The design of composite structures is considered here. The approximation concepts approach is used to solve the optimisation problem. The convex approximations of the MMA family are briefly described. Modifications of these approximations are presented. They are based on gradient information at two successive iterations, avoiding the use of the expensive second order derivatives. A two points fitting scheme is also described, where the function value at the preceding design point is used to improve the approximation. Numerical examples compare these schemes to the existing ones for the selection of optimal fibres orientations in laminates. It is shown how these two points based approximations are well adapted to the problem and can improve the optimisation task, leading to reasonable computational efforts. A procedure is also derived for considering simultaneously monotonous and non monotonous structural behaviours. The resulting generalised approximation scheme is well suited for the optimisation of composite structures when both plies thickness and fibres orientations are considered as design variables. All the studied approximations are convex and separable: the optimisation problem can then be solved using a dual approach.

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