Aluminium Design and Construction John Dwight

design does not, because stress at working load is not necessarily an
indication of how near a component is to actual failure.
5.1.4 Serviceability limit state
The reason for considering this limit state is to ensure that the structure
has adequate stiffness, the requisite calculations being usually performed
with the structure subjected to unfactored nominal loading. It is usually
concerned with the performance of members rather than joints.
When a member is first taken up to its nominal working load, its
deformation comprises two components: an irrecoverable plastic deflection
and a recoverable elastic one (Figure 5.2). The main causes for the
plastic deflection are the presence of softened zones next to welds (Chapter
6) and the rounded stress-strain curve. Further factors are local stress
concentrations and locked-in stresses, which also lead to premature
yielding (as in steel).
The serviceability check for a member simply consists of ensuring
that its elastic deflection does not exceed an acceptable value:
where � E =predicted elastic deflection under nominal loading, and �
� E � L (5.5)
L =limiting or permitted deflection.
A specific design calculation for the plastic deflection (under the initial
loading) is never made. This is because it is usually small, and disappears
on subsequent applications of the load. However, with materials having
a very rounded stress-strain curve, the initial plastic deformation tends
to be more pronounced, and there is a danger that it may be unacceptable.
We cover this possibility in design by arbitrarily decreasing the limiting
Figure 5.2 Elastic (� E ) and plastic (� P ) components of deflection at nominal working load.
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.