Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 1 1 INTRODUCTION 1.1 Definition A truss is essentially a triangulated system of (usually) straight interconnected structural elements; it is sometimes referred to as an open web girder. The individual elements are connected at nodes; the connections are often assumed to be nominally pinned. The external forces applied to the system and the reactions at the supports are generally applied at the nodes. When all the members and applied forces are in a same plane, the system is a plane or 2D truss. F 1 2 1 2 1 Compression axial force 2 Tension axial force Figure 1.1 Members under axial forces in a simple truss The principal force in each element is axial tension or compression. When the connections at the nodes are stiff, secondary bending is introduced; this effect is discussed below. 1.2 Use of trusses in single-storey buildings In a typical single-storey industrial building, trusses are very widely used to serve two main functions:  To carry the roof load: - Gravity loads (self-weight, roofing and equipment, either on the roof or hung to the structure, snow loads) - Actions due to the wind (including uplift due to negative pressure).  To provide horizontal stability: - Wind girders at roof level, or at intermediate levels if required - Vertical bracing in the side walls and/or in the gables. Two types of general arrangement of the structure of a typical single-storey building are shown in Figure 1.2 and in Figure 1.3. In the first case (Figure 1.2), the lateral stability of the structure is provided by a series of portal trusses: the connections between the truss and the columns provide resistance to a global bending moment. Loads are applied to the portal structure by purlins and side rails.