Use of thin-gauge galvanized C-purlin sections in long-span roof truss construction

Abstract

Cold-formed thin-gauge galvanized steel lipped C-sections are widely used as secondary structural elements such as purlins and side rails in building construction because of its low weight and ease of handling and erection. However, these sections are frequently misconceived as having low member stiffness and poor clear spanning ability, and hence, perceived as limited in terms of alternative applications and uses. This paper examines alternative applications of such sections and an innovative example of how such sections can be used to form the long-span primary roof trusses for a church building is described in details. The structural roof layout consists of 35 numbers of main primary trusses spaced at 1.2m centres with a long clear span of 25m. One of the issues that are often raised with regards to this form of thin-gauge construction is the use of self-drilling screws which are often proprietary and system-specific in nature.To support such construction, it is often necessary to carry out some single and double lap shear joint tests to assess the shear capacity of the proprietary selfdrilling screws and to verify its design values against code calculations. In this connection, this paper reports on the direct shear tests which are conducted on a particular type of self-drilling screws. In addition, the paper also describes a full-scale proof load test which is carried out on another long-span roof truss as part of the developmental work to convince the Building Authority on the acceptance of such construction. The test objectives, methods, observations and analysis of the load test are discussed in details and the results show convincingly that such ultra-light cold-formed thin-gauge steel roof trusses can be used without any problem.