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3. Purlins and Rails
Either a cold-formed lipped channel or lipped Z-section could be used for the purlins and guis. The major disadvantage of a Z-section is that the principal axes are
inclined to the web. If subjected to unrestrained bending in the plane of the web, out-of-plane displacements occur; if these are restrained, out-of-plane forces are generated. The purlins are assumed to be continuous over two spans of 4.5 m each and spaced at a maximum of 1.65 m centres, 1.8 m centres and 1.9m centres for the 10, 11 and 12 m span frame respectively. This is in compliance with the generally notion that cold-formed sections should be spaced at 1.5-2 m centres. A combination of cladding performance, erectability and lateral restraint requirements for economically-designed main frames dictates that the purlin and rail be spaced in the stated range. The spacing of the purlins is influenced by the end and internal spans of the sheeting. Although the minimum spacing is proportional to the span of the frame, experimental tests actually incorporated larger spacing in some cases. A 100×75×20×2 Channel section (or 125× 75×20×2 Z-section) is found to be suitable both as a purlin and a rail. Sag bars are excluded from this study
because of the small purlin and rail-span suggested, and the constructional complications that will arise in the structures as a result of their inclusion.
The purlins are spliced after every two spans and staggered as shown in Fig. 3. The splices are staggered to avoid having possible points of weakness along one frame as a result of the splicing connection. At the conceptual stage it was decided to have a fully continuous purlin. This was done to reduce the bending moments and
the large deflections associated with simply supported beams, thereby allowing a lighter section to be used. For this to be successful, the splice should be capable of resisting the purlin bending moment applicable at the splice location, and also the bolts had to be able to transfer the shear developed between the splicing member
and the purlin. Figure 4 shows the splicing that was initially adopted for the purlins of a 12 m span frame to be fully continuous. Since the splicing flat bar is not
subject to lateral torsional instability, the full limiting bending stress was used in designing the connection. The bolts connecting the splice to the purlin are
sufficiently spaced in the horizontal direction to keep the bolt shear forces within acceptable limits and to avoid excessive rotation of the purlin due to bolt slip. The South African Steel Construction Handbook (2005) recommends that the spacing between the bolts on one side of the splicing member should not be less than 2.2 h, where h is the depth of the purlin under consideration. This is done to ensure that the shear force in the bolts is not excessive. K-bracings in the plane of the roof and walls were used to stabilise the frames. They were chosen over crossbracings
so that there is less obstruction in the bracing area. Another characteristic, which made this bracing system an obvious choice, is its ability to share any force
equally between the members. All bracings were connected at the bottom flange of the purlins.
4. Purlin-Frame Connection
Purlins and rails are normally connected to the frames by bolting them to angle cleats. The angle cleats are in turn either bolted or welded to the flanges of the rafters
and columns respectively. Connecting the purlin/rail to the flanges of the frame channel section weakens the tension flange through drilling or punching of bolt-holes. Under tension, flanges can easily fail by tearing. Thus a different approach to connecting the purlins and rails to the frame had to be developed. Channel-purlins and rails are connected to the main frame through 100×75×20×3
cold-formed lipped angle cleats as shown in Fig. 5(a). This is a better approach to restraining the main frame as the connection restrains both lateral and torsional