For a change, let’s look at a topic about a fun benefit of trees: swings.
Upon seeing a client’s coast live oak with a swing on it, attached with chains wrapped around a limb, I recommended revising the attachment to avoid girdling wounds around the limb by having a tree service drill vertical holes through it, install eyebolts and hang the swing from the eye-bolts.
The client later responded:
We have questions about eyebolts, chains, swings and total weight an oak tree branch can handle.
Our tree service will put in eye-bolts but they say the current chains are inadequate. We agree but how much weight can the oak tree branch handle? We have calculated that the eyebolts, chains, and swings will weigh approx. 60-70 lbs. plus the weight of the riders (150-400 lbs.). This seems like a lot but we don’t have a clue.
Could you give us your opinion as to how to proceed? We would greatly appreciate it.
As far as I know, there is no formula to answer that question. I am an arborist, not an engineer! I rely on International Society of Arboriculture Best Management Practices book coupled with experience and common sense. For one thing, the weight of the hardware is negligible compared to the weight and momentum of the swinger.
Trees evolved to handle dynamic loads. Think about the forces exerted in a wet and windy storm. They can handle a lot more than the static weight of wood and foliage, depending on structural integrity in relation to the load.
Here are a couple of key points:
Movement of the limb under load will tell you a lot.
The load is multiplied as the swinger swings higher and comes down with increasing force.
The International Society of Arboriculture Best Management Practices booklet on Tree Support Systems provides help, at least with respect to attaching hardware to a tree limb.
It verifies that eye through-bolts, eye-lags, or J-lags are acceptable (at least for cable systems) and provides a table for hardware size in relation to the limb diameter and expected load.
For example: A 10-inch diameter limb would get a 3/8-inch minimum diameter forged eye-bolt or a 5/8-inch J-lag for an estimated load of 900 pounds.
Looking into it further I found a good source of guidance from “International Association of Certified Home Inspectors (InterNACHI).
It mentions, as a general guideline, to put swings on limbs at least 8 inches in diameter and to limit swing installations to certain hardwoods: species of oak, sycamore and maple and cautions about the increasing load as the rider swings downward.
Key points covered in the article include inspection for:
Decay and defects in the limb and tree and branch attachment.
Dead or broken branches above the swing.
Ground beneath the swing should be loose mulch or other soft material, and free of potentially harmful obstacles.
Above-ground clearance below the loaded swing should be about 10 inches.
The rope (if not a chain) should be thick enough for the load and thin enough for a child to easily grip—about 1—1/1/2 inches.
To evaluate the limb, test it with increasing weight loads. Watch its bending response. When it no longer bends further downward and only wobbles from side to side, more or less, it is nearing maximum load.
Unfortunately, we’re struck with a “straw that broke the camel’s back” phenomenon. A small increase in load could act on a small, possibly unnoticed defect that propagates into a complete failure.
If you want a swing to serve as heavy-duty playground equipment or circus trapeze for big people, build it of steel or timbers!
Otherwise, take it easy.