Basis of 2.4 (or 3) factor used for Beta in Table 5.5 of ASME Section VIII, Division 2

[misc]

I am interested in the basis for the 2.4 and 3 factors used for Beta in Table 5.5 of ASME Section VIII, Division 2.
I believe the 1.5 factor in Table 5.4 for Limit Load Analysis is based on the acceptable load being 2/3 of the collapse load calculated for example via FEA (EPP material behavior). Is there an analogous explanation for the 2.4 (or 3) being used for Beta in Table 5.5? How were these values decided upon?
The load required to be carried for EP Analysis is 2.4 / 1.5 = 1.6 times greater than for Limit Analysis.
I suppose this is the ratio of the ultimate stress over the yield stress for some materials, but not all so I am not confident this is the reason.
Thank you in advance for your input.

 

[TGS4]

The load factor, β, in Table 5.5 (and from Appendix 46-4 in VIII-1) is indeed the design margin against ultimate for the respective design codes and classes.

This is well-detailed and discussed in ASME PTB-1.

Note that the difference in Limit Load and Elastic-Plastic is that in the first the material response is elastic-perfectly-plastic up to the pseudo-yield stress (1.5*S), whereas in the latter the material response is elastic-plastic up to the true ultimate stress.