NEW STRATEGY FOR ASSUMED STRESSES FOR 4-NODE HYBRID STRESS MEMBRANE ELEMENT

An oblique (skew) co-ordinate system defined at the origin of the isop arametric co-ordinates is introduced as the reference co-ordinate syst em for assumed stresses in the formulation of a 4-node hybrid membrane element. The initially uncoupled stresses in these reference co-ordin ates can be constrained to satisfy the equilibrium conditions pointwis e by using the procedure of Pian and Wu7, when suitable incompatible d isplacements are introduced. Such strategy allows the element stiffnes s to be formulated by using either the Hellinger-Reissner or the compl ementary energy functional. Numerical results show that the performanc es of three elements, constructed by using different sets of incompati ble displacements in this paper, are practically identical and are all of slightly higher accuracy than that of the Pian-Sumihara element.6