Section VIII Div 2 Part 5 Table 5.5 Loads

[pogoGo]

Are the loads excluded from each load combination meant to have a factor of 1 or meant to be excluded altogether? For example, Global Case 1 does not show a factor for live loads. If I have been given live loads as a design input, does that mean Global Case 1 is

2.4*(P + Ps +D) + 0*L
or
2.4*(P + Ps +D) + 1*L

 

[jte]

pogoGo-

That would be zero, so the way you wrote it in the first line is correct. If the load is not mentioned in a load case, then it does not exist in that load case.

 

[pogoGo]

Thanks, jte. Out of curiosity, do you know how the code justifies live loads being neglected during the local check? Based on what I've seen, the live loads are much higher than anything else (specifically sub-sea) thus protection against local fracture strains would be critical during live loading. This seems especially true in applications with low pressures and high live loads.

 

[jte]

I'll stay out of justifying the code's reasoning. That's best left to other, more qualified posters in this forum.

 

[TGS4]

pogoGo - first, welcome to eng-tips.com.

Excellent questions. The answer that jte provided to your first question is sufficient.

Regarding your second question about the reasoning for the load case combinations and load factors thereof; first I would point you to ASME PCCPTB-1. There are some very good explanations in there. A summary is that we are trying to be analogous to the Structural Code ASCE-7 and we take guidance from their load factors.

For Protection Against Local Failure - you will notice that the thermal loads are also neglected from the load case combination, as well as the live loads that you pointed out. That was a judgement call by the committee. When situations such as yours arise, we concluded that, if the design engineer considers it important, then they are empowered to include them by 1.1.1.2, and the User should include them in the UDS by 2.2.2.1(h) (in which case they do form a mandatory part of the calculations).

The Code represents a minimum standard, but should not be considered the final arbiter of all things pressure vessel-related, because there are simply too many possible situations that cannot be foreseen. If, in your opinion, your live loads are significant and important, then I would suggest, perhaps as a start to reclassify them as dead loads. If that causes problems, then I would add some factored live loads to Required Factored Load Combination (1), such that it is consistent with Required Factored Load Combination (2). And, by all means, feel free to add some factor of L into the Required Factored Load Combination for Protection Against Local Failure. There is no prohibition on being more conservative than the Code.

 

[jte]

pogoGo-

While I appreciate TGS4's nod to the PCC, I believe he meant to refer to ASME PTB-1 2014 edition.

 

[TGS4]

jte is correct. I shouldn't write these overly early in the morning. Post edited.

 

[MrPDes]

I would just add that that if your vessel is an ASME VIII Div 1 vessel using U-2(g) to conduct your elastic-plastic analysis then:

2.4*(P + Ps +D)
should be changed to
3.5*(P + Ps +D)