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DNV-OS-C201 Structural Design of Offshore Units (WSD method)
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SECTION 5
Structural StrengthSec.5
A. General
Sec.5
A 100 General
Sec.5 A
101 This section gives provisions for checking of ultimate strength
for typical structural elements used in offshore steel structures.
Sec.5 A
102 The ultimate strength capacity (yield and buckling) of structural
elements shall be assessed using a rational, justifiable, engineering
approach.
Sec.5 A
103 Structural capacity checks of all structural components shall
be performed. The capacity checks shall consider both excessive
yielding and buckling.
Sec.5 A
104 Simplified assumptions regarding stress distributions may
be used provided the assumptions are made in accordance with generally
accepted practice, or in accordance with sufficiently comprehensive
experience or tests.
Sec.5 A
105 Gross scantlings may be utilised in the calculation of hull
structural strength, provided a corrosion protection system in accordance
with Sec.10 is installed and maintained.
Sec.5 A
106 In case corrosion protection in accordance with Sec.10 is
not installed (and maintained) corrosion additions as given in Sec.10 B407 shall be used. The
corrosion addition shall not be accounted for in the determination
of stresses and resistance for local capacity checks.Sec.5
A 200 Structural analysis
Sec.5 A
201 The structural analysis may be carried out as linear elastic,
simplified rigid-plastic, or elastic-plastic analyses. Both first
order or second order analyses may be applied. In all cases, the
structural detailing with respect to strength and ductility requirement
shall conform to the assumption made for the analysis.
Sec.5 A
202 When plastic or elastic-plastic analyses are used for structures
exposed to cyclic loading e.g. wave loads, checks shall be carried
out to verify that the structure will shake down without excessive
plastic deformations or fracture due to repeated yielding. A characteristic
or design cyclic load history needs to be defined in such a way
that the structural reliability in case of cyclic loading e.g. storm
loading, is not less than the structural reliability for ultimate
strength for non-cyclic loads.
Sec.5 A
203 In case of linear analysis combined with the resistance formulations
set down in this standard, shakedown can be assumed without further
checks.
Sec.5 A
204 If plastic or elastic-plastic structural analyses are used
for determining the sectional stress resultants, limitations to
the width to thickness ratios apply. Relevant width to thickness
ratios are found in the relevant codes used for capacity checks.
Sec.5 A
205 When plastic analysis and/or plastic capacity checks
are used e.g. cross section Type I and II, according to Appendix
A, the members shall be capable of forming plastic hinges with sufficient
rotation capacity to enable the required redistribution of bending
moments to develop. It shall also be checked that the load pattern will
not be changed due to the deformations.
Sec.5 A
206 Cross sections of beams are divided into different types dependent
of their ability to develop plastic hinges. A method for determination
of cross sectional types is found in Appendix
A.Sec.5
A 300 Ductility
Sec.5 A
301 It is a fundamental requirement that all failure modes are
sufficiently ductile such that the structural behaviour will be
in accordance with the anticipated model used for determination
of the responses. In general all design procedures, regardless of
analysis method, will not capture the true structural behaviour.
Ductile failure modes will allow the structure to redistribute forces
in accordance with the presupposed static model. Brittle failure
modes shall therefore be avoided or shall be verified to have excess
resistance compared to ductile modes, and in this way protect the
structure from brittle failure.
Sec.5 A
302 The following sources for brittle structural behaviour may
need to be considered for a steel structure:
| — | unstable fracture caused by
a combination of the following factors: brittle material, low temperature
in the steel, a design resulting in high local stresses and the
possibilities for weld defects |
| — | structural details where ultimate resistance is reached
with plastic deformations only in limited areas, making the global
behaviour brittle |
| — | shell buckling |
| — | buckling where interaction between local and global
buckling modes occurs. |
Sec.5
A 400 Yield check
Sec.5 A
401 Structural members for which excessive yielding are possible
modes of failure shall be investigated for yielding.Individual stress components and the von Mises equivalent
stress for plated structures shall not exceed the permissible stress,
see Sec.2 E.
Guidance note:
- For plated structures the von
Mises equivalent stress is defined as follows:
where sx and sy are membrane stresses in x-
and y-direction respectively, t is shear stress in the
x-y plane, i.e. local bending stresses in plate thickness not included. - In case local plate bending stresses
are of importance for yield check, e.g. for lateral loaded plates,
yield check may be performed according to DNV-RP-C201 Part 1 Sec.
5.
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Sec.5 A
402 The coefficient b, defined in Sec.2
E as the ratio between the permissible and basic usage factors,
shall be equal to 1.0 for the yield checks.
Sec.5 A
403 Local peak stresses from linear elastic analysis in areas
with pronounced geometrical changes, may exceed the yield stress
provided the adjacent structural parts has capacity for the redistributed
stresses.Guidance note:
- Areas above yield determined
by a linear finite element method analysis may give an indication
of the actual area of plastification. Otherwise, a non-linear finite
element method analysis may need to be carried out in order to trace
the full extent of the plastic zone.
