Help regading ASME section VIII Div-2, section 5.4

[Ashok kumar]

HI,
I do have following queries about ASME section VIII Div-2, section 5.4 , protection against buckling collapse.

(1) For analysis method 'type 1', are we required do a simply eigenvalue analysis?. For what type of material this analysis is provision valid? I am using stainless steel material at 525 degree centigrade, is this provision of code is valid for such application.

(2) For 'type 3' analysis, I shall be using load combination in table 5.5, which say applied load shall be 2.5 times of dead load (assuming only dead is acting). Does it indirectly says there a factor of safety 2.5 on buckling collapse load calculated by this method?. Is there any further factor of safety shall be applied on this calculated load factor. How we determine failure load from analysis result, is it the load at which Netwon-Raphson method fails to converge (i.e last converged solution), or we need to apply TES method.


(3) How to do 'type 2' analysis. Can you suggest some guidelines or source which I can refer.

(4) In one of discussion thread on this forum, it is indicated that after 0.55 sy, there is elastic plastic buckling.In this regime people generally uses tangent modulus theory, where they replace Young's modulus with Tangent Modulus. Now tangent modulus reduces to almost 0.1 times or lesser of elastic modulus. Does it mean that effective buckling load will be 10 times lesser. That is very conservative. It means that if my structure is in compression, my structure will buckle even if eigen value predicts pretty high buckling load.

Thanks in advance for help.

Ashok Kumar

 

[TGS4]

A couple of general notes before answering your specific questions. First, the current rules in ASME Section VIII, Division 2, Part 5 are only valid for components NOT in the creep regime. This is very important because the rules in the creep regime are very different. There are two general ways to determine whether or not the component is in the creep regime:
1) The allowable stresses in the appropriate Table of ASME Section II, Part D at the applicable temperature are in italics.
2) Reference Table 4.1 in API 579-1/ASME FFS-1 "Temperature Limit Used To Define The Creep Regime"

I generally prefer to use 2). For 304/304H stainless steels, the creep regime starts at 510°C (950°F), whereas for 316/316H/321/321h/347/347H, the creep regime starts at 538°C (1000°F).

Also, the buckling rules in VIII-2 will be changing (hopefully for the 2019 Edition). In general, eigenvalue buckling, especially the design margins for eigenvalue buckling, are only appropriate for stresses less than 0.55Sy (i.e. elastic buckling only). Otherwise, you are into a complicated plastic-buckling regime where elastic eigenvalue buckling is simply incapable of providing an answer.

My preferred approach to demonstrating Protection Against Failure From Buckling is to use the Type 3 analysis - Elastic-Plastic Buckling. The load factors applied to the loads are indeed the design margin against buckling. Until we update the buckling design factors (see above re 2019 Edition), use the current factors.

There are multiple ways of detecting "collapse", but my preferred approach is to simply use the Newton-Raphson method, as opposed to a Riks method. Although, if you use such an approach, you could also apply a twice-yield approach to the load-displacement curve.