The spanners must be at least 72 inches long, 24 inches wide, and no more than 4 inches thick, with a minimum physical weight of 78 lbs each. Each spanner must have a rated capacity of at least 20,000 lbs (40,000 lbs per pair/axle) and be designed with a 2-to-1 safety factor, meaning an 80,000-lb axle load per pair causes no permanent deformation. Structural tubes must be 3x3 to 4x4 inches, with a top surface of high-traction rungs no more than 2 inches wide and spaced no more than 4 inches apart. Each end requires a minimum 12-inch reinforced stub toe tapering to a 2-inch blunt end, and one side must have a reinforced kickplate for flat surfaces. Each spanner must be etched with the part number and capacity.