Creating and simulating a structure in python

I have included what I have so far; the part I’m stuck on has a comment labeled ‘M1 and M2 fixed’. I need to finish Part 2 ‘first steps’ steps 2 – 4, which can be found of the last page of the document labeled ‘MECH203_Design_Istruction_Part2_Final’. And I need to do Part 3 which is creating a structure.

I want to make a truss bridge that is as cheap as possible to support a given load (the load is up to you, as long as it’s reasonable. Reasonable meaning, the force is what you would normally expect a bridge to hold, with traffic).