Aluminium Design and Construction John Dwight

CHAPTER 5
Limit state design and
limiting stresses
British Standard BS.8118 follows steel practice in employing the limit
state approach to structural design, in place of the former elastic (‘allowable
stress’) method [14]. Limit state design is now accepted practice in
most countries, the notable exception being the USA. In this chapter,
we start by explaining the BS.8118 use of the limit state method, and
then go on to show how the required limiting stresses are obtained.
5.1 LIMIT STATE DESIGN
5.1.1 General description
In checking whether a component (i.e. a member or a joint) is structurally
acceptable there are three possible limit states to consider:
Limit state of static strength;
Serviceability limit state;
Limit state of fatigue.
Static strength is usually the governing requirement and must always
be checked. Serviceability (elastic deflection) tends to be important in
beam designs; the low modulus (E) of aluminium causes it to be more
of a factor than in steel. Fatigue, which must be considered for all cases
of repeated loading, is also more critical than for steel.
In the USA, when limit state design is mentioned, it is given the
(logical) title ‘Load and Resistance Factor Design’ (LRFD).
5.1.2 Definitions
Some confusion exists because different codes employ different names
for the various quantities that arise in limit state design. Here we
consistently use the terminology adopted in BS.8118, as below.
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.