Environmental Engineering Reference
In-Depth Information
independently of each other for the same specified return period and then combined
with each other. It should be assumed that there is a correlation between average wind
speed and significant wave height, but not the short-term extreme values. The extreme
wave height and the extreme gust are not considered to act simultaneously. Instead,
they are assumed to act with a random distribution [11] (see also Section 4.6.3).
The various design load cases for offshore wind turbine structures are dealt with in
Section 4.6.4.
4.6.3
Fundamental considerations regarding the safety concept
4.6.3.1 Safety analysis
a) A probabilistic safety analysis with the help of the first-/second-order reliability
method (FORM/SORM) can be carried out on the given loadbearing structure when
the problem is a non-linear one or is formulated in general terms. More advanced
methods, for example simulations, are generally not worthwhile.
The limit state function is defined based on the failure model to be used. Using
derivatives with respect to the standard deviations
s
X
and the mean values m
X
of
to be determined as a measure of
the probability of failure P
f
. This is then compared with the normative target value
according to [44,45] irrespective of the reference period.
b) If linearisation is possible, or the problem itself is a linear one, then the safety
analysis can be carried out semi-probabilistically according to [44,45,46] with the
help of partial safety factor
g
F
or
g
M
and combination factor
c
.
The design values of the basic variables (X) derived from the linear limit state equation
are used to determine the specified safety elements [44,45,58,25]. These depend on the
parameters for the distribution functions (m
X
and
the basic variables (X) enables the reliability index
b
s
X
) and the weighting factors (
a
X
).
Fixed weighting factors (
a
E
¼
0.7 and
a
R
¼
0.8) are generally used here in order to
separate actions (E) and resistances (R).
If actions with high standard deviations dominate (as is the case for sea state and wind),
then
a
E
¼
1.0 and
a
R
¼
0.4 should be used as fixed weighting factors.
Partial safety factors
g
Q
for variable actions (Q) depending on the coefficient of
variation (V
Q
¼s
Q
/m
Q
) are given in Figure 4.13.
Fig. 4.13 Partial safety factors for variable actions [25]
