Abstract: Hierarchy of a structure is introduced, the force-displacement relation of a lower level is the constitutive relation of the upper level. In plate-like space truss, the compression-shortening relation of a chord is the constitutive relation of its compressed region when it is simplified as a plate. The compressed bars are of brittle nature, similar to concrete in a slab, while tension chord is similar to the tensile reinforcement. For brittle failure, the required reliability index is 3.7, greater than 3.2 for ductile failure, this implies that an addition safety factor is required for compression chords. For seismic design, brittle failure is prevented by capacity design technique, which is realized by strong column factor and strong joint factors in design check. This demands that a strong chord factor is necessary to achieve a ductile space truss. The paper proposes the value for this strong compression factor.