What is static indeterminacy of beams?

In statics and structural mechanics, a structure is statically indeterminate when the static equilibrium equations - force and moment equilibrium conditions - are insufficient for determining the internal forces and reactions on that structure.

What is indeterminacy of beam?

Indeterminacy Degrees

For example, beams three and four in the illustration are said to be statically indeterminate to the first degree because they have four unknown reaction forces, while there are only three equations of equilibrium: 4 - 3 = 1.