How to calculate ASME B31.3 -2010 Occasional stress (Sus+E1) using AutoPIPE?
A common mistake, Do NOT use the reported In-plane moment, Out-plane moment, or Torsional moment from the Occasional load-case (ex. Sus+E1) when calculating the code stresses. These reported absolute values are the summation of individual load case (Gr) and (E1) from the forces and moments report section:
Example:
Instead, start by reviewing the following help section:
Please see the following AutoPIPE help section: Help > Contents> Contents Tab> Reference Information> Code Compliance Calculations> ASME B31.3 (2010)>
a. Stresses Due to Occasional Loads (ASME B31.3 - 2010)
b. Stresses Due to Sustained Loads (ASME B31.3 - 2010)
Notice code stress results for Occasional load case (Sus+E1) are a calculated by adding the stress due to Sustain (Gr+MaxP) and the stresses due to occasional (E1) together.
A. Stresses Due to Occasional Loads (ASME B31.3 - 2010):
The stress due to combined sustained and occasional loads (sLO) is calculated in a similar way as per code equation 23a as shown below:
Where, SL = MAX Stresses due to sustained loads
Notes:
1. Sa term may be ignored if all of the following Results options settings are unchecked:
> Include axial force
2. Following values are taken from Occasional (E1) Forces and Moments report:
> Fa> Mo> Mi> Mt
3. Following values are taken from Code stress report for Occasional load case (Sus+E1):
> Ii> Io
4. Section Modulus
AutoPIPE reports sLO and the allowable stress (k× SH).
If the model result option "Use Alt Occ Allow" is checked, the calculated allowable stress will be based on hot yield stress Syt and the material composition for temperatures above Tcr.
B. Stresses Due to Sustain Loads (ASME B31.3 - 2010):
For case with pressure extension cases and with Pcase in sustained, the equation to be used for calculating Sustained stress will be given as per code equation 23a (code case 178 in earlier editions):
> Use code case178> Include axial force> Inc Axial,Pcase in Sus
2. St term may be ignored if all of the following Results options settings are unchecked:
> Use code case178> Include torsion
3. Sa term should account for Results option "Long Press (P/A/M/N)" setting:
Sa = [ Ia * Long Press (P/A/M/N) ] - Sa (above)
4. Following values are taken from Sustain Forces and Moments report:
> Fa> Mo> Mi>Mt
5. Following values are taken from Code stress report for Sustain load case (Gr+MaxP):
AutoPIPE reports sL and the allowable stress (w * SH)., where w = Weld Efficiency Factor for value\
Is there a place in the B31.3-2010 Code where it discusses the lack of applicability of the Ia index for occasional loading? By virtue of the description of the indeces as being “sustained index”, one might almost conclude that none of the indeces apply for occasional loads?
As mentioned above, see section ASME B31.3-2010 section 320.2: Stress Due to Sustained Loads. The stress due to combined sustained, and occasional loads (sLO) is calculated in a similar way as per code equation 23a as shown below:
where:
sLo = occasional load stress calculated per code equation 23a
sL = sustained load stress calculated per code equation 23a.
Sbo = bending stress due to occasional loads
Sao = axial stress due to occasional loads.
Sto = stress due to torsional moment due to occasional loads.
Notice the definitions of Ia and It:
In the absence of more applicable data, It is taken as 1.00.
Therefore AutoPIPE calculations for this code and year will default to 1.00 unless a user defined "Joint type & User SIF" is entered changing the stress indices.
ASME B31.3 Piping Code Calculation Issues
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