Page 27 - Marine Structural Design

4 Part I Struchlral Design Principles

strengthened, and structural analysis re-conducted until the strength and fatigue requirements
are met. The use of FEM technology has been supported by the fast development of computer
and information technology. Information technology is widely used in structural analysis, data
collection, processing, and interpretation, as well as in the design, operation, and maintenance
of ship and offshore structures. The development of computer and information technology has
made it possible to conduct a complex structural analysis and process the analysis results. To
aid the FEM based design, various types of computer based tools have been developed, such
as CAD (Computer Aided Design) for scantling, CAE (Computer Aided Engineering) for
structural design and analysis and CAM (Computer Aided Manufacturing) for fabrication.

Structural design may also be conducted based on performance requirements such as design
for accidental loads, where managing risks is of importance.
1.1.2 Limit-State Design
In a limit-state design, the design of structures is checked for all groups of limit-states to
ensure that the safety margin between the maximum likely loads and the weakest possible
resistance of the structure is large enough and that fatigue damage is tolerable.
Based on the first principles, limit-state design criteria cover various failure modes such as:
Serviceability limit-state
Ultimate limit-state (including bucklingkollapse and fracture)
Fatigue limit-State
Accidental limit-state (progressive collapse limit-state)
Each failure mode may be controlled by a set of design criteria. Limit-state design criteria are
developed based on ultimate strength and fatigue analysis as well as use of the risWreliabi1it.y
methods.
The design criteria have traditionally been expressed in the format of Working Stress Design
(WSD) (or Allowable Stress Design, ASD), where only one safety factor is used to define the
allowable limit. However, in recent years, there is an increased use of the Load and Resistance
Factored Design (LRFD), that comprises of a number of load factors and resistance factors
reflecting the uncertainties and safety requirements.
A general safety format for LRFD design may be expressed as:
sd <% (1.1)
where,
Sd = D~k.yf, Design load effect
& = m&m, Design resistance (capacity)
sk = Characteristic load effect
Rk = Characteristic resistance
yf = Load factor, reflecting the uncertainty in load
ym = material factor = the inverse of the resistance factor
Figure 1.1 illustrates use of the load and resistance factors where only one load factor and one
material factor are used in the illustration for the sake of simplicity. To account for the