Question about required load combinations from Table 5.5 in ASME VIII 1

[Espen Lund]

Hi,

So, I am analyizing a drive shaft that is pressurized thru several bores, but also transfers a significant torque. In ASME VIII table 5.5, all the required load combinations are defined. Here, I define the pressure as P and the torque as L, because the torque is a result of relative rotation that is coming from wave motion (the ship is rotating). In the "Protection Against Plastic Collapse" design check, I multiply pressure P by 2.1 and torque L by 2.7, and if the solution converges, then it is ok. But in the "Protection Against Local Failure", I see that the load combination is only defined as 1.7(P+Ps+D). So do I only need to apply the pressure, and then if I am below the triaxial strain criterion I am fine? The reason I am asking is that I find it strange that I should not include design torque in this design check, because it is a significant amount, and is actaully causing greater strains that the pressure action.

Will appreciate any help, and if you need more information to answer then I'll of course provide this.

Thanks.

Espen

 

[TGS4]

From 5.2.4.4:

Step 4. Determine the load case combinations to be used in the analysis using the information from Step 2 in conjunction with Table 5.5. Each of the indicated load cases shall be evaluated. The effects of one or more loads not acting shall be investigated. Additional load cases for special conditions not included in Table 5.5 shall be considered, as applicable. emphasis added

Just because these are the bare minimum required design load case combinations, does not preclude the User from specifying additional combinations. If this torque load is always present and is significant, there is nothing stopping you from defining it as D, either.

I can guarantee that such a situation was not foreseen by the Code writers.

 

[jte]

I can guarantee that such a situation was not foreseen by the Code writers.

What? Are you trying to say that the Committee did not anticipate the Vessel code being used to design pressurized shafts which help to propel ocean going vessels? Well, I suppose that's what industry gets with a bunch of volunteers writing the codes...

 

[NRP99]

jte
For your kind information, please refer the table 5.2 highlighted attachment from code. Code has already considering maximum loads required to design the vessel.

http://www.eng-tips.com/viewthread.cfm?qid=414381

Any way, it is designer's responsibility to consider all load cases and not the code/code writer's. Generally speaking, code is like guideline book(Although this statement is debatable because most of all jurisdictions use this as STANDARD. But in context of general engineering, I am going to state it).

Even if some loads are not given in the Table 5.5, would you not consider it for design? Some clients/process licensor specify their own load case combinations. What would you do in that case?

The OP's vessel is shaft pressurized through its bore. This is analogous to nozzle transmitting the piping loads such as twisting.

Question is not about writing guidelines but of use of logic and engineering judgment and common sense which we rarely find in abundance.

 

[Espen Lund]

Thank you for your answers, all. But how do I then identify the correct load factor for the torque action? "D" does not exactly fit the description. Do I then simply add L to the local load criterion, such that it becomes 1.7(P+Ps+D+L), or how do I know that 1.7 is the correct/intended safety factor for torque in this design check?

 

[TGS4]

This is where you earn your money as a mechanical engineer. You need to be adequately-versed in the theory and practice of Protection Against (the various failure modes), sufficiently knowledgeable in both Strength of Materials and this specific problem, and experienced enough to know what are appropriate design margins in order to be consistent with the specific and overall design philosophy of the Code..

(You do understand what Protection Against Local Failure is, right?)

No one but you can answer those questions.

 

[jte]

Hi NRP99, and welcome to Eng-Tips - I see you've recently joined. As info, you can click on the "jte" at the top of this thread to see a bit of my background.

Thanks for the education... But I can assure you that I'm more than somewhat familiar with Table 5.5. I can also assure you that I'm not nearly as familiar with it as TGS4 is! And yes, I agree with your assessment of the OP's circumstance and the need for engineers to think on their own rather than rely on codes and standards exclusively. My poke at TGS4 was a bit of an inside joke coupled with a touch of sarcasm.

 

[NRP99]

Hello jte,

Thanks for welcoming.

My poke at TGS4 was a bit of an inside joke coupled with a touch of sarcasm.

Never mind.