# 2.2 Important Concepts

- Stable/Unstable
- A
*stable*structure is one that will not collapse when disturbed.*Stability*is the capability for a structure to recover equilibrium. In general, there are many ways that a structure may become unstable, including buckling of compression members, yielding/rupture of members, or nonlinear geometric effects (P-Delta effects); however, for linear structural analysis, the main concern is instability caused by insufficient reaction points or poor layout of structural members. - Internally Stable
- An
*internally stable*structure is one that would maintain its shape if all the reactions supports were removed. A structure that is internally unstable may still be stable if it has sufficient external support reactions. An example is shown in Figure 2.1. - External Determinacy
- In an
*externally statically determinate*structure, all of the external reaction component forces may be calculated using only static equilibrium. A structure for which the external reactions component forces cannot be calculated using only equilibrium is externally statically*indeterminate*. - Internal Determinacy
- In an
*internally*statically determinate structure, all of the external reaction component forces*and*internal forces may be calculated using only static equilibrium. A structure for which the internal forces cannot be calculated using only equilibrium is internally statically*indeterminate*.Typically if one talks about 'determinacy' (without specifying internal or external), then it is*internal*determinacy that is meant. - Redundant
- Indeterminate structures effectively have more unknowns than can be solved using the three equilibrium equations (or six equilibrium equations in 3D). These extra unknowns are called
*redundants*. - Degree of Indeterminacy
- The
*degree of indeterminacy*for a structure is equal to the number of redundants. An indeterminate structure with 2 redundants may be said to be statically indeterminate to the second degree or "$2^\circ$ S.I."