Page 29 - Marine Structural Design

6 Part I Siruciural Design PrincipIes

1.2.1 Ultimate strength Criteria
Ultimate strength criteria are usually advocated in design codes for various basic types of the
structural components such as:
columns & beam-columns
plates and stiffened panels
shells and stiffened shells
structural connections
hull girders
An illustration of the Euler buckling strength is given in Figure 1.2 for pinned columns under
compression. Due to combination of axial compression and initial deflection, the column may
buckle when the axial compression approaches its critical value,
Z~EI
PCR =-
I=
where 1 and EI are column length and sectional bending rigidity respectively. Due to buckling,
the lateral deflection Swill increase rapidly.
Initiation of yielding usually occurs in the most loaded portion of the structural members. As
the yielding portion spreads, the bending rigidity of the structural component decreases and
hence buckling is attained. For structural members other than un-stiffened thin-walled shells,
ultimate strength is reached when inelastic buckling occurs.
The design of components in ship and offshore structures is mainly based on relevant
classification rules and API and IS0 codes. The classification rules are applicable to ocean-
going ships, mobile offshore drilling units (MODU) and floating structures. For offshore
structural design, however, API and IS0 codes are more frequently applied.

6
---_
Pcr - , ------___ ____----- -j-pcr
~
Buckled Shape

Figure 1.2 Buckling of Pinned Columns
It should be pointed out that final hcture is also part of the ultimate strength analysis. The
assessment of final fracture has been mainly based on fiacture mechanics criteria in British
standard PD6493 (or BS7910) and American Petroleum Institute code AFT 579. In fact there is
a similarity between buckling strength analysis and fiacture strength analysis, as compared in
the table below: