4.1 Structural resistance may be established by
testing or by complete analysis taking account of both elastic and
plastic material properties. Safety margins for ultimate strength
shall be introduced by partial factors of safety taking account of
the contribution of stochastic nature of loads and resistance (dynamic
loads, pressure loads, gravity loads, material strength, and buckling
capacities).
4.2 Appropriate combinations of permanent loads,
functional loads and environmental loads including sloshing loads
shall be considered in the analysis. At least two load combinations
with partial load factors as given in table 2 shall be used for the
assessment of the ultimate limit states.
Table 2 Partial load
factors
|
Load combination
|
Permanent loads
|
Functional loads
|
Environmental loads
|
| 'a'
|
1.1
|
1.1
|
0.7
|
| 'b'
|
1.0
|
1.0
|
1.3
|
The load factors for permanent and functional loads in load
combination 'a' are relevant for the normally well-controlled
and/or specified loads applicable to cargo containment systems such
as vapour pressure, cargo weight, system self-weight, etc. Higher
load factors may be relevant for permanent and functional loads where
the inherent variability and/or uncertainties in the prediction models
are higher.
4.3 For sloshing loads, depending on the reliability
of the estimation method, a larger load factor may be required by
the Administration or recognized organization acting on its behalf.
4.4 In cases where structural failure of the cargo
containment system are considered to imply high potential for human
injury and significant release of cargo, the consequence class factor
shall be taken as γC
= 1.2. This value
may be reduced if it is justified through risk analysis and subject
to the approval by the Administration or recognized organization acting
on its behalf. The risk analysis shall take account of factors including,
but not limited to, provision of full or partial secondary barrier
to protect hull structure from the leakage and less hazards associated
with intended cargo. Conversely, higher values may be fixed by the
Administration or recognized organization acting on its behalf, for
example, for ships carrying more hazardous or higher pressure cargo.
The consequence class factor shall in any case not be less than 1.0.
4.5 The load factors and the resistance factors
used shall be such that the level of safety is equivalent to that
of the cargo containment systems as described in sections 4.21 to
4.26 of this Code. This may be carried out by calibrating the factors
against known successful designs.
4.6 The material factor γm
shall
in general reflect the statistical distribution of the mechanical
properties of the material, and needs to be interpreted in combination
with the specified characteristic mechanical properties. For the materials
defined in chapter 6 of this Code, the material factor γm
may
be taken as:
-
1.1 when the characteristic mechanical properties
specified by the recognized organization typically represents the
lower 2.5% quantile in the statistical distribution of the mechanical
properties; or
-
1.0 when the characteristic mechanical properties
specified by the recognized organization represents a sufficiently
small quantile such that the probability of lower mechanical properties
than specified is extremely low and can be neglected.
4.7 The partial resistance factors γsi
shall in general be established based on the uncertainties
in the capacity of the structure considering construction tolerances,
quality of construction, the accuracy of the analysis method applied,
etc.
4.7.1 For design against excessive plastic deformation
using the limit state criteria given in paragraph 4.8 of this standard,
the partial resistance factors γsi
shall
be taken as follows:
Factors A, B, C and D are defined in section 4.22.3.1 of
this Code. Rm
and Re
are
defined in section 4.18.1.3 of this Code.
The partial resistance factors given above are the results
of calibration to conventional type B independent tanks.
4.8
Design against excessive plastic
deformation
4.8.1 Stress acceptance criteria given below refer
to elastic stress analyses.
4.8.2 Parts of cargo containment systems where
loads are primarily carried by membrane response in the structure
shall satisfy the following limit state criteria:
where:
|
σm
|
= |
equivalent primary general membrane stress |
|
σL
|
= |
equivalent primary local membrane stress |
|
σb
|
= |
equivalent primary bending stress |
|
σg
|
= |
equivalent secondary stress |
|
f
|
= |
|
|
F
|
= |
|
With regard to the stresses σm
, σL
, σb
and σg
, see also the definition of stress categories in section
4.28.3 of this Code.
-
Guidance Note:
The stress summation described above shall be carried
out by summing up each stress component (σx
, σy
, τxy
), and subsequently the equivalent stress shall be calculated
based on the resulting stress components as shown in the example below.
|
4.8.3 Parts of cargo containment systems where
loads are primarily carried by bending of girders, stiffeners and
plates, shall satisfy the following limit state criteria:
σms
+ σbp
+ σbs
+ σbt
+ σg
≤ 3.0F
where:
|
σms
|
= |
equivalent section membrane stress in primary structure |
|
σbp
|
= |
equivalent membrane stress in primary structure and stress in
secondary and tertiary structure caused by bending of primary structure |
|
σbs
|
= |
section bending stress in secondary structure and stress in
tertiary structure caused by bending of secondary structure |
|
σbt
|
= |
section bending stress in tertiary structure |
|
σg
|
= |
equivalent secondary stress |
|
f
|
= |
|
|
F
|
= |
|
The stresses σms
, σbp
, σbs
, and σbt
are
defined in 4.8.4. For a definition of σg
,
see section 4.28.3 of this Code.
-
Guidance Note:
The stress summation described above shall be carried
out by summing up each stress component (σx
, σy
, τxy
), and subsequently the equivalent stress shall be calculated
based on the resulting stress components.
|
Skin plates shall be designed in accordance with the requirements
of the Administration or recognized organization acting on its behalf.
When membrane stress is significant, the effect of the membrane stress
on the plate bending capacity shall be appropriately considered in
addition.
4.8.4 Section stress categories
Normal stress is the component of stress normal to the plane
of reference.
Equivalent section membrane stress is the component of the
normal stress that is uniformly distributed and equal to the average
value of the stress across the cross section of the structure under
consideration. If this is a simple shell section, the section membrane
stress is identical to the membrane stress defined in paragraph 4.8.2
of this standard.
Section bending stress is the component of the normal stress
that is linearly distributed over a structural section exposed to
bending action, as illustrated in figure 1.
4.9 The same factors γC
, γm
, γsi
shall be
used for design against buckling unless otherwise stated in the applied
recognized buckling standard. In any case the overall level of safety
shall not be less than given by these factors.