Static Indeterminacy of Structures

If the number of independent static equilibrium equations (refer to Section 1.2) is not sufficient for solving for all the external and internal forces (support reactions and member forces, respectively) in a system, then the system is said to be statically indeterminate . A statically determinate system, as against an indeterminate one, is that for which one can obtain all the support reactions and internal member forces using only the static equilibrium equations. For example, for the system in Figure 1.10, idealized as one-dimensional, the number of independent static equilibrium equations is just 1 ( ), while the total number of unknown support reactions are 2 ( ), that is more than the number of equilibrium equations available. Therefore, the system is considered statically indeterminate. The following figures illustrate some example of statically determinate (Figures 1.11a-c) and indeterminate structures (Figures 1.12a-c).