Determinate rod

The rod system shown above is DETERMINATE.

• Member loads depend on only equilibrium relations. For this problem, F₁ = F₂.
• Due to the different cross-sectional areas, the stress in (2) is twice that of (1).
• Material properties, cross-sectional areas and member lengths do not factor in on the member loads.

Indeterminate rod

The rod system shown above is INDETERMINATE.

• Both equilibrium and deformation must be included in the member load analysis.
• For this indeterminate rod, the compatibility equation relating the two member elongations is:  e₁ = -e₂ .
• Solving for member loads gives: F₁ = -2F₂ (member(1) carries twice the load of (2) since (1) because it is twice as stiff). The "minus" sign says that (2) will be in compression where (1) is in tension.
• The magnitudes of the stress in (1) and (2) will be the same.