Liberal allowable seems to turn off for maximum expansion range cases only, in B31.1 only.

Load Cases:
23 (EXP) L23=L6-L5 - High temp expansion case
24 (EXP) L24=L7-L5 - Sub-zero cold expansion case
25 (EXP) L25=L23-L24 - Maximum Expansion Range Case

When analyzing per ASME B31.1 the code allowable stress calculated by CII for case 25 is the same with the liberal allowable turned on or off. The same system analyzed per 31.3 shows as expected a big increase in allowable stress with the liberal allowable turned on. This problem does not occur for load case 23 or 25.

Is this a software error or does B31.1 somehow require the liberal allowable to be applied differently than the basic install to hot/cold cases?

-Matt Stewart

If the Sustained case is overstressed, you can't use the liberal allowable (and yes CAESAR II will turn it off). Is that the situation?

If not, open a Support Request on Smart Support and send in the model.

The sustained case is not over stressed.

I will open a ticket, in the mean time I have sent a copy of the model to D. Diehl. Thanks for your help as this is affecting our construction schedule.

Regards
Matt Stewart

It looks like CII automatically input a value of zero for Sh T2 since it is below the minimum temperature listed in ASME B31.1 Table A-1. It then automatically turned off the liberal allowable because in this unique circumstance it made for a lower allowable stress range than the basic allowable per ASME B31.1 equation 1a.

Thank you to Intergraph for helping to explain this!

Some supplementary information on the calculation:

In B31.1 section 102.3.2 B) Displacement Stress Range, the allowable is calculated as S_A = f(1.25*S_c + 0.21*S_h) [equation 1A]

The code goes on to state, when S_h is greater than S_L, the difference can be added to the 0.25*S_h term so the equation becomes S_A = f(1.25*S_c + 1.25*S_h - S_L) [equation 1B]

Since the sustained stress must be less than the hot allowable (i.e. S_L < S_h) the residual allowable not used in the sustained case is (S_h - S_L). Adding this residual back into to eq. 1A gives eq. 1B

S_A = allowable stress range
S_L = sustained stress
S_h = hot allowable
S_c = cold allowable
f = cyclic reduction factor (f = 6/N^0.2 <= 1.0)
N = total number or cycles