Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 7 Figure 1.7 N-truss (also with N-truss purlins) 1.4 Aspects of truss design for roof structure 1.4.1 Truss or I -beam For the same steel weight, it is possible to get better performance in terms of resistance and stiffness with a truss than an I-beam. This difference is more sensitive for long spans and/or heavy loads. The full use of this advantage is achievable if the height of the truss is not limited by criteria other than the structural efficiency (a limit on total height of the building, for example). However, fabrication of a truss is generally more time consuming than for an I-beam, even considering that the modernisation of fabrication equipment allows the optimisation of fabrication times. The balance between minimum weight and minimum cost depends on many conditions: the equipment of the workshop, the local cost of manufacturing; the steel unit cost, etc. Trusses generally give an economic solution for spans over 20 or 25 m. An advantage of the truss design for roofs is that ducts and pipes that are required for operation of the buildings services can be installed through the truss web. 1.4.2 General geometry In order to get a good structural performance, the ratio of span to truss depth should be chosen in the range 10 to 15. The architectural design of the building determines its external geometry and governs the slope(s) given to the top chord of the truss.