How are is the Time Independent Design Stress, f, calculated according to EN 13480-2017 in AutoPIPE for Austenitic Steels (A > 35%)?
As seen above, help indicates 2 equations, but appears that we use the first equation only for calculating the allowable, why?
Is there a setting to using the 2nd equation?
To answer the question, Is there a setting to using the 2nd equation?, No there is no setting direct the use of equation #1 over equation #2.
Calculations for EN 13480 design stresses are done in AutoPIPE as stated in the standard ( EN13480 - 2017 5.2.2-1). When Austenitic Steels are selected with elongation (A) larger than 35%, the program uses the two equations mentioned above.
Decision to use Equation #1 or #2 is controlled by the information available in the model. Look again at the help information, notice the subscript after equation #2, If Rmt is available. Therefore, if Rmt information is missing from the model the program will automatically use equation #1. Otherwise, if Rmt information is present, then the program will automatically use equation #2.
This is demonstrated in the example below:
1. Set Rp1.0t and Rmt in the ‘Operating Pressure & Temperature’ dialog as shown:
2. Generate the code compliance report:
Using equations (5.2.2-1) we would have the following answers to choose from in this case:
From the code compliance report above, we can see that AutoPIPE uses the correct design stress as required when Rmt is defined.
In addition, there may be another reason for choosing one equation over the other. AutoPIPE takes the minimum of the longitudinal and transverse elongation into account when using one of the equations.
These are shown in the pipe properties dialog:
If the user has set one of both below 35.00, AutoPIPE will use the next applicable equation as stated in the standard:
Please also review the help note below to guide the user:
(Contents>Reference Information>Code Compliance Calculations>EURO EN 13480 (2017) > Design Stresses)
Question: Per EN13480-3:2017, The allowable design stress, ff, is the design stress for flexibility analysis per code equation 12.3.2-1,
Per Code equation 12.1.3-3,
f = Design stress calculated according to 5.2
fcr = calculated according to 5.3 at calculation temperature tc.
Answer:
First, ff is used for sustained combinations only. However, default sustained combinations do not have a temperature load case. But AutoPIPE calculates allowables for sustained at the temperature corresponding to the pressure used. Thus, this ff is calculated at temperature corresponding to the pressure load used.
The variable in question is the design stress, as explained in section 5 of part 3. Here, it uses variables such as Rmt, ReHt, Rp0.2t, and Rp1.0t, which are at combination temperature. Rm would be used at room temperature always.
AutoPIPE help 12.7 and higher will be changed to:
That f is the calculated design stress at temperature corresponding to the pressure load of the sustained combination.
En13480 Piping calculations
Bentley AutoPIPE