Structural Engineering

One whose support reactions and internal forces can be found from equilibrium equations alone (no compatibility needed).

One with more unknown reactions/internal forces than available equilibrium equations, so compatibility conditions are also required.

Three: ΣFx = 0, ΣFy = 0, ΣM = 0.

Number of unknown reactions/internal redundants minus the number of available equilibrium equations.

If m + r = 2j it is determinate; m + r > 2j is indeterminate; m + r < 2j is unstable (m members, r reactions, j joints).

It lacks sufficient or properly arranged supports/members to resist applied loads, allowing rigid-body motion or mechanism formation.