Based on Newton's laws of motion, the equilibrium equations available for a two-dimensional body are:
- : the vectorial sum of the forces acting on the body equals zero. This translates to:
- Σ H = 0: the sum of the horizontal components of the forces equals zero;
- Σ V = 0: the sum of the vertical components of forces equals zero;
- : the sum of the moments (about an arbitrary point) of all forces equals zero.
In the beam construction on the right, the four unknown reactions are VA, VB, VC and HA. The equilibrium equations are:
- Σ V = 0:
- VA − Fv + VB + VC = 0
- Σ H = 0:
- HA = 0
- Σ MA = 0:
- Fv ⋅ a − VB ⋅ (a + b) − VC ⋅ (a + b + c) = 0.
Since there are four unknown forces (or variables) (VA, VB, VC and HA) but only three equilibrium equations, this system of simultaneous equations does not have a unique solution. The structure is therefore classified as statically indeterminate.
To solve statically indeterminate systems (determine the various moment and force reactions within it), one considers the material properties and compatibility in deformations.
If the support at B is removed, the reaction VB cannot occur, and the system becomes statically determinate (or isostatic). Note that the system is completely constrained here. The system becomes an exact constraint kinematic coupling. The solution to the problem is
If, in addition, the support at A is changed to a roller support, the number of reactions are reduced to three (without HA), but the beam can now be moved horizontally; the system becomes unstable or partly constrained—a mechanism rather than a structure. In order to distinguish between this and the situation when a system under equilibrium is perturbed and becomes unstable, it is preferable to use the phrase partly constrained here. In this case, the 2 unknowns VA and VC can be determined by resolving the vertical force equation and the moment equation simultaneously. The solution yields the same results as previously obtained. However, it is not possible to satisfy the horizontal force equation unless .
Statical indeterminacy is the existence of a non-trivial (non-zero) solution to the homogeneous system of equilibrium equations. It indicates the possibility of self-stress (stress in the absence of an external load) that may be induced by mechanical or thermal action.[disputed ]