OS-C501
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Offshore Standard DNV-<strong>OS</strong>-<strong>C501</strong>, November 2013<br />
Sec.6 Failure mechanisms and design criteria – Page 119<br />
8.1.2 Determination of the elastic critical load of a structure or member that experiences bifurcation buckling<br />
corresponds to the solution of an 'eigenvalue' problem in which the elastic buckling load is an 'eigenvalue' and<br />
the corresponding mode of buckling deformation is described by the corresponding 'eigenvector'.<br />
8.1.3 Elastic buckling may occur at different levels:<br />
— global level for the structure; this involves deformation of the structure as a whole.<br />
— global level for a structural member; this is confined mainly to one structural member or element but<br />
involves the whole of that member or element.<br />
— local level for a structural member; only a part of a structural member or element is involved (e.g. local<br />
buckling of the flange of an I-beam or of a plate zone between stiffeners in a stiffened plate).<br />
8.1.4 Resistance of a structural member to elastic buckling is normally expressed as a critical value of load<br />
(applied force, or stress resultant induced in a member) or as a critical value of a nominal average stress (e.g.<br />
axial or shear force divided by area of cross-section). However, such resistance may also be expressed as a<br />
critical value of mean strain induced at a cross-section in a member.<br />
8.1.5 Initial geometrical imperfections (out-of-straightness, out-of-roundness, or eccentricity of applied<br />
loading) that lead to a situation where compressive forces in a structural part are not coincident with the neutral<br />
axis of that part may influence significantly the buckling behaviour. An idealised structure without such<br />
imperfections is referred to as “geometrically perfect”.<br />
8.1.6 Bifurcation buckling is essentially a feature of geometrically perfect structures. Geometrical<br />
imperfections generally destroy the bifurcation and lead to a situation where bending deformations begin to<br />
grow as the load is increased from zero. An elastic critical load may still be associated with the structure, and<br />
may provide a good indication of the load level at which the deformations become large. However, some<br />
structures with unstable post-buckling behaviour are highly sensitive to geometric imperfections. In the<br />
presence of imperfections, such structures may experience limit point buckling at loads that are significantly<br />
lower than the elastic critical load of the geometrically perfect structure.<br />
8.1.7 Elastic buckling deformation of a geometrically perfect or imperfect structure may trigger other failure<br />
mechanisms such as fibre failure (compressive or tensile) or matrix cracking.<br />
8.1.8 The presence of failure mechanisms such as matrix cracking or delamination may influence significantly<br />
the buckling behaviour of structures and structural members.<br />
8.2 General requirements<br />
8.2.1 Resistance of structures or structural members in the presence of buckling may be determined by means<br />
of testing or analysis.<br />
8.2.2 The effects of initial geometrical imperfections shall always be evaluated for structures or structural<br />
members being checked for buckling.<br />
8.2.3 Assumptions regarding geometrical imperfections shall wherever possible be based on<br />
— knowledge of production methods and corresponding production tolerances<br />
— knowledge of how imperfections of given shape and magnitude influence the structural behaviour, and<br />
— experience from previous measurements and tests.<br />
If an adequate knowledge base does not exist, a programme of measurement and/or testing shall be agreed to<br />
demonstrate that the design assumptions are justified.<br />
8.3 Requirements when buckling resistance is determined by testing<br />
8.3.1 If the resistance of a structure or structural member is determined by testing, the requirements in 302 to<br />
306 shall be satisfied. Testing shall be done as described in Sec.10 'Component Testing'.<br />
8.3.2 Sufficient tests shall be performed to provide statistical data so that both the mean resistance and the<br />
COV may be determined.<br />
8.3.3 The structures or members tested shall incorporate the least favourable geometrical imperfections that<br />
are possible within the specified production tolerances. Alternatively, the structures or structural members<br />
tested may incorporate a representative range of geometrical imperfections that may arise in the intended<br />
production process; however, in such a case it must be demonstrated that the imperfections considered are<br />
representative in terms of their distributions of shape and amplitude.<br />
8.3.4 The design criterion shall normally be applied at the level of overall load for a structure, or either force<br />
(stress resultant) or nominal stress or strain (averaged over the member cross-section) for a structural member.<br />
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