The space-equilibrium generator
Three legs, three equations, one answer. A rigid top on three legs carrying 100 kN at (0.4, 0.25) m. The three equilibrium equations available — one vertical and two moments — leave three unknowns, so the system is exactly determinate and the reactions are 60.4, 3.1, 36.5 kN. Move the load anywhere and the answer moves with it; nothing about the legs' stiffness enters.
2 essays call
space-equilibrium. The drawing above is what it returns with no arguments at all; every
call below passes it something, because a placement that passes nothing draws whichever
member of the family the generator happens to default to rather than the one its essay
argues about.
Where it is called
Changing this generator changes every one of these figures.
Six equations, and the drawing shows three
Ninety essays have taken place on a piece of paper, where equilibrium is three equations and a structure is a diagram. The real object has six, the extra three are the ones nobody writes down, and the difference between three legs and four is not a matter of degree.
Internal forcesThe internal force with no diagram
A cut through a member reveals four things, and this collection has drawn diagrams for three of them. The fourth is a torque, it obeys exactly the same rules, and whether it exists at all can depend on a decision the designer is free to make.