ASME VIII-2: Protection Against Plastic Collapse in Operating Condition
ASME VIII-2: Protection Against Plastic Collapse in Operating Condition
(OP)
Hi,
I'm performing an elastic analysis under operating conditions and rereading ASME VIII-2 I noticed that with regard to Protection Against Plastic Collapse the limits provided in Figure 5.1 do not match in par. 5.2.2. However, in the latter Figure, following the dotted lines (Operating Loads) leads to Pm, Pl and Pb, as well as obviously (as expected in operating condition) Q and F.
My doubt is, by taking advantage of an elastic analysis (Par. 5.2.2) can I use the operating loads, or should I use another check (not PAPC?
In this case any stress value (except to consider F) must be compared PL+Pb+Q with Sps (equal to 3*S) (why in Figure 5.1 does it not also consider Pm+Pb+Q?)?
So my main question is whether I can proceed as in Design Condition, but in Operating Condition (changing only the loads and the condition to compare with allowable).
I'm performing an elastic analysis under operating conditions and rereading ASME VIII-2 I noticed that with regard to Protection Against Plastic Collapse the limits provided in Figure 5.1 do not match in par. 5.2.2. However, in the latter Figure, following the dotted lines (Operating Loads) leads to Pm, Pl and Pb, as well as obviously (as expected in operating condition) Q and F.
My doubt is, by taking advantage of an elastic analysis (Par. 5.2.2) can I use the operating loads, or should I use another check (not PAPC?
In this case any stress value (except to consider F) must be compared PL+Pb+Q with Sps (equal to 3*S) (why in Figure 5.1 does it not also consider Pm+Pb+Q?)?
So my main question is whether I can proceed as in Design Condition, but in Operating Condition (changing only the loads and the condition to compare with allowable).
RE: ASME VIII-2: Protection Against Plastic Collapse in Operating Condition
For Protection Against Plastic Collapse, you need to focus on the Design Load Combinations. In this failure mode you will find the limits on Pm, PL, and PL+Pb - equations 5.2, 5.3, and 5.4. There is no use in applying operating loads for this failure mode - it is simply not permitted. Pay special attention to the General Note in Table 5.3.
(Caveat - with Design Load Combinations 6, 7, and 8, there is a parameter ΩPP, which has the effect of being the operating pressure (to be used in combination with the wind and seismic occasional loads to form the indicated Design Load Combinations). However, great care was taken in the ASME Code Committees while creating this parameter so as to not confuse it with PO).
Likewise, you use the design load combinations for Protection Against Local Failure and Protection Against Collapse From Buckling.
You use the operating loads for the failure modes of Protection Against Failure From Cyclic Loads: Fatigue and Ratcheting.
RE: ASME VIII-2: Protection Against Plastic Collapse in Operating Condition
For all intents and purposes, please consider Pm as a hand-calc value. You should use your hand-calc to validate your FEA where you would categorize a stress as Pm - you never use FEA to calculate Pm.
In the context of the P+Q stresses, it matters not whether the membrane stress is general or local.
And again, the limit on PL+Pb+Q is not 3*S, it it SPS. Pay special attention to 5.5.6.1(c) - particularly the last two sentences:
RE: ASME VIII-2: Protection Against Plastic Collapse in Operating Condition
With your last message, you "took the earth out from under my feet" with this statement:
Do you mean that the Pm stress you get from FEA may not be correct? Or rather, do you mean for example a cylindrical tank? But in the case, for example, of an Air Cooled API 661 with a flanged cover or in the case of a device with more complex shapes, how can the Pm value be obtained with hand-calc?
I don't want to ask stupid questions, but I've never asked myself this kind of problem.
Thank you in advance
RE: ASME VIII-2: Protection Against Plastic Collapse in Operating Condition
I will share some of my thoughts with respect to my contention that Pm should be a hand-calc. First, it is not my contention that the value of Pm is wrong - in fact if you re-read my 21 Jun 22 03:44 post, you will see that I said Validating your FEA is extremely important, but the work process ought to be one where you use hand-calcs to validate your FEA - and the hand-calc for Pm is ideally suited for this situation.
Accordingly, Pm is calculated as F/A, M*c/I, and P*r/t or C*P*d^2/t^2 (for a flat head)- or any of the other hand-calc method in Part 4. If you have a complex shape that you can't perform a hand-calc, then you are into a situation where paragraph 5.2.1.2 comes into play:
Basically, if you can't do a hand-calc to calculate Pm, then your geometry may be too complex for the elastic stress analysis method for demonstrating Protection Against Plastic Collapse.
RE: ASME VIII-2: Protection Against Plastic Collapse in Operating Condition