2.5 Statically Indeterminate Problems If the equilibrium equations are sufficient to calculate all the forces (includ ing support reactions) that act on a body, these forces are said to be statically determinate. In statically determinate problems, the number of unknown forces is always equal to the number of independent equilibrium equations. If the number of unknown forces exceeds the number o independent equilibrium equations, the problem is said to be statically indeterminate
Static indeterminacy does not imply that the problem cannot be solved; it simply means that the solution cannot be obtained from the equilibrium equations alone. A statically indeterminate problem al has geometric restrictions imposed on its deformation. The mathematical expressions of these restrictions, known as the compatibility equations, provide us with the additional equations needed to solve the problem (the term compatibility the geometric compatibility between deformation and the imposed constraints). Because the source of the compatibility equations is deforma- tion, these equations contain as unknowns either strains or can, however, use Hooke's law to express the deformation measures in terms of stresses or forces. The equations of equilibrium and compatibility can then be solved for the unknown forces.