A typical backyard zip line can easily put 800-2000lbs of tension on its anchor points during use, so make sure your anchors are sturdy and secure.
Typical methods for anchoring include trees, wood poles, or metal poles.
•A healthy tree with a 12” or larger diameter is a great anchor.
•Trees with cracks with pervasive rot, visible disease, exposed roots, or excessive lean are not suitable for bearing the tension of a zip line.
•Protecting your tree's health will preserve its structural integrity. Put blocks between your cable and your tree to ensure this. Because the living, vital part of a tree is the thin layer under the bark, a tree can slowly starve if constricted by cable. It may take years to notice the damage, but fewer leaves, fewer branches are eventual collateral. We've seen tree-houses and zip lines completely kill their host trees. Dead trees become a safety hazard. Watch out for your beautiful tree.
•Flash flooding, and erosion can make a once reliable tree a hazard.
•DO NOT partially imbed an eyebolt into your tree and expect it to hold a zip line and rider. Eyebolts are technically less invasive to a tree's health than wrapped cable and are an acceptable anchor method. The bolt must penetrate the trunk entirely to be reinforced with a washer and nut on the back. We've heard of zip lines failing because the installer didn't back up the anchor bolt; don't let it be you!
Sometimes you will need to install your own anchor when a suitable tree is not available.
•ACCT standards for zip lines prescribe that a free-standing pole be a minimum of 12” in diameter at the top, sunk a minimum of 4’ in the ground, or 2’ plus 10% of the pole’s total length if it is greater than 20’.
•For wooden poles, eyebolts or slings should be anchored 12” or lower from the top of the pole, and any attachment that passes through the wood should have a redundant backup (i.e. a washer) on the outside that can be visually inspected.
•Supported poles can be a minimum of 8" diameter. These poles must be anchored via cable (i.e. guy wires) to other poles or ground anchors.
Ask your local power company about "retired" power poles and possible installation.
Additional depth, bracing, or guy wires may be needed for soft or unusual soil types.
Some recommended penetrator ground anchors for utility poles are the Screw and the Arrowhead.
When in doubt, a local contractor is usually a good resource for understanding soil types in your area.
Dimensional Lumber Anchors
In some circumstances when the necessary anchoring height is not very high, dimensional lumber may be appropriate for constructing a launching or landing anchor. A typical design will incorporate two 12’ treated 6x6’s or 8x8’s concreted into the ground 4’ deep with a beam spanning the two posts like a doorframe. The zip line cable is anchored to the crossbeam, and the frame is either supported from the back with guy-wires, or from the front with additional posts bracing the frame at a 45 degree angle.
Most buildings or backyard play structures are not designed to withstand the horizontal loading of a zip line cable. Design or retrofit accordingly to ensure your structure is not damaged and participants are not put at risk.
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