Abstract: A rigid caltrop structure is formed of two metallic members abutting each other and welded together, each of the members including all of a single triangular planar portion and parts of two other adjoining triangular planar portions, the pairs of adjacent corners of sides of the triangular portions forming penetration points so that when three of the penetration points rest on a horizontal surface and the fourth penetration point projects upward, a force applied to that fourth penetration point will not directly tend to shear the weld.