Procedure for sizing a structure using extracted shell forces

[enghelp2806]

Hello, I'm trying to understand how to size a structure using the loads (shell forces?) that flow through a structure. I've created a very simple representation (essentially a beam) of the actual structure (aircraft intercostal or keel beam). I know I could determine the distribution of loads given the boundary conditions and input loading, but I want to take a stab at using Patran to extract forces in order to determine sizing.

How would I do this? The way I understand it is that I'd extract the shell forces per element. Then I could sum all of the forces along one section, giving me the resultant load in a given direction? But where along the part would I take a section? For example, summing the forces at "A" inches along the length provides me with a different resultant force than at "B" inches along the length.

Thanks