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Steel Designers' Manual - 6th Edition (2003)
32 Single-storey buildings
WI
Fig. 1.29 Typical double-span purlin with cladding restraint
collapse load/span
x x
L : L
Fig. 1.30 Collapse mechanism for a two-span purlin system
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. 6 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. 7
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.
From the above mechanism it can be shown that: