Page 446 - Marine Structural Design

422 Part IV Structural Reliabilify

(23.25)

It is seen from the above tow equations that the evaluation of the failure probability of series
and parallel systems amounts to evaluation of the standard multi-noma1 integral. This is
however a difficult task for problems of large dimensions.
To demonstrate the reliability calculation of a simple parallel system, an example is given in
Section 23.11.

23.6 Combination of Statistical Loads

23.6.1 General
In general, loads can be grouped into the following three classes based on statistical
characteristics of their form and history
Time-invariant loads: e.g. dead loads
0
Random loads: e.g. wave loads
Transient random loads: e.g. earthquake loads
When two or more random loads acting on the structures, the combination of statistical loads
must be considered based on the statistical characteristics of individual loads.
For instance, the primary types of load combinations for ship structures are:
Hull girder loads
Hull girder loads and local pressure
Hull girder loads and transient loads
A simple load combination problem can be expressed as, e.g. for hull girder collapse
(d (23.26)
M, (4 = Ms (d f M w
where,
M,(t) = total bending moment acting ship hull girder;
M.,(t) = still water bending moment;
M,(t) = vertical wave bending moment;
In most of the current ship design Rules, the peak coincidence method for the combination of
still water bending moment and vertical wave bending moment is applied as follows
M,, (4 = M5.m (d+ Mw,, (4 (23.27)
This is based on the very conservative assumptions that the maximum values of the two
bending moments occur simultaneously.
However, the combination of statistical loads is fairly complex and a number of methods have
been derived to solve this problem. Here, only the application of Turkstra’s rule (Turkstra,
1972) and the Ferry Borges-Castanheta (1971) model are presented.