In this paper, a mixed spectral element formulation is presented for planar, linear elasticity. The degrees of freedom for the stress are integrated traction components, ie, surface force components. As a result, the tractions between elements are continuous. The formulation is based on minimization of the complementary energy subject to the constraints that the stress field should satisfy equilibrium of forces and moments. The Lagrange multiplier, which enforces equilibrium of forces, is the displacement field and the Lagrange multiplier, which enforces equilibrium of moments, is the rotation. The formulation satisfies equilibrium of forces pointwise if the body forces are piecewise polynomial. Equilibrium of moments is weakly satisfied. Results of the method are given on orthogonal and curvilinear domains, and an example with a point singularity is given.

In structural mechanics the geometry is a crucial factor in the derivation of the governing force equilibrium equations, which describe the balance of forces in a discrete setting. In conventional discretization techniques the quantities are approximated through nodal expansions, which lead to global force equilibrium, but not local. This paper shows that by considering the geometry of the problem the equilibrium of forces can be satisfied globally as well as locally.