Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 9 connections. In order to reduce these consequences (typically, the increase of the deflections), solutions are available such as use of pre-stressed bolts, or limiting the hole size. 1.4.5 Lateral stability It is necessary to design the chords in compression against the out-of-plane buckling. For simply supported trusses, the upper chord is in compression for gravity loading, and the bottom chord is in compression for uplift loading. For portal trusses, each chord is partly in compression and partly in tension. Lateral restraint of the upper chord is generally given by the purlins and the transverse roof wind girder. For the restraint of the bottom chord, additional bracing may be necessary, as shown in Figure 1.8. Such bracing allows the buckling length of the bottom chord to be limited out of the plane of the truss to the distance between points laterally restrained: they serve to transfer the restraint forces to the level of the top chord, the level at which the general roof bracing is provided. This type of bracing is also used when a horizontal load is applied to the bottom chord (for example, forces due to braking from a suspended conveyor). A A A A A A Truss AA Cross bracing between trusses Thick black dots: two consecutive trusses Blue The purlin which completes the bracing in the upper region GreenThe longitudinal element which closes the bracing in the lower region Red Vertical roof bracing Figure 1.8 Lateral bracing The roof purlins often serve as part of the bracing at the top chord. Introduction of longitudinal members at the lower chord allows the trusses to be stabilised by the same vertical bracing. It is possible to create a horizontal wind girder at the level of the bottom chords, with longitudinal elements to stabilize all the trusses. 1.5 Design of wind girders 1.5.1 Transverse wind girder In general, the form of a transverse wind girder is as follows (see Figure 1.2):  The wind girder is arranged as an X truss, parallel to the roof plane.