Will this bracket support a swingset?

[Kate R.]

My questions are:
- Will a proposed bracket be able to support 7500 lb. without undue or permanent distortion or failure?
- Can the proposed dimensions be improved?

The bracket will be attached to a large tree and will support one end of the 4x6 redwood swingset beam. (On the other end: an A-frame of 4x4s.) The attachment includes bracket sketches and a rough drawing of the swingset. Appropriate brackets are not commercially available, so we need to get one made.

Constraints:

- For the tree's health, one bracket attached by one big bolt is better than several brackets.

- To allow for tree movement the beam is not fixed to the bracket, and the bracket is wide enough to allow side-to-side slippage. To keep the beam from popping out the top, the bracket is oversized in height and will have four small additional holes to hold cables or bolts across the top.

- The biggest force on the bracket will stem from use. Based on information on other websites, I've calculated that if 220 lb of children swinging in unison on an 8' radius fast enough to do a 360 around the beam, the centripetal force at the bottom of the swing will be less than 1400 lb. The beam and swing equipment will likely be under 100 lb. A 5x safety factor yields 7500 lb. Clearly the bracket will get a lot of movement in several directions.

- The bracket will likely be 1/4" stainless steel -- could be cold- or hot-rolled plate but custom galvanizing costs $$$. Presumably the bracket will be made of plate welded at the corners.

The proposed bracket will have a J shape with dimensions (assuming 1/4" steel):
- Bottom 5.5" (x 6"?)
- Short vertical side 8" high (x 6"?)
- Tall vertical side of 10" high (x 6"?)

The least-certain dimensions are the width of the bracket (i.e., parallel to the long dimension of the bracket), and the thickness of the steel. For example, would we be better off with a bracket of 6" width and 1/4" steel, or 4" width and 3/8" steel? Is the whole thing over- or under-engineered?

I will be grateful for any help you can give me. Thanks!

 

[Studiot]

- For the tree's health, one bracket attached by one big bolt is better than several brackets.

The bracket looks good, I prefer the 6" version. Quarter plate with the stiffening gusset as shown will be fine.

However I don't like the single point fixing for two reasons.

Firstly it could fail suddenly without warning.

Secondly the swinging action will cause leverage of the bracket against the fixing point tending to work the fixing loose.

I am not a tree specialist, but I do know that you need to be careful with the fixing materials. Copper, for instance will kill the tree.

Will you be serving ice cream on open days?

 

[RandomGuy88]

Be sure to calculate the proper bolt diameter since only one bolt will be taking all of the load in shear.

I would be worried about the bolt being ripped out of the tree by the loads, especially if the bolt is not very deep in the tree. Although I have never bolted anything to a tree so I don't know what they are capable of taking.

But I would like to say that this would have been a great problem in some of the mechanical engineering courses I took. Very practical.

 

[nucleus]

This reminds me of this cartoon:
http://www.businessballs.com/treeswing.htm

 

[xxChrisxx]

My questions are:
- Will a proposed bracket be able to support 7500 lb. without undue or permanent distortion or failure?
- Can the proposed dimensions be improved?

Do you know if that design will holf 7500lb as are you asking? Is it your own design or copied from somewhere else?

Constraints:

- For the tree's health, one bracket attached by one big bolt is better than several brackets.

Sod the trees health. Health of the humans going to be swinging on it takes precedence. Use more bolts. If you really don't want the tree to be harmed don't put a swing there.

- The bracket will likely be 1/4" stainless steel -- could be cold- or hot-rolled plate but custom galvanizing costs $$$. Presumably the bracket will be made of plate welded at the corners.

Just use a mild steel and paint it.

The least-certain dimensions are the width of the bracket (i.e., parallel to the long dimension of the bracket), and the thickness of the steel. For example, would we be better off with a bracket of 6" width and 1/4" steel, or 4" width and 3/8" steel? Is the whole thing over- or under-engineered?

Don't worry about that, this isn't precision stuff so it can be massively over engineered. Factor of acceptence of 5 seems like a sensible amount. I'd be more worried about fatigue if the FOA was below 5.