- The yield checks do not refer to local stress concentrations
in the structure or to local modelling deficiencies in the finite
element model.
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Sec.5 A
404 For large volume hull structures gross scantlings may be applied
for calculation of stresses in connection with the yield checks.
Sec.5 A
405 For yield check of welded connections, see Sec.9.Sec.5
A 500 Buckling check
Sec.5 A
501 Elements of cross sections not fulfilling requirements to
cross section type III shall be checked for local buckling.Cross sectional types are defined in Appendix
A.
Sec.5 A
502 Buckling analysis shall be based on the characteristic buckling
resistance for the most unfavourable buckling mode.
Sec.5 A
503 The characteristic buckling strength shall be based on the
5th percentile of test
results.
Sec.5 A
504 The coefficient b, defined in Sec.2
E as the ratio between the permissible and basic usage factors,
shall be equal to 1.0 for buckling check of flat plated structures
and stiffened panels, beams, columns and frames. The coefficient b to
be applied for buckling of shell structures are presented in C103.
Sec.5 A
505 Initial imperfections and residual stresses in structural
members shall be accounted for.
Sec.5 A
506 It shall be ensured that there is conformity between the initial
imperfections in the buckling resistance formulas and the tolerances
in the applied fabrication standard.
Guidance note:
If buckling resistance is calculated in accordance with DNV-RP-C201
for plated structures, DNV-RP-C202 for shells, or Classification
Note 30.1 for bars and frames, the tolerance requirements given
in DNV-OS-C401 should not be exceeded, unless specifically documented.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.5
B. Flat Plated Structures and Stiffened
Panels
Sec.5
B 100 Yield check
Sec.5 B
101 Yield check of plating and stiffeners may be performed as
given in Sec.6.
Sec.5 B
102 Yield check of girders may be performed as given in Sec.6.Sec.5
B 200 Buckling check
Sec.5 B
201 The buckling stability of plated structures may be checked
according to DNV-RP-C201.Sec.5
B 300 Capacity checks according to other
codes
Sec.5 B
301 Stiffeners and girders may be designed according to provisions
for beams in recognised standards such as AISC-ASD.Guidance note:
The principles and effects of cross section types are included
in the AISC-ASD.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.5
C. Shell Structures
Sec.5
C 100 General
Sec.5 C
101 The buckling stability of cylindrical and un-stiffened conical
shell structures may be checked according to DNV-RP-C202.
Sec.5 C
102 For interaction between shell buckling and column buckling,
DNV-RP-C202 may be used.
Sec.5 C
103 For shell bucking the coefficient b, defined in Sec.2 E as the ratio between the
permissible and basic usage factors, shall be taken as shown in
Table C1.Sec.5 C
| Table C1 The coefficient b for
shell buckling |
| Type of structure | l £ 0.5 | 0.5 < l < 1.0 | l ³ 1.0 | Girder, beams stiffeners on shells | 1.0 | 1.0 | 1.0 | | Shells of single curvature (cylindrical shells,
conical shells) | 1.0 | 1.2 - 0.4 l | 0.8 | | Spherical shells | 0.8 | 0.96 - 0.32 l | 0.64 |
Note
that the slenderness is based on the buckling mode under consideration | |
| l | = | reduced slenderness parameter |
| | |
|
| fy | = | specified minimum yield stress |
| fE | = | elastic buckling stress for the buckling mode
under consideration. |
Sec.5
D. Tubular Members, Tubular Joints
and Conical Transitions
Sec.5
D 100 General
Sec.5 D
101 Tubular members may be checked according to Classification
Note 30.1 or API RP 2A - WSD.For interaction between local shell buckling and column buckling,
and effect of external pressure, DNV-RP-C202 may be considered.
Sec.5 D
102 Cross sections of tubular member are divided into different
types dependent of their ability to develop plastic hinges and resist
local buckling. Effect of local buckling of slender cross sections
shall be considered.
Guidance note:
- Effect of local buckling of
tubular members without external pressure, i.e. subject to axial
force and/or bending moment) are given in Appendix A, cross
section type IV.
Section 3.8 of DNV-RP-C202 may be used, see C100. - Effect of local buckling of tubular members with external
pressure need not be considered for the following diameter (Dm) to thickness (t) ratio:
where| E | = | modulus of elasticity and |
| fy | = | minimum yield stress. |
In case of local shell buckling, see C100, section 3.8 of
DNV-RP-C202, or API RP 2A-WSD may be used.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.5 D
103 Tubular members with external pressure, tubular joints and
conical transitions may be checked according to API RP 2A-WSD.Sec.5
E. Non-Tubular Beams, Columns and
Frames
Sec.5
E 100 General
Sec.5 E
101 The design of members shall take into account the possible
limits on the resistance of the cross section due to local buckling.Guidance note:
Cross sections of member are divided into different types
dependent of their ability to develop plastic hinges and resist local
buckling, see Appendix A. In case of local buckling, i.e. for cross
sectional type IV, DNV-RP-C201 may be used.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.5 E
102 Buckling checks may be performed according to Classification
Note 30.1, or other recognised standards such as AISC-ASD.
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