Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: Subarea: Original Author: Bentley Technical Support Group Problem: 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 the user combinations that can be created. So what constitutes a valid or invalid combination? Answer: 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 any gaps, friction, or soil properties. 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 (0.0" to the left, but a total 0.4" to the right), The pipe displaced 0.306" to the right and now there is no lateral load on 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. An example of an invalid combination: Item #1 Same eample 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 who much gap space is allowed 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? Short answer: your first question after reading the the above information on this WIKI page should have been, what type of analysis is being 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 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: --------------------------- Error --------------------------- E87-20: Invalid load case for initial state. Initial state cannot be a user load case. --------------------------- OK --------------------------- 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 functionality. 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. See Also Bentley AutoPIPE External Links Bentley Technical Support KnowledgeBase Bentley LEARN Server Comments or Corrections? Bentley's Technical Support Group requests that you please submit any comments you have on this Wiki article to the "Comments" area below. THANK YOU!
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