How to build a 2 post wooden swing for kids & adults?

[rebelliouskiwi]

I was wondering what timber I should use to create a 2 post swing into the ground, similar to the picture below? It will need to withhold up to 150kilos I guess? It will be a swing for kids and adults. 

Should I be protecting the timber somehow? Ideally, I want it to be smooth, so if I sand it, it's going to need protection again isn't it? I don't want to paint, as I want it to keep the timber look. But varnish/ oil / stain is fine. 

This picture shows black L brackets, is that something that would be recommended here, or would timber braces across the corners (from top beam and post) be better?

Which concrete mix would be recommended for this project? The swing will be going into the ground. Currently grass and very sandy soil. 

Hardware - bolts for the frame & hardware for the swing to attach to, something that is going to withstand harsh sun as there is no shade or trees around the spot where it will be positioned.

I am located near Blue Mountains in NSW. I notice that some timbers that get recommended in past projects haven't been available to me in NSW. 

Many thanks! Hopefully I will be able to make this for my tween & teen kids for Christmas, that will last a long time, that Mum & Dad can enjoy too 😉😂

[JacobZ]

Hi @rebelliouskiwi,

Being outdoors and in direct contact with the ground, you will need to use H4-treated timber, which is treated for exposure to weather and direct contact with the earth. My recommendation would be this 88 x 88mm 3.0m Post F7 H4 Treated Pine Premium KD, which is unfortunately not stocked at Bunnings Katoomba, but is available at Bunnings Lithgow and Bunnings Valley Heights. These posts are DAR, meaning dressed all round. This means they were originally milled to 90x90 and then sent through a planer to create a smooth, finished surface.

Being H4 treated timber, there is no need to coat the timber with anything, although the cut ends of the timber would benefit from being coated with Tanalised Clear Enseal Timber Treatment for some added protection.

The general rule of thumb with posts, especially when they need to carry weight, is that no less than 30% of the overall height of the post is underground, surrounded by concrete, so if you were using 3 metre posts, 900mm would be below ground, encapsulated in concrete, with 2100mm above ground. This will ensure the posts are good and sturdy, and they will stay where they are long-term.

For concrete, post-mix concrete is the way to go. 
 

For the brackets at the top, the closest I could find are these Simpson Strong-Tie 210 x 210 x 75mm Black Outdoor Accents Strap Tie L Plates. These brackets are typically used with pergolas and similar outdoor structures, so they are very strong. They use Simpson Strong-Tie 5.6 x 90mm Outdoor Accents Structural Screws with these Simpson Strong-Tie Black Outdoor Accents Hex Head Decorative Washers. Looking at the availability in your area, it is likely you'd need to Special Order them.

Let me know what you think and if you've got further questions or any concerns, please let me know so I can assist further.

Jacob

[MitchellMc]

Hi @rebelliouskiwi,

I just wanted to add that apart from the weight of the two adults, you need to factor in the immense amount of force applied to the structure whilst swinging back and forth. This is exponentially higher than the weight of the occupants. I'd actually recommend you speak with your local store's Special Orders team to see if you can at least order 125mm or, better still, 150mm posts. I'd also be not only be using the brackets Jacob suggested, but some long landscape screws through the top beam and down into the posts.

If you haven't built something like this before, I strongly suggest enlisting the services of a professional to assist or at least look over your plans.

Mitchell