The disclosure notably relates to a computer-implemented method for designing a three-dimensional (3D) finite element mesh of a 3D part that includes a lattice structure. The method includes superposing a regular tiling of cells with a solid representation of the 3D part, partitioning the cells into two groups, a first group of cells, each in contact with the solid representation, and a second group of cells, none in contact with the solid representation. The method also includes computing a Boolean union of the first group of cells and the solid representation, the Boolean union forming a volume, finite element meshing the volume of the computed Boolean union while preserving the set of faces of the first group of cells that are shared with the second group of cells, and merging the finite element meshes of the cells of the second group and the meshed volume of the computed Boolean union.