Thanks for the answer.
In fact I already succeed to do what I wanted by doing what you mention.
What mingled me is the help page on the Load.GetNodalLoads that says that the force arrays (FXArray, FYArray...) "return the load array for all the load cases and all directions". I understood that each case of the arrays corresponds to the force applied in one load case.
In fact, the function Load.GetNodalLoads retrieve the loads in the active load case for a specific node and strore the in the different cases of the arrays depending of the order.
One last typo I found is in the Load.GetActiveLoad help page. It said that we specify the function that way "lActiveLoad = objOpenSTAAD.GetActiveLoad" but it should be "lActiveLoad = objOpenSTAAD.Load.GetActiveLoad" instead.
Julien Richard
HATCH