A structural element core adapted to space apart opposed first and second, laterally and longitudinally extended, generally planar surface structures, the core comprising a closed figure pattern of ribbing defined by a plurality of periodically intersecting laterally or longitudinally disposed and vertically stacked fibers and a fiber supporting plastic matrix, the fibers and matrix being radially deflected at their locus of intersection with oppositely disposed fibers and matrix sufficiently to be vertically stacked to the same height at their locus of intersection as between. comprising opposed first and second. The core is usefully combined with one or more planar surface structures to form a structural element e.g., an aircraft panel.