Analysis to try and find the required sections for a telescopic beam

[sunbite]

Homework Statement

Hi all. This is for a piece of Design coursework to design a rowing machine. I want to have the central beam the seat slides on be constructed from four individual beams the can telescope down, each with a single welded support. I'm really struggling on starting my (probably rather basic) analysis. The first step is to make a free body diagram but I am confused as to whether I should take the beam as a whole or as each individual section. Can anybody offer any tips?
The main problem I have just now is that if I take each individual beam as a section they only have one point of support (imagine an L rotated by 90°) surely any analysis would result in it tipping over. Also do I apply the force of the user 1090.9N to each section individually as in reality that force is only applied once across all four beams

Homework Equations

Just now I don't have any as until I get to shear force and bending moment diagrams its all graphical techniques or trigonometry. Eventually I think I use σ=My/I (the engineers bending formula) but that's later on

The Attempt at a Solution

I sketched the free body intially as the outline of the entire frame, but now I'm confused as to whether I have drawn it correctly

 

[Valkarie]

. The force of the user (1090.9N) is only applied once, but am I right in saying that the force will be distributed across each individual beam? And if so, how much force should I put down for each beam? Also how do I account for the fact that each beam has only one point of support?