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Steel Designers' Manual - 6th Edition (2003)
the collapse load, Wc = f(
M1, M2, x, L)
x = f( M1, M2, L)
q1 = f( Wc, M1, L)
Design of common structural forms 33
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, M 1, based on an upper limit rotational capacity.
A similar expression can be found for M 2, 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 fullscale
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.
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