Page 460 - Marine Structural Design

43 6 Part IYStructural Reliability

Weibull Distribution
The Weibull distribution is defined by

PW= (x - Uy-' it exp[ - (~11 for x 2 u (24.16)
aA

forx2u (24.17)

where
U = location parameter
a = scale parameter
it = shape parameter
The mean value and standard deviation for the Weibull distribution are defined by

(24.18)

(24.19)

24.3 Uncertainty Analysis
24.3.1 Uncertainty Classification

For the purpose of reliability analysis, the uncertainties may be classified as
Inherent uncertainty
0
Measurement uncertainty
Statistical uncertainty
Model uncertainty
0
Inherent Uncertainty
Inherent uncertainty is also known as fundamental or physical uncertainty. It is a natural
randomness of a quantity, such as the variability in wave and wind loading. The uncertainty
source cannot be reduced by more information. These uncertainties are the results of natural
variability inherent in physical wave and wind processes, or man-made processes. For instance,
the variability of wave height for a sea state with a given significant wave height and period, is
fundamentally random. Also, the occurrence of sea states, characterized by Hs and Ts is
fundamentally random. Examples of man-made phenomena that are fundamentally random are
functional loads on structures and structural resistance. Contrary to natural processes, man-
made processes can be influenced by human intervention, by QNQC of human activities and
the fabricated structure itself.