06. Combinations involving User 1-140 load cases appear to be incorrectly added in combinations with

Applies To
Version(s):2004, XM, & V8i
Area: Load Case
Original Author:Bentley Technical Support Group

Logged November 2014, AutoPIPE V8i


When assigning Impose Support Displacement to Load-case = GR, analyze the model, and note the results for Gr. 

Then change the assignment of the Impose Support Displacement from Load-case = GR to U1 (User1), analyze the model, and note the results for Gr+U1.

it has been noted that the results from GR does not equal GR+U1. Why?

Also, can I add more than 1 user load case in a combination (ex. Gr+U1+U2)?


Using a very simple model like Walkthru.dat (copied to the AutoPIPE folder when the program was installed, default = C:\Bentley\AutoPIPE V8i SELECTseries\Examples) and performing a Linear analysis. 

Open the file and perform the steps:

1. Change the spring support at B01 to a V-stop support with 0.00 gap up and down.

2. Make the V-stop support the current node point in the model, Insert> Xtra Data> Impose Support Displacement> "Load-case to combine with" = GR, Translation = Y=1.0 in (25.4mm).

3. Delete the Anchor at B04

4. Select Load> Static Analysis Sets..> Modify analysis set #1, Remove T1, E1,and W1 load-case.

5. Analyze the model.

6. Add a new user Code Combination, GR:

7. Review the results for GR. 

8. Save the file as, GR.DAT, and close the model.

Now let us reassign the impose support displacement from Gr to U1 and compare the results:

1. Open GR.DAT file from above.

2. Save file as GrU1.dat.

3. Modify the impose support displacement at node B01 from GR to U1. 

4. Select Load> Static Analysis Sets..> Modify analysis set #1, Add U1 load-case.

5. Add a new user Code Combination, GR+U1 :

6. Review the results for GR+U1. 


 7. Compare the results from both model, the results were found to be Identical. 


AutoPIPE correctly calculates the same stress regardless of how which load case the "Impose Support displacement" is assigned to Gr or U1. The key observation here is to be sure that the combinations being reviewed / compared are correct. 

Question: Can I add more than 1 user load case in a combination (ex. Gr+U1+U2)?

Answer: Yes and No. 

Another words, Yes more than one user load case can be added to a combination assuming that a Linear analysis is being performed. However, if a Non-linear analysis is being performed; than, NO, a combination cannot have more than one user load case. Remember that a non-linear analysis is performed following a defined load sequence; Gr then Pressure then Temp then Etc..

A nonlinear analysis requires successive iteration to achieve an accurate result set for each load case. In the end the results from the individual load cases are added together as described by the combination to calculate the results. Therefore the results from a Non-linear analysis where multiple Impose Support Displacements are assigned to a single combination (ex. GR) will differ when compare to a combination that is made up of multiple Impose Support Displacements assigned to different User cases (ex. Gr+U1+U2+Ux). 

Keep in mind, User load cases are considered as an Occasional load case and at this time AutoPIPE does not allow Occasional load cases to be the initial case for another Occasional load cases (ex. U1 is the initial load case for U2). When trying to specify, U1 as the initial case for U2, the following would appear:

E87-20: Invalid load case for initial state.
Initial state cannot be a user load case.

In addition, this scenario applies to all occasional load cases (ex. earthquake, wind, user). Again if performing a non-linear analysis you must set the load sequence, and at this time AutoPIPE does not allow an Occasional case to be the initial case for another Occasional load-case.

A new enhancement has been logged for this scenario. 

See Also

Results Post Processing in AutoPIPE

Bentley AutoPIPE