What is statically determinate structure?

A statically determinate structure is one that is stable and all unknown reactive forces can be determined from the equations of equilibrium alone. A statically indeterminate structure is one that is stable but contains more unknown forces than available equations of equilibrium.

How do you know if a structure is statically determinate?

A truss is considered statically determinate if all of its support reactions and member forces can be calculated using only the equations of static equilibrium. For a planar truss to be statically determinate, the number of members plus the number of support reactions must not exceed the number of joints times 2.