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Comparison standard DNV vs ABS.

The user of the plug-in software should be aware that there are found differences in calculation results between the code according to DNV or ABS. This can result in "red spots" in one code, while the other code gives acceptable unity checks. This can have different causes as pointed out below.

But as also mentioned in the ABS buckling standard:

"ABS does not seek to inhibit the use of an alternative technological approach that is demonstrated to produce an acceptable level of safety."

To our experience this is a general accepted approach by certification authorities.  As long as the engineer can show that the structure is verified by generally recognized methods, which are based on solid proven literature, tests and practical tolerances, it can be made acceptable.

Nevertheless, we want to emphasize some differences between the code DNV-RP-C201 and ABS buckling guide, which can lead to differences in outcome. Differences due to other approaches or due to (in our opinion) shortcomings in the code:

Some differences due to different approaches are:

  • The DNV code includes a check on built-in tolerances, which is defined in the code by a minimum lateral pressure. This pressure implies an initial deformation in the panel, which always includes a moment in the stiffener / girder. ABS doesn't include such a "minimum" deformation;
  • The ABS code contains some checks on dimension relations. This limits the code, which in some cases prevent calculation of answers. In many cases this indicates that the DNV code is at the edge of its limitations, e.g. increasing the stresses by a few MPa results in UC>1 or an error message as well. However, in our opinion, proportion ratios in ABS section 3/9.7 and 3/9.9 are not very practical for dimensioning girders, since it leads to a heavy design.
  • At low stresses or at low slenderness ratios (β<~2), ABS shows higher allowables due to the fact that ABS only uses the effective width in the calculations when the panel buckles elastically. DNV always uses the effective width, which shows in that case lower allowables.
  • DNV allows for loadline correction, which increases the allowable. (Optimization of the z* value in DNV code is, in the software of FEMDS, limited to the distance from the neutral axis to center line skin plate or from neutral axis to center line stiffener / girder flange. It is felt unpractical to assume that the working load line can be outside this range.)
  • Both standards allow shear allowables beyond elastic buckling. DNV only allows this post buckling behavior when the y-stress is in tension.  If this stress is in compression, DNV doesn't allow shear stresses beyond elastic buckling. This can lead to abrupt differences in shear allowable dependent on whether the y-stress is in tension or compression. ABS always allows the calculation of an ultimate shear allowable which includes post buckling behavior.  This can result in big differences between both standards with slender designs (with increased ratio; width over length).  This is also made visible in the figure below.

l= length of the panel
s= width between the stiffeners
t= thickness of the panel
β= slenderness ratio
τ= allowable edge shear stress

With:

comparison parameters

Post buckling leads to diagonal tension and thus to extra compression loads in stiffeners and or girders. In DNV this seems to be covered but it is unclear how this is covered by ABS when checking the stiffeners and girders. As of today, discussions with ABS did not lead to a satisfactory answer in this.

The DNV code contains more checks on the structure than ABS. This also leads to differences in outcome:

  • ABS code only considers compression stresses and gives almost no directions when tensile loads are involved.
  • The ABS code doesn't include a check on the shear connection in the stiffener due to lateral pressure. ABS checks the skin only but doesn't include to check the shear strength of the stiffener or girder.  Probably this is covered in the code by the 'stiffness and proportions' check in section 3/9, but it is unclear how. DNV also incorporated a decrease in allowable if the shear load is more than 50% of its capacity.
  • ABS omits to check torsional buckling when the stiffener (or girder) is loaded in bending due to pressure loading.
  • The ABS code only incudes the lateral pressure on the plate side only. (This is conservative when the pressure load is applied at the stiffener side.)

Considering all the above and to cover the omissions in ABS code, we prefer the DNV code above the code of ABS and recommend to check the structure with DNV code before publishing the results according to the ABS.