 

[nvn]

Kate R.: Because the bracket is wider than the beam, it appears the beam will wobble in and out of the page while someone is swinging. Furthermore, even if the J bracket were snug against the beam, the entire bracket could exhibit this same wobbling problem, in and out of the page, because the bracket currently has only one lag screw. Therefore, the connection currently appears unstable. I would say, make the J bracket the same width as the beam. And use two lag screws; one at the top of the bracket in the current location, and one at the bottom of the bracket, below the beam, directly below the top lag screw.

It seems any design that allows the tree to grow outward will cause the connection to be unstable, and will therefore allow the beam to wobble in and out of the page when someone is swinging.

 

[blimkie.k]

I'm not a licensed engineer...yet, but here is my 2 cents on fastening the bracket.

It seems any design that allows the tree to grow outward will cause the connection to be unstable, and will therefore allow the beam to wobble in and out of the page when someone is swinging.

Yea every time the swing comes outward its going to pull that bracket with it. I would consider putting a bolt long enough to go completely through the tree with enough extrusion at either end to allow for future tree growth. Fasten on the side opposite the bracket it with a large enough washer that it cannot pull through the tree and 2 bolts. 2 bolts allow you go tighten them against each other to lock them in place. If you want an added safety factor cap the ends of the bolts with some rebar capping. This is used in construction almost everywhere to prevent cuts and scrapes and prevent a potential falling worker from impalement. Check out my attached ms paint sketch.

Every year you can loosen the bolts off a couple mm or however needed to allow the tree to grow. You see trees growing around old pieces of metal or decks all the time so I figure after a couple years of growth the tree will tighten up around the bolt like its not even there. You could even spray the hole around the bolt with a pruning agent to prevent bugs from entering the hole.

 

[PaulS1950]

I am more concerned that the beam is not fixed to the bracket nor is it supported on the other end from colapse to the side.
I would either slot the beam and install a bolt through it or use a tripod at the "A" frame end to prevent it from pulling away from the bracket.

 

[Kate R.]

Wow! Thanks for the thoughtful replies.

Re the single bolt for the sake of the tree:
In this case, protecting the bolt support means protecting the tree. The live growing tissue (cambium) and vascular structure is just beneath the bark. Nearly all vascular transport is parallel to the trunk or branch; very little flow occurs laterally.* If there are two bolts near each other in the same branch or trunk, the tree is likely to "compartmentalize" the damage, that is, abandon and wall off the cambium and vascular tissue between the bolts. The abandoned area may shrink as the wood dries and ultimately rot. Net result: no bolt support.
The plan has been to run the bolt through the tree and use broad washers on both ends. One option would be using an eye-bolt or eye-nut, and cabling the far end (with turnbuckle) to another branch (see picture). A bolt all the way through is much less of a problem for the tree than two on the same side, and it's much like what is done when a huge old tree is cabled to support heavy branches. The resulting support isn't the same as two supports on the swing beam, but it may help.
I'm looking at a 3/4" or 7/8" bolt. Also at getting the bracket made with the open top of the J closed (would this be called a "d" bracket?).

Re allowing the swing beam to move inside the J-bracket:
Trees move every time the wind blows. If the swing beam is fixed to the J-bracket, the support on the other end has to compensate for the movement. My thought has been instead to overbuild the A-frame support for the other end of the swing beam. This could include anchoring the posts in cement, like a fence. The suggested tripod is also an option.
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* A nice illustration of this flow up and down rather than sideways: if one leaf on a sunflower plant is given a pulse of CO2 labeled with carbon-14, the radioactivity will show up in the flower in only one radial section (like a pie slice, only curved). If the labeled CO2 is applied to a tomato leaf, it shows up in a small part of the root system.
(See: M. Alkio, W. Diepenbrock, E. Grimm, 2002, Evidence for sectorial photoassimilate supply in the capitulum of sunflower (Helianthus annuus), New Phytologist (good illustrations); and T.M. Bledsoe, C.M. Orians, 2006, Vascular pathways constrain 13C accumulation in large root sinks of Lycopersicon esculentum (Solanaceae), American Journal of Botany (free download on Google Scholar).