Sometimes we need to use different stress analysis codes for different parts of the model. For example:
How to combine different codes, that may use even different loads cases, different load safety factors, different elastic modulus in load cases?
We can offer 3 approaches.
Option 1. Approximate method, not very accurate, but quick. Put an anchor in the points, where code should be changed. Put the first part of the model into one project file and analyze using first code. Put the second part of the model into other project file and analyze using second code.
Option 2. This method is the best when one part of the model is made of flexible material like HDPE, thermoplastic of GRP/FRP/GRE material, but the other part is made of rigid material like steel. The steel pipe has significantly greater stiffness, so we can ignore the stiffness of plastic piping during analysis of steel piping.
Create the steel piping model with free ends at the points where it is connected to plastic pipes. Remove plastic pipes from the model, only steel pipes exist. Run analysis and remember the displacements of that connection points (where plastic connects to steel) DX, DY, DZ, RX, RY, RZ.
After that create plastic piping model and put anchors at the same points (where plastic connected to steel). Add movements to the anchors, that was previously calculated from steel piping model DX, DY, DZ, RX, RY, RZ.
This trick should work fine if stiffness of steel piping is significantly greater than HDPE piping. Otherwise the option 3 is recommended.
Some codes has a different bend and tee flexibility k-factors, and different load safety factors. Support loads, internal element forces, displacements will be different for each code. Therefore the option 3 is most accurate in this case.
Option 3. Most accurate.
1. Choose first pipe stress analysis code, for example ASME B31.1 (or plastic piping code). Create full piping model, for example including ASME B31.3 part.
2. Copy material properties from ASME B31.3 database into ASME B31.1 database. In case of plastic material, create new material in plastic piping database and call it for example "steel". Specify the elastic modulus and thermal expansion factor of steel material. Run analysis. The stiffness of steel part will be considered correctly.
3. Copy previous piping model into the new project file and change code to ASME B31.3 for example. The idea is the same, copy materials from ASME B31.1 to ASME B31.3 database or create a new material and call it "Thermoplastic" for example. Specify the elastic modulus and thermal expansion factor of plastic material. Run analysis. The stiffness of plastic part will be considered correctly.
4. Copy analysis results into MS Word or Excel and remove the steel part (or ASME B31.3) stresses in the first model, and remove plastic model (or ASME B31.1) part stresses in the second model. Or just use the cylindrical shell elements instead of pipes for the part which stresses should be ignored in the report.