Page 28 - Marine Structural Design

Chapter I Introduction 5

uncertainties in strength parameters, the design resistance & is defined as characteristic
resistance Rk divided by the material factor ym. On the other hand, the characteristic load effect
Sk is scaled up by multiplying a load factor yf.
The values of the load factor yrand material factor ym are defined in design codes. They have
been calibrated against the working stress design criteria and the inherent safety levels in the
design codes. The calibration may be conducted using structural reliability methods that allow
us to correlate the reliability levels in the LRFD criteria with the WSD criteria and to assure
the reliability levels will be higher or equal to the target reliability. An advantage of the LRFD
approach is its simplicity (in comparison with direct use of the structural reliability methods)
while it accounts for the uncertainties in loads and structural capacities based on structural
reliability methods. The LRFD is also called partial safety factor design.
While the partial safety factors are calibrated using the structural reliability methods, the
failure consequence may also be accounted for through selection of the target reliability level.
When the failure consequence is higher, the safety factors should also be higher. Use of the
LRFD criteria may provide unified safety levels for the whole structures or a group of the
structures that are designed according to the same code.

Load Effect Caoacitv
Char. value Sk Char. value &
is factored up is factored down

Figure 1.1 Use of Load and Resistance Factores for Strength Design

1.2 Strength and Fatigue Analysis

Major factors that should be considered in marine structural design include:

Still-water and wave loads, and their possible combinations
Ultimate strength of structural components and systems

Fatigue/fracture in critical structural details.
Knowledge of hydrodynamics, bucklinglcollapse, and fatiguehacture is the key to
understanding structural engineering.