Protection Against buckling (ASME VIII-2 Part 5) 2

[victorpbr]

Hi,

I'm studying Part 5 of ASME VIII-2 and am in doubt about if I am interpreting this correctly. Could you please confirm if I got the idea right?

Here is what I understand.
Type 1 analysis is a eigenvalue buckling situation where pre-stress is from an elastic approach.
Type 2 analysis is a eigenvalue buckling situation where pre-stress if from an elastic-plastic approach with nonlinearities.

[1] However, I do not understand fully Type 3. In the case where I`m carrying an elastic-plastic approach and checked protection against plastic collapse does it means that the vessels passes buckling checking?

The code says

Type 3 – If a collapse analysis is performed in accordance with 5.2.4, and imperfections are explicitly considered in
the analysis model geometry, the design factor is accounted for in the factored load combinations in Table 5.5. It should
be noted that a collapse analysis can be performed using elastic or plastic material behavior. If the structure remains
elastic when subject to the applied loads, the elastic–plastic material model will provide the required elastic behavior,
and the collapse load will be computed based on this behavior.

The part where it says that the collapse analysis can be performed using elastic or plastic material behavior is confusing me, as 5.2.4. is adressed for elastic-plastic analysis. So how could I do it using elastic behavior?

English is not my first language and sometimes some terms are not quite familiar to me. But in this case (Type 3) if what I said in [1] is false, which kind of buckling analysis should be carried on this scenario, linear or nonlinear?

Thank you for your attention.

 

[TGS4]

Victor - I am conflicted about how best to answer your very thoughtful question. The rules are in flux and will be changed (if not in the 2019 Edition, then certainly in the 2021 Edition). As it stands right now, these rules are both unconservative and excessively conservative.

Please contact me offline and I will assist you.

 

[NRP99]

victorpbr

My Understanding-

Type 1- Do Elastic Analysis without any imperfections and geometric non-linearity followed by buckling analysis. Apply Load factor Φ[sub]B[/sub] on the calculated Buckling load. Note that for the Pre-stress elastic analysis use Table 5.3- load and load case combinations.

Type 2-Do Elastic-Plastic Analysis with imperfections and geometric non-linearity followed by buckling analysis. Apply the load factor Φ[sub]B[/sub] on the calculated buckling load. Here also Use table 5.3 for load and load case combinations.


Type 3-Do elastic-plastic analysis with imperfections and geometric non-linearity followed by buckling analysis. Here table 5.5 is used hence no need to apply load factor. Rather loading case itself consider load factor β. This means if the non-linear analysis converges using the factored loads, it means the component is stable and can withstand the factored loads. Then the applied load/load case itself is critical buckling load.

Note the difference of Φ[sub]B[/sub] and β for cylinders subjected to external pressure:Φ[sub]B[/sub]=2.5 and β=2.4. The type 1 and 2 procedure is more stringent than type 3 since we are making assumptions.

I am not sure but elastic response is anticipated for the external pressure since external pressure(which is generally a full vacuum pressure) multiplied by the factor load will still give elastic response. Hence the statement- It should be noted that a collapse analysis .....collapse load will be computed based on this behavior.

 

[DriveMeNuts]

Type 3 is a model of the actual real life equipment with geometric imperfections. The load applied to the model is multiplied by the factor in Table 5.5. The result is the equipment has a buckling 'safety' margin of 2.4 for a Div 2 vessel and 3.5 for div 1 vessel.

Allowing type 3 to use linear material behaviour doesn't quite make sense to me. I would expect this to make the structure stiffer at its discontinuities where peak stresses occur, hence making it slightly non-conservative. I presume that the contribution of these peak stress regions towards buckling is considered negligible.