Purlins and Siderails - Structural Forms.

Due to the increasing awareness of the load-carrying capabilities of sections formed from thin-gauge material, proprietary systems for both purlins and siderails have been developed by several manufacturers in the United Kingdom. Consequently, unless a purlin or siderail is to be used in a long-span or high-load application (when a hot-rolled angle or channel may be used), a cold-formed section is the most frequently used cladding support member for single-storey structures.

Cold-formed sections manufactured from thin-gauge material are  particularly prone to twisting and buckling due to several factors which are directly related to the section’s shape. The torsional constant of all thin-gauge sections is low (it is a function of the cube of the thickness); in the case of lipped channels the shear centre is eccentric to the point of application of load, thus inducing a twist on the section; in the case of Zeds the principal axes are inclined to the plane of the web, thus inducing bi-axial bending effects. These effects affect the load-carrying capacity of the section.

When used in service the support system is subject to downward loading due to dead and live loads such as cladding weight, snow, services, etc., and uplift if thedesign wind pressure is greater than the dead load of the system. Therefore, for  a typical double-span system as shown in Fig. 1.29, the compression flange is restrained against rotation by the cladding for downward loading, but it is not so restrained in the case of load reversal.

Typical double-span purlin with cladding restraint
Fig. 1.29 Typical double-span purlin with cladding restraint

In supporting the external fabric of the building, the purlins and siderails gain some degree of restraint against twisting and rotation from the type of cladding used and the method of its fastening to the supporting member. In addition, the connection of the support member to the main frame also has a significant effect on the load-carrying capacity of the section. Economical design, therefore, must take account of the above effects.

There are four possible approaches to the design of a purlin system:

(1) Design by calculation based on an elastic analysis as detailed in the relevant code of practice BS 5950: Part 5.

This approach neglects any beneficial effect of cladding restraint for the wind uplift case.

(2) Empirical design based on approximate procedures for Zeds as given in the codes of practice. This approach leads to somewhat uneconomic design.

(3) Design by calculation based on a rational analysis which accounts for the  stabilizing influence of the cladding, plasticity in the purlin as the ultimate load is approached, and the behaviour of the cleat at the internal support. The effects, however, are difficult to quantity.

(4) Design on the basis of full-scale testing. Manufacturers differ in the methods: the example used here is from one manufacturer who has published the method used.

For volume production, design by testing is the approach which is used.Although this approach is expensive,maximum economy of material can be achieved and the cost of the testing can be spread over several years of production.

Design by testing involves the ‘fine-tuning’ of theoretical expressions for the collapse load of the system.The method is based on the mechanism shown in Fig. 1.30 for a two-span system.

Collapse mechanism for a two-span purlin system
Fig. 1.30 Collapse mechanism for a two-span purlin system

From the above mechanism it can be shown that:

The performance of, for example, a two-span system is considerably enhanced if some redistribution of bending moment from the internal support is taken into account. The moment–rotation characteristic at the support is very much dependent on the cleat detail and the section shape. The characteristics of the central support can be found by testing a simply-supported beam subject to a central  point load, so as to simulate the behaviour of the central support of a double-span system.

From this test, the load–deflection characteristics can be plotted well beyond the deflection at which first yield occurs.A lower bound empirical expression can then be found for the support moment, M1, based on an upper limit rotational capacity. A similar expression can be found for M2, the internal span moment, again on the basis of a test on a simply-supported beam subject to a uniformly distributed load, applied by the use of a vacuum rig, or perhaps sandbags.

The design expressions can then be confirmed by the execution of numerous full-scale tests on double-span systems employing pairs of purlins supporting proprietary cladding.

As described earlier, load reversal under wind loading invariably occurs, thereby inducing compressive forces to the flange in the internal span, which is not restrained by the cladding as is the case in the downward load case. Anti-sag rods or the like are placed within the internal span, thereby reducing the overall buckling length of the member. The system is again tested in load reversal conditions and, as before, the design expressions can be further refined.

In some instances, sheeting other than conventional trapezoidal cladding (which is invariably through fixed to the purlin by self-drilling fastenings in alternate troughs) will not afford the full restraint to the compression flange: examples are standing seam roofs, brittle cladding, etc.The amount of restraint afforded by these latter types of cladding cannot easily be quantified. For the reasons outlined above, further full-scale testing and similar procedures of verification of design expressions are carried out.

The results of the full-scale tests are then condensed into easy to use load–span tables which are given in the purlin manufacturers’ design and detail literature.The use of these tables is outlined in Table 1.1.

Typical load table
Table 1.1 Typical load table

The tabular format is typical of that contained in all purlin manufacturers’ technical literature.The table is generally prefaced by explanatory notes regarding fixing condition and lateral restraint requirements, the latter being particularly relevant to the load-reversal case. Conditions which arise in practice, and which are not covered in the technical literature, are best dealt with by the manufacturers’ technical services department, which should be consulted for all non-standard cases.

Siderail design is essentially identical to that for purlins: load capacities are again arrived at after test procedures.

Self-weight deflections of siderails due to bending about the weak axis of the section are overcome by a tensioned wire system incorporating tube struts, typically at mid-span for spans of 6–7m, and third-points of the span for spans of 7–8m and above (Fig. 1.31).

Anti-sag systems for side wall rails. tube struts, wire ropes

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