The set GU_f of possible effective elastic tensors of composites built from two materials with positive definite elasticity tensors _1 and _2=_0 comprising the set U={_1,_0} and mixed in proportions f and 1-f is partly characterized in the limit →∞. The material with tensor _2 corresponds to a material which (for technical reasons) is almost rigid in the limit →∞. The paper, and the underlying microgeometries, have many aspects in common with the companion paper "On the possible effective

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... city tensors of 2-dimensional printed materials". The chief difference is that one has a different algebraic problem to solve: determining the subspaces of stress fields for which the thin walled structures can be rigid, rather than determining, as in the companion paper, the subspaces of strain fields for which the thin walled structure is compliant. Recalling that GU_f is completely characterized through minimums of sums of energies, involving a set of applied strains, and complementary energies, involving a set of applied stresses, we provide descriptions of microgeometries that in appropriate limits achieve the minimums in many cases. In these cases the calculation of the minimum is reduced to a finite dimensional minimization problem that can be done numerically. Each microgeometry consists of a union of walls in appropriate directions, where the material in the wall is an appropriate p-mode material, that is almost rigid to 6-p≤ 5 independent applied stresses, yet is compliant to any strain in the orthogonal space. Thus the walls, by themselves, can support stress with almost no deformation. The region outside the walls contains "Avellaneda material" that is a hierarchical laminate which minimizes an appropriate sum of elastic energies.