Page 491 - Marine Structural Design

Chapter 26 Reliability-Based Design and Code Calibration

A more efficiently balanced design results in weight savings andor an improvement of
reliability.
Uncertainties in the design are treated more rigorously.
Because of an improved perspective of the overall design process, development of
probability-based design procedures can stimulate important advances in structural
engineering.
The codes become a living document. They can be easily revised periodically to include
new sources of information and to reflect additional statistical data on design factors.
The partial safety factor format used herein also provides a framework for extrapolating
existing design practice to new ships where experience is limited.
Experience has shown that adoption of a probability-based design code has resulted in
significant savings in weight. Designers have commented that, relative to the conventional
working stress code, the new AISC-LRFD requirements are saving anywhere from 5% to 30%
steel weight, with about 10% being typical. This may or may not be the case for ships and
other marine structures.
In reliability-based marine structural design, the effect of uncertainties in loads, strength and
condition assessment is accounted for directly. Safety measures are calculated, for assessing
designs or deciding on design targets.

26.3.2 Application of Reliability Methods to ASD Format
A design equation may be formulated using ASD format as
R, 2q-S, (26.7)
Alternatively, the safety factor could be referenced to the capacity of the entire structure
system Based on characterization of the demands and capacities as being log-normally
distributed, the usage factor, q, in ASD can be expressed as @ea, et al, 1997)

Bs
q = a-exp[(po - 2.330,)] (26.8)
BR
where,
q = usage (safety) factor
a = factor that incorporates the interactive dynamic effects - transient loading and
dynamic behavior of the system
Bs = median bias in the maximum demand (loading)
BR = median bias in the capacity of the element
/3 = annual safety index
0- = total uncertainty in the demands and capacities
os = uncertainty in the annual expected maximum loadings
The number 2.33 in Eq. (26.8) refers to 2.33 standard deviations from the mean value, or the
99'h percentile. This is equivalent to the reference of the design loading to an average annual
return period of 100 years. In case of installation conditions are defined on the basis of a 10-
year return period condition, a value of 1.28 could be used (goth percentile).