In this study multiple cores sandwich composites undergoing impact loads are optimised in order to improve their strength at the onset of delamination. The optimisation is carried out finding the distribution of the stiffness properties which minimizes the interlaminar stresses. The importance of using refined structural models is emphasised. A 3D Zig-Zag model and a mixed solid element with interlaminar stresses and displacements as nodal d.o.f. are considered as structural models. The former is used within the optimisation process. The latter, which uses a unique set of C° standard tri-linear shape functions for all the d.o.f., i.e. fulfils the intra-element equilibria in an approximate integral form, is used to assess the stress fields of the optimised solution. The onset of delamination is predicted using stress-based criteria. It appears that the optimisation process is able to reduce the interlaminar stress concentrations, with beneficial effects on the delamination resistance for all the sample cases considered.