working stress vs. limit state design

Question

working stress vs. limit state design

chaboche (Mechanical)

(OP)

8 Aug 07 11:31

Is it still possible to perform an assessment to DNV using working stress design?  I understand that the new code DNV-OS-C101 uses limit state design instead.

I am looking at lifting of a component.  These are the factors that I have come up with:

Using the working stress design method:
- 1.2 DAF
- 1.35 consequence factor
- 0.8 factor on Von-Mises stress.

Using the limit state design method:
- 1.2 DAF
- 1.3 consequence factor
- 1.3 load factor
- 1.15 material factor

If I combine these factors:

working stress design:
1.2x1.35/0.8 = 2.02

limit state design:
1.2x1.3x1.3x1.15 = 2.3

For the assessment I had in mind, it would appear to be advantageous to use the older working state design as it allows more onerous stresses.  I am aware that I am being overly simplistic in my comparison and would welcome your comments.

Ussuri (Civil/Environmental) · Answer 1 · 2007-08-08 12:30:41

DNV being Norwegian will tend towards limit state design as opposed to working stress and they dont mention working stress approaches in their documents. However, I dont think there is anything against using a working stress approach. Certainly some of the people at my work use it in conjunction with DNV codes, they just leave out the load factor.

Your comparison is not quite correct as the limit state is compared against the design resistance (around 0.9 x yield)where your working stress is checked against (0.4-0.75 x yield)

Other factors I include are:
Contingency on base weight of 1.1 from ISO 19901-5
Reserve Factor 1.05
Skew Factor 1.25 if not included in a physical model.

Does C101 cover lifting operations? I always took it to be structural steel design. I tend to use the DNV Rules for Marine Operations for lifting

chaboche (Mechanical) · Answer 2 · 2007-08-09 05:38:02

Thanks for your help Ussuri. I now feel more confident about using either approach. I was worried that the working stress method may no longer be acceptable.

You are correct, the lifting operations are covered in the DNV rules for planning and execution of marine operations.

The assessment is for a ~130Te reel. I have built an FE model and am considering several loadcases, including lifting.

Ussuri (Civil/Environmental) · Answer 3 · 2007-08-09 08:35:30

And the americans still tend to use working stress.

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working stress vs. limit state design