Properties of stiffness matrix

• Order of stiffness matrix corresponds to total dofs
•  Singular stiffness matrix means structure is unconstrained and rigid body motion
• Each column of stiffness matrix is an equilibrium set of nodal force required to produce unit respective dof
• Symmetric stiffness matrix shows force is directly proportional to displacement
• Diagonal terms of the matrix are always positive i.e. force directed in say left direction cannot produce a displacement in right direction. 
• Diagonal terms will be zero or negative only if the structure is unstable.