5. RELIABILITY CONCEPT
In principle, the errors and uncertainties involved
in the estimation of the loads action and the capacity
behaviour of a structure may be allowed for in
strength design by using load factors to increase the
nominal loads and using capacity factors to decrease
the structural strength. The purpose of using any
factors is to ensure that the probability of failure
under the most adverse conditions of structural
overload remains very small, which may be implicit
or explicit in the rules written in a code. In outdated
structural design codes that employed the traditional
working stress design (e.g. AS 1250-1981 Steel
Structures), and in the current AS1085.14 sleeper
code, safe design was achieved by using factors of
safety to reduce the failure stress to permissible
working stress values, but ultimately the purpose