>>137I have experience at university with Stage 2 'Fluid Mechanics' and 'Solids & Structures'
I'd say it's a simple resultant force of the distributed pressure loads acting on beams in the x and y directions (where z is vertical).
How do you analyse a large 3D structure? Idk. It's either something simple similar to the 2D Method of Sections but in 3D or it's something complicated with Spreadsheets 3.