1. Derivation of element stiffness matrices by assumed stress distributions
2. Derivation of element stiffness matrices
3. Melosh, R. J. "Basis for derivation of matrices for direct stiffness method/' AIAA J. 1, 1631-1367 (1963).
4. de Yeubeke, J. "Upper and lower bounds in matrix structural analysis," Matrix Methods of Structural Analysis (Pergamon Press, New York, 1964), pp.163-202.