Is this a Valid combination using AutoPIPE?

Applies To
Area: Combination
Original Author:Bentley Technical Support Group


AutoPIPE provides the user the ability to create user Code and Non-code combinations. When creating these combination the user can specify any one of 10 combination methods and add up to 10 load cases to be applied to the combination as indicated on the Code and Non-code Combinations dialogs below: 

User beware, not all combinations are valid combinations. There are limitations on combinations that can be created. So what constitutes a valid or invalid combination?


First, start by reading the following AutoPIPE help section: 

Help > Contents> Search Tab> enter "Rules for Creating User Combinations" (include the quotes), press List Topics button, double click on the selected topic from the list provided to see more information.

Before creating any user defined combination, the user should know which type of analysis was performed on the model; a Linear Analysis or a Non-linear analysis. Knowing this is key to creating user defined combination. For Linear Analysis, the model analysis does not consider any gaps, friction, or soil properties. However for a Non-linear analysis the model does consider gaps, friction, or soil properties seetings in the model. In addition, a Non-linear analysis is subjected to a load sequence (i.e. first apply Gr, then apply T1, etc..) where a Linear analysis is not (Load cases GR, T1, are all applied at the same time). 

Almost any combination is valid for a Linear analysis, but that is not the case for a Non-linear analysis. Please see the following WIKI page to completely understand Load sequencing here. After reading this documentation, you will understand that the results of the individual load cases are added together to get the combined operating condition results.

Example: (seen in the "Non_Linear_Load_Sequencing.pdf"): 

The results from the individual load cases, Gr...T1....E1, are added together to get the results of the operating combination GrT1E1

Because of AutoPIPE's Non-linear load sequence, for each load case the initial location of the pipe is critical before applying the current Load case. Example: consider a pipe resting on a V-stop support with a gap of 0.2" both sides:

1. Typically the initial load case for Gravity (GR) is the non deformed shape of the piping / support arrangement that has been modeled. 

2. Gravity (GR) is applied, a downward force is applied on the support, but there is no vertical or lateral movement of the pipe. AutoPIPE calculated the results due to this load and are saved under the GR load case

3. Temperature (T1) is now applied, this causes the pipe to move, 0.2" to the left:

Again, AutoPIPE calculated the results due to this load and are saved under T1 load case. The calculations take into account the friction force due to pipe movement to the left and allows the pipe to move no more than the specified gap distance = 0.2". In this case, the pipe movement closed the 0.2" gap and as a result, there was a lateral load on the support.

4. Earthquake (E1) is now applied, this causes the pipe to move, 0.306" to the right:

Again, AutoPIPE calculated the results due to this load and are saved under E1 load case. The calculations take into account the friction force dues to pipe movement to the right and allows the pipe to move no more than the specified gap distance (0.4"). In this case, the pipe is restrained from moving to the left because of the initial load case pipe position (currently there is 0.0" to the left, and a total 0.4" to the right), The pipe displaced 0.306" to the right and now there is no lateral load on the left side of the support. 

5. Analysis is completed and now combinations can be created. The combination of GrT1E1 adds the results from the save information during the analysis phase above. The results of GrT1E1 = has a total displacement of 0.106" to the right with no lateral load on the support.  

This a valid combination and the results are correct.

*** Important Note ***

Remember the importance of Load sequencing for NonLinear analysis. Review your model's Analysis summary report for Load cases analyzed and load sequence used by the program. This is vital information to understanding why a combination may be considered valid / invalid:

An example of an invalid combination:

Item #1

Same example shown above with the following twist, instead of 1 static earthquake being analyzed there are 3 (i.e. E1, E2, E3). As a result there are 3 different locations for the pipe movement due to these load cases. After the analysis was completed 2 new user defined combinations were created: SRSS(E1,E2,E3) and GT1+SRSS(E1,E2,E3).


Are the results from this new set of combinations valid?


In short, the answer is: No. 

Reason, when Load case E1 was applied, the program knew the location of the pipe and the allowed gap spacing in all directions, the same applies for E2, and E3. However, the combination SRSS (E1,E2,E3) is calculating a new resultant based on the results of these 3 load cases. This new resultant does not take into consideration the position of the piping to know how much of a gap that may or may not exist in all direction before coming into contact with the support. SRSS (E1,E2,E3) in a Non-linear analysis is not a valid combination nor should it be used in other combinations (i.e. GT1+SRSS (E1,E2,E3). The results from these combinations are wrong.

However, these 2 new user combinations would be a legitimate combination in a Linear analysis.  

Item #2:

Are both of the following combinations correct?





In short, the answer is depends.

One should be asking  / thinking, what type of analysis was used; Linear or Non-linear

Again the type of analysis is very important.

Non-linear analysis

Due to load sequence, combination GT1U1 is valid, but GT1U1U2U3 is NOT valid. The reason is that the initial case for U1, U2, and U3 are all set to T1 (see image below): 

In order for GT1U1U2U3 to be a valid combination the initial case for U2 would need to be set to U1, and the initial case for U3 needs to be set to U2. However, the following is displayed when trying to set the initial case for U2 and U3 to another occasional case:

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

This was originally added at the inception of the programming to prevent looping or nesting issues, However a new enhancement has been logged to allow this type of combination. 

Linear Analysis

If a linear analysis was performed, since there is no load sequence in a Linear analysis both of these combinations would be valid. 

