Aluminium Design and Construction John Dwight

stress for such materials, when checking the ultimate limit state (Section
5.3.1).
The type of member for which the serviceability limit state is most
likely to be critical is a beam, especially if simply supported, for which
� E can be calculated employing conventional deflection formulae (Section
8.8). It is rarely necessary to check the stiffness of truss type structures.
The designer must decide on a suitable value for � L , preferably in
consultation with the client. The important thing is not to insist on an
unduly small deflection, when a larger one can be reasonably tolerated.
This is especially important in aluminium with its relatively low modulus.
A general idea of the deflection that can be tolerated is given by the
value suggested for purlins in BS.8118, namely � L =span/100 (under
dead+snow+wind).
For a component that has to carry a combination of loads, the strict
application of equation (5.5) may be thought too severe. A more lenient
approach is to base � E on a reduced loading, in which the less severe
imposed loads are factored by � f2 as given in Table 5.1.
Turning to joints, it is never necessary to check the deformation of
welded ones, and even for mechanical joints an actual calculation is
seldom required. With the latter, if stiffness is important, a simple solution
is to specify close-fitting bolts or rivets, rather than clearance bolts.
Alternatively, for maximum joint stiffness, a designer can call for
frictiongrip (HSFG) bolts, in which case a check must be made to ensure
that gross slip does not occur before the nominal working load is reached.
In so doing the calculated friction capacity is divided by a serviceability
factor (� s ), as explained in Section 11.2.7.
5.1.5 Limit state of fatigue
For a structure or component subjected to repeated loading, thousands
or millions of times, it is possible for premature collapse to occur at a
low load due to fatigue. This can be a dangerous form of failure without
prior warning, unless the growth of cracks has been monitored during
service.
Fatigue is covered in Chapter 12. The usual checking procedure is
to identify potential fatigue sites and determine the number of loading
cycles to cause failure at any of these, the design being acceptable if
the predicted life at each site is not less than that required. The number
of cycles to failure is normally obtained from an endurance curve,
selected according to the local geometry and entered at a stress level
(actually stress range) based on the nominal unfactored loading.
Alternatively, for a mass-produced component, the fatigue life can
be found by testing.
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