After determining the unknown displacements, the other forces are calculated satisfying the compatibility conditions and force-displacement relations. Primary unknowns are the displacements and force-displacement relations are computed and subsequently, equations are written satisfying the equilibrium conditions of the structure in this method. Three categories of engineering structures are considered: a) Frames: contain at least one one multi-force member, i.e., member acted upon by 3 or more forces. Once the redundant forces are calculated, the remaining reactions are evaluated by equations of equilibrium. (vertical reaction at support D ) and each one will help us.

Solving these equations, redundant forces are calculated. For example, we can construct influence lines for (shear force at B ) or (bending moment at) or. Primary unknown are forces in the members, and compatibility equations are written for displacement and rotations (which are calculated by force displacement equations) in this method.

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