How to find worst case of concentrated forces?


I am modelling a pipe system where concentrated forces and moments are applied (in Gravity case) at the interface connection points. Is there a way to check which sign combination that gives the worst result regarding stresses and restraint forces? F.ex. for applied F1, F2, M3, M4, the worst combination could be -F1, +F2, -M3, +M4, etc.

Appreciate any answer.


  • Hi Heidi,

    I do not fully understand your question. Do you want to see the effects of changing the sign of your input forces/moments on the results that are generated? Can you provide a small example?

    If you want to see the effect of changing the sign of your applied concentrated forces/moments, it may be a good idea to apply these using different load cases (for example user load cases). This will ensure that you get to see different load cases which you can then combine in the post processor to determine your worst combination.

    If you can elaborate the question, I may be able to help further.



    Bilal Shah

  • Yes, my intention was to see the effects of changing signs of input forces and moments on the results.

    As I am to design the pipe supports for a pipe system onboard a gas ship, I want to find the maximum restraint/support load occurring for each support point. The loads (from a flexible riser) are applied at interface flanges at four points, and their signs change.

    How can load cases be combined using the post processor? Is the post processor included in the Advanced version - do you mean the Result grid/Output file?

    Thank you.
  • Heidi,

    I think in this case you would probably need to define the combinations of these applied concentrated loads/moments using different load cases. For example, if you had two input force values say FX and FY, both of which could reverse in direction you can do either of the following:

    Insert the forces in individual load cases:
    U1 = FX
    U2 = -FX
    U3 = FY
    U4 = -FY

    Then you can create the following result non-code combinations (Load > Combinations > Non-Code combinations) with Combination method as Sum:
    Summation method: Sum
    +FX+FY [Case 1 = U1, Case 2 = U3]
    +FX-FY [Case 1 = U1, Case 2 = U4]
    -FX+FY [Case 1 = U2, Case 2 = U3]
    -FX-FY [Case 1 = U2, Case 2 = U4]

    You may then be able to output the maximum load on your support out of these combinations using another combination with combination method set to say Sign Max:

    Summation method: Sign Max
    MaxFXFY [Case 1 = +FX+FY, Case 2 = +FX-FY, Case 3 = -FX+FY, Case 4 = -FX-FY]

    Alternatively, you may define the input load combination +FX+FY in your load case U1, +FX-FY in load case U2 and so on. And then create another combination with U1, U2, U3 and U4 to get your worst case scenario.

    From post processor I meant the combination dialog, which is part of all AutoPIPE editions.

    Hope this helps.


    Bilal Shah

  • Thank you for suggesting a solution. However, it will take a while to implement as I have six forces (FX, FY, FZ, MX, MY, MZ) and need 2^6 = 64 load cases. I will make the best of it.

    Again, thanks for your quick reply.