models are used, it may be necessary, in order to verify the resistance of certain elements, to combine the results of several analyses; example: the upper chord of a truss also serves as chord of the wind girder.  If a global 3D model is used, “parasitic” bending can be observed, which often only creates an illusory precision of the structural behaviour process. That is why 2D models are generally preferable. In the worked example, where the truss is simply supported on the columns, the design model chosen is that of the truss only.