Item #3:

Whether it is correct to use a combination of the maximum load in one case for code and non-code comb.?


T1 - design temp

P1 - design pressure

Can we created loads based on DNV-RP-D101 APPENDIX G?


Again, tt will depend on what type of analysis was performed; linear analysis or Non-linear analysis.

If using Non-linear analysis, then the load sequencing is important, and combining results like that mentioned above would be inconsistent.

However, if you are using linear analysis then the principle of load superposition applies and you can create code and non-code combination as mentioned.

Item #4: 

Is it possible to combine a Non-linear load case with linear load cases Example: GT1E1+S1 (Gravity + Temp+ Static Earthquake + SAM? What if the Non-linear combination closed the gaps before the SAM was applied? 


Assuming, Gravity, Temperature, and Static earthquake were performed as a Non-linear analysis while S1 (SAM - Seismic Anchor Movement) would always be performed as a Linear  analysis.

It may not be a consistent approach to combine Non-linear load case with a linear load case. Non-linear analysis need to have initial states defined for subsequent load cases that are to be analyzed. This type of combination (Non-linear load case combined with linear load case) would not be an accurate representation of the system.  

As a potential workaround, a SAM case could be modeled as impose support displacement. Consider removing the actual SAM load case from the combination and replace the SAM load case with individual impose support displacement assigned to the respective Static Seismic load case.  

Item #5:

What does Purple highlighted combinations mean?


Using AutoPIPE CONNECT and higher, there is a new color legend feature. Please see the following AutoPIPE help section: 

Help > Contents> Contents Tab> Command Reference> Tools & Utility Commands> Combinations> Display Combinations> scroll down to see Color Legend:

Select the hyperlink for Combination Validation for complete details about Purple highlighted combinations. This leads to another page in the help that discusses in details the logic behind the coloring of a particular default or user combination.

Note: Nonlinear analysis Load cases like Thermal 1, Pressure 1, Wind 1, etc.. will be colored purple. Why?

Because, these are individual load cases and not actual operating conditions which are comprised of multiple individual loads (Gravity, Tempe., Press.etc..) The results for these individual load cases are based on an initial case being applied. AutoPIPE will calculate a value based on combination settings,  However, one cannot just add random individual load cases together and expect the results to be correct. Users must understand how individual load case results are calculated and their contribution to a given combination / operating condition.

Remember, consider operating conditions (i.e. GrT1P1) for analysis, and only review the individual load cases (ex. Gr, T1, P1) to understand why the operating condition results are such. And, individual load cases maybe considered inconsistent if initial loads are not apart of the combination. See the Combination Validation document for complete details. 

Item #6:

Why are these combinations inconsistent?


First step would be to review the Inconsistent report. On the Load combination dialog screen press the "Combination Options" button, and select "Show Inconsistent Combinations Report"

Press OK to close the Load Combination dialog, the following report will then appear:

Next review the load sequence for the Analysis set. This is best viewed from the Analysis Summary sub-report:

Now using the aforementioned rules in AutoPIPE help, determine why the combinations are listed as inconsistent.

Item #6:

Why are these ASME B31.8 combinations inconsistent?


As mentioned above, first step list combinations that are not consistent by using one of the following:

1. See Analysis Summary output report, CODE COMPLIANCE COMBINATIONS, Notice the last character on the row where the combination is listed, C = Y or N.

2. On the Load combination dialog screen press the "Combination Options" button, and select "Show Inconsistent Combinations Report". Then press OK to close the Load Combination dialog,  The program will automatically display a report identifying all inconsistent combinations. 

Now generate a report that includes the Analysis Summary sub report and look at the CODE COMPLIANCE COMBINATIONS section for inconsistent combination(s) as reported. Review each inconsistent combination for load sequences:

In each case the given combination was listed as inconsistent by the program because the initial case for more than one load case in the combination was the same (ex. GR).

Question: How to correct this issue for analysis set #2 so combinations are consistent?

Answer: The problem was the initial load case for the occasional load was GR, however GR load case was already an initial load case for Temperature. Therefore, need to update the load sequence to place the occasional load after Pressure. How? Correct, using a default method of setting the initial load case for OCC loads to OP1..OPn ( ex.Gr+T1+P1).

For this example, from the Analysis set dialog, change Nonlinear Analysis dialog setting "Initial case for Occ loads" as indicated below:  

Then run an analysis, create an output report, and now notice that the combinations are all consistent.  

Done.. the combination now meets the requirements by AutoPIPE to be considered consistent. 

Always remember the following:

1. For Non-linear analysis, all combinations made with

a. A single load cases (except Dynamic analysis loads) will be considered inconsistent (only exception may be a combination made with GR only).

b. Multiple load cases that share the same initial load case as shown above will be considered as inconsistent.

c. Missing load cases from the middle of a load sequence will be inconsistent (ex. GRT1W1 where P1 was missing from the combination). 

2. Inconsistent combinations in AutoPIPE does not necessarily mean wrong for the code.  This is where the confusion may occur for user.

POP Quiz... 

Question: Is this default code combination consistent or inconsistent?

Answer: It is actually inconsistent, why?

Correct because the load sequence was incorrect, it was missing the thermal load case T1:

Item #8:

Additional Examples:

See Also

Combination Load Cases

"Non-Code Comb." tab

Bentley AutoPIPE