Statically indeterminate structure

Summary

A structure is statically indeterminate when the equations of static equilibrium are not enough to find all its support reactions and internal forces. The extra unknowns are fixed only once the deformation of the members is taken into account, through compatibility conditions and the material law. The number of extra unknowns is the degree of indeterminacy. Such structures are common because the redundant members give stiffness and safety; they are analysed by the flexibility method or the direct stiffness method.