Define redundant force

A redundant force (or redundant) is a reaction or internal force that is not needed for the static equilibrium of a structure, so it cannot be found from the equations of equilibrium alone. The number of redundants equals the degree of static indeterminacy $D_s$.

The redundants are chosen and released to form the primary (determinate) structure, are treated as unknown external loads $X_1, X_2, \ldots$, and are solved from the compatibility equations $\sum f_{ij}X_j + \Delta_{i0} = 0$. Example: the prop reaction $R_B$ of a propped cantilever, or the horizontal thrust $H$ of a two-hinged arch.