Multi-Point Highline Anchors
The difference between multi-point and single-point anchors is that multi-point anchors share load across multiple points to create a solid masterpoint, wherein a single point anchor only has one point to carry the load. This masterpoint should be very strong and easily handle the load of the entire highline rig. To create a multi-point anchor system we need main anchoring materials that are capable of connecting all points together. You also need backup anchoring material to protect against a main anchor failure. If any one point in the system were to fail you want something to be in place to catch it, limit extension, limit shock loads and/or protect against complete anchor failure.
There are a few different styles of anchor configurations emerging throughout the world. Each are safe and effective but there are some common themes running throughout.
Multi-point Anchor Configurations
To build a the main anchor across multiple points there are two commonly used methods: The Sliding-X configuration and the figure eight master point (also called BFK, which stands for Big Freaking Knot). Both have their benefits and drawbacks.
A Sliding-X configuration works by connecting all the bolts/anchor points in a way that if one were to fail, the anchor would remain intact, extend and then catch on the connector at the masterpoint. There would be no way for the entire anchor to come undone.
The big advantage of this method is that the masterpoint can move and distribute the load across the different anchor points rather well. It also gives better departure angles with the same amount of rope compared to the BFK because there is no knot involved taking up space. The main drawback to this system is the potential for extension in case of an anchor point failure. Imagine one of the bolts in the picture above failing. This would lead to the rapid extension of the anchor and cause a shock load when the rope from the failed point catches on the masterpoint. To check that your sliding-X is rigged correctly, you simply make sure that each strand of rope lines up with the same side of the bolt and device; right to right, left to left.
The BFK is slightly trickier to create, as you have to be very sure of the highlines direction of pull and orient the knot accordingly. Additionally, there is a certain reduction of the breaking strength in the knot but because the knot contains so many strands of rope and is so big that the reduction is minimal. If you manage to distribute the load evenly, it has an advantage over a sliding-x of not being able to extend. In case of bolt or anchor material failure, the additional load is immediately transferred to the other bolts with minimal shockloading and extension, all the legs of the knot are isolated from each other.
Don't Do This
A method that you should avoid in every case is the American Death Triangle (ADT). This is already an infamous anchor in climbing and we would hate to see that happen in highlining too.