Anchor rigging: Getting a grip on pre-tensioning.
For anchors a long way back from an edge, we often need to bring the anchor rigging and focal point forward to a better working position. An effective way to achieve this is by using multiple strands and pre-tensioning our rigging. When the load becomes live ideally there is little to no movement in the rigging.
If anchors need to be held in a particular alignment, a pre-tensioned front tie is a tool that can be used. As an example, I have used this effectively for rock anchors where the alignment of the load is important.
The techniques shown here could be useful for any type of rescue, rigging or pursuit and can be adapted to any environment.
Anchor rigging types
1. Pre-tensioned anchor rigging
Pre-tensioned rigging is usually set up as a 3:1 CD (CD=change of direction) using carabiners. Three strands of static rope or cord are used to reduce stretch over a longer rigging distance. For five metres+ use 3 strands of 10-11mm static rope. For less than five metres, an 8mm accessory cord can be used. Another fast and easy alternative is a small pre-made pulley system (e.g. 4:1CD/5:1 jigger/set of fours).
When tensioning the back tie you need a method of easily capturing the rope. For 10-11mm static rope capture with a 7-8mm Prusik. For 8mm accessory cord capture with a 6mm Prusik or two round turns tied off. A small pre-made pulley system usually comes with a 6mm progress capture Prusik. Note: An Italian/Munter hitch is not a suitable method as it is very hard to capture enough tension.
Top tips:
- You need to capture the last rope so that tension is across all three lines.
- To finish, tie off with a half hitch and overhand.
- Have the capture, tie-offs and excess rope away from the focal point to reduce clutter.
- Tension (haul) on the ropes so they are drum tight.
- Vector one of the rigging legs to remove stretch.
- Work as a team when tensioning – vector, haul, load capture locked on.
2. Adjustable anchor rigging
Adjustable front ties are used for positioning a floating focal point. You can use one 10-11mm rope strand (adjusted with a 7-8mm Prusik) or two 8mm cord strands (adjusted with a 6mm Prusik or two round turns tied off).
Top tips:
- To finish, tie off with a half hitch and overhand.
- Have the capture, tie-offs and excess rope away from the focal point to reduce clutter.
- Anchors for the front ties can be marginal as they only need to be able to resist the pre-tensioning force.
Anchor rigging methods
1. Fixed focus anchor rigging
A situation may exist where you have a forward anchor in the right position but is not bombproof e.g. a small tree. However, this small tree is strong enough in compression but not for a bending (moment) force. The small tree can be back tied and pre-tensioned to one or two larger trees (or other suitable anchors) further back.
Top tips:
- You can raise the working height of the focal point.
- Interlock the slings on the focal point anchor.
2. Floating focus anchor rigging
Another way to use back ties is when you have a floating focal point such as a rigging plate. A common way to accomplish this is to have adjustable front ties (1 or 2) for positioning and then tension with multiple strands in the back (back ties).
Top tips:
- Position the plate (focal point) just in front of where it needs to be. The pre-tensioning will bring it back due to the stretch of the front tie(s).
- Each back tie leg can be individually adjusted to get even tension.
- The focal point can be easily positioned where you want it.
3. Pre-tensioned front tie
You can use a pre-tensioned front-tie with fixed 2 or 3-point anchor rigging. This method is ideal for shorter rigging legs (less than 3m). Make sure you rig the fixed anchors with double strands on each leg to reduce stretch.
The pre-tensioned front-tie may need to be a pulley system to take the stretch out of 4-6 legs (e.g. 4:1CD/5:1 jigger/set of fours). Otherwise, rig a 3:1CD with 7-8mm accessory cord, carabiners and a suitable load capture (e.g. Prusik or two round turns tied off).
Top tips:
- Used to hold short legs in place in a particular position e.g. multiple rock anchors.
- Don’t over-use this method for short legs when gravity will do the job for you e.g. a 2-point snow anchor.
- It can be difficult to get even tension on all legs.
- For longer legs use pre-tensioned back ties as a preference.
Pre-tensioned front tie
Conclusion
Pretensioned anchor rigging is a tool.
Think before you use this style of anchor rigging; “Is this improving what I am doing or just adding more equipment and complexity?”
Use pre-tensioned anchor rigging when appropriate to bring the focal point forward, or when you need rigging to be held in a particular alignment. Otherwise, let gravity do the work for you.
By Grant Prattley, Over The Edge Rescue, 2016
