1. Type A ――composed of a top board with double hooks, a holding down plate with teeth and a M8 inner-hexagon-bolt.
2. Type B ――composed of a top board with double hooks (one of hooks can draw together the two neighboring steel gratings),and a holding down plate with teeth and a M8 inner-hexagon-bolt.
3. Type C ――composed of a top board with double hooks and a M8 hexagon-bolt which can be welded in the supporting structure.