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How plate buckling standards of DNV and ABS differ…

…and why you should check structures using both.

logo DNV vs ABS
One of the most used structural standard for certification in the field of offshore structural engineering in The Netherlands is DNV-RP-C201. This is one of the results of a study organized by FEM Data Streamliner. There are major differences between standards, especially if you compare DNV and ABS. Edgar Peetam (FEM Data Streamliner) explains.

FEM Data Streamliner provides a tool that can be used in addition to software that checks structures using the Finite Element Method (FEM). FEM Data Streamliner also contains the strength requirements as laid down in standards provided by major certification authorities. The two most often used standards are DNV, which is the most popular standard in Europe, and ABS, the biggest global player. These standards differ and comparing structures to the standards of DNV and ABS will lead to different results.

Red spots

“The user of add-on software should be aware that there are found differences in calculation results between the standards according to DNV and ABS”, Edgar Peetam, FEM specialist and developer of FEM Data Streamliner, explains. “This can result in ‘red spots’ in one standard, while the other standard gives acceptable unity checks. These differences are the result of a different approach but also, in my opinion, to shortcomings in the standards.”

FE model with red spots

Peetam: “In general ABS is based on test results, while DNV is based on both test results and theory. The test configurations in ABS include dimensions normally used in ship configurations. In the offshore industry dimensions can differ considerably from the ones on which the ABS standard is based. This is one of the reasons the ABS standard is, in my opinion, more limited.”

Minimum lateral pressure

“For example the DNV standard includes a check on built-in tolerances which is defined in the standard by a minimum of lateral pressure”, Peetam states. “This pressure implies an initial deformation in the panel, which always includes a moment in the stiffener or girder. ABS doesn't include such a ‘minimum’ deformation.”

“A second difference is that the ABS standard contains some checks on dimension relations”, he continues. “This limits the standard, which sometimes prevent calculation of answers. In many cases this indicates that the DNV standard is at the edge of its limitations. For example increasing the stresses by a few megapascal can result in an error message.”

Effective width

Peetam: “Also, at low stresses or at low slenderness ratios 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 in that case will show lower allowables. The Unity Check should generally not be considered as a reserve factor on the allowables, but more as an indication of how much residual strength the structure contains.”

“A fourth difference is that DNV allows for load line correction, which increases the allowable”, he knows. “In the software of FEM Data Streamliner, we limited the optimization of the z-value in the DNV standard to the distance from the neutral axis to the center line skin plate, or from the neutral axis to the center line stiffener or girder flange. It is unpractical to assume that the working load line can be outside this range.”

Slender designs

Peetam: “Both standards allow shear allowables beyond elastic buckling. DNV only allows this post buckling behavior when the y-stress is in tension and not when this stress is in compression. This can lead to abrupt differences in shear allowable. 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.”

Besides differences, sometimes there is also lack of clarity. Peetam gives an example: “Post buckling leads to diagonal tension and thus to extra compression loads in stiffeners 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 on this point.”

More checks

In general, the DNV standard contains more checks on the structure than the standard of ABS. This also leads to differences in outcome. “For example: ABS standard only considers compression stresses and gives almost no directions when tensile loads are involved. Besides that, the ABS standard 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 connection of the stiffener or girder. Probably this is covered in the code, but it is unclear how. DNV also incorporated a decrease in allowable if the shear load is more than fifty percent of its capacity.”

Peetam finished with two more differences. “ABS omits to check torsional buckling when the stiffener or girder is loaded in bending due to pressure loading. And the ABS standard only incudes the lateral pressure on the plate side only.”

Favorite

Peetam: “Because of the differences between ABS and DNV, we advise to check structures with both standards if ABS is requested. With add-on software such as FEM Data Streamliner this is now, unlike it was before, within hand’s reach of structural engineers. But we have our favorite. Considering everything I mentioned we prefer the DNV standard because it is the most complete one. We recommend to check the structure with the DNV standard before comparing the structure to the standard of ABS.”

For more explanation see also:

Comparison plate buckling DNV vs ABS.

Previous articles

Need for better and faster strength analysis of structures.

Code checkers needed in structural engineering.

SOFTWARE TESTERS

The start-up company FEM Data Steamliner is looking for people to test the software with the new implemented standards. "The more feedback we can get from end users, the better", Edgar Peetam explains. People that are interested can contact Edgar Peetam at edgarpeetam@femds.com

About the study

The study was organized by FEM Data Streamliner and executed by Nasano Management in May 2016. 36 experts in the field of structural engineering had an extensive interview in which they were asked about important subjects in their field. The questioned engineers work in offshore industry, marine industry, crane construction and heavy mechanics. 31% of the interviewees is an analyst or calculator. Others hold positions as team leader (28%), constructor (19%) and project leader (17%).

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