Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 10  The chords of the wind girder are the upper chords of two adjacent vertical trusses. This means that the axial forces in these members due to loading on the vertical truss and those due to loads on the wind girder loading must be added together (for an appropriate combination of actions).  The posts of the wind girder are generally the roof purlins. This means that the purlins are subject to a compression, in addition to the bending due to the roof loading.  It is also possible, for large spans of the wind girder, to have separate posts (generally tubular section) that do not act as purlins.  The diagonal members are connected in the plane of the posts. If the posts are the purlins, the diagonal members are connected at the bottom level of the purlins. In a large X truss, diagonals are only considered in tension and it is possible to use single angles or cables. It is convenient to arrange a transverse wind girder at each end of the building, but it is then important to be careful about the effects of thermal expansion which can cause significant forces if longitudinal elements are attached between the two bracing systems, especially for buildings which are longer than about 60 m. In order to release the expansion of the longitudinal elements, the transverse wind girder can be placed in the centre of the building, but then it is necessary to ensure that wind loads are transmitted from the gables to the central wind-bracing. Transverse wind girders are sometimes placed in the second and penultimate spans of the roof because, if the roof purlins are used as the wind girder posts, these spans are less subject to bending by roof loads. The purlins which serve as wind girder posts and are subject to compression must sometimes be reinforced:  To reinforce IPE purlins: use welded angles or channels (UPE)  To reinforce cold formed purlins: increase of the thickness in the relevant span, or, if that is not sufficient, double the purlin sections (with fitting for the Zed, back to back for the Sigma). 1.5.2 Longitudinal wind girder It is necessary to provide a longitudinal wind girder (between braced gable ends) in buildings where the roof trusses are not “portalized”. The general arrangement is similar to that described for a transverse wind girder:  X truss  The chords are two lines of purlins in small buildings, or additional elements (usually tubular sections)  The posts are the upper chords of the consecutive stabilized roof trusses.