Page 514 - Marine Structural Design

490 Part IY Structural Reliability

Considering S,, A, and K are random variables, the following safety check expression may be
defmed,

(27.33)
Ys
where the subscript n refers to the nominal or design values. Reliability methods may be
applied to calibrate the partial safety factors: stress factor ys, damage safety factor yA and
material property safety factor yK . See Stahl and Banon (2002) for latest development on this
subject.

27.6 Numerical Examples
27.6.1 Example 27.1: Fatigue Reliability Based on Simple S-N Approach
Problem
Assuming that the fatigue strength is described by a S-N curve and the fatigue loads
are described by a Weibull distribution, then the fatigue damage can be obtained by
equation (27.7) given by

D=-.
K (In No ),"
If only A, SO and K are considered as random variables, the failure probability may be
written as

(27.34)

g@)= X, -k- X; (27.35)
X,
and k is a constant.
Assuming m=3, k=106 and XI, X2 and X, are independent and specified in Table 27.3.
Find the distribution of go and calculate failure probability directly using simple
approach.
Table 27.3 InDut Data

Solution
Before using FORM, it is shown that a simple approach can be applied to calculate Pf
in the case. The Eq. (27.5 1) can be rewritten as