Adult garden swings are generally two or three seaters. Most stand alone, hung from a frame. The seats tend to be made of wood or lightweight aluminium. If you're building a single-seat swing, your insurance company will prefer you use lightweight rubber swings rather than solid wood or metal swings. Solid swings, however, are far more comfortable on older bottoms. This version is simpler to build, very strong for multiple adult-sized swingers and with simple elegant lines.
Dig two post holes, each 3 feet deep, with the post hole digger. Set them 16 feet apart. Dig the holes wider at the bottom than at the top so the hole is bell shaped.
Set the two 12-foot posts in the holes. Pour 3 to 4 inches of pea gravel around the base of the posts. Mix concrete in the bucket or wheelbarrow. Do not make it too soupy or it won't be as strong when set. Pour the holes full of wet concrete to within 3 inches of the top of the hole. Square up the posts so their sides are parallel with each other, then allow them to sit overnight. Brace the posts with scrap 2-by-4s if needed.
Nail the two 16-foot cross beams to either side of the posts at the top. Bore a 5/8 inch hole through both cross beams and the post at both ends. Drive a 5/8-inch carriage bolt through the holes in the cross beams and posts. Screw on washer and a nut and bolt the ends of the cross beams tight.
Tap the two short 3-foot-6-inch posts between the cross beams. Nail them in place. Leave a gap between their ends of 3 feet in the centre and 3 feet 6 inches at either end. Drill two 5/8 inch holes 8 inches from each end of both support boards, through the cross beams and bolt the carriage bolts through cross beams and support.
Drill a single vertical 5/8 inch hole in each support board if you are hanging a two or three seater swing. If hanging a pair of individual swings, drill two vertical holes in either end for the eye bolts from which you will hang the swing chains.
Insert the 5/8 inch eye bolts up through the support beams and bolt them at the top with a washer, lock washer and nut.
Attach the swing chains to the eye bolts. A two- or three-seat garden swing will hang from the centre of the swing frame about 3 feet from the posts at either side. Two single swings will hang about 3 feet apart and almost 3 feet from the posts.
Always wear eye protection when working with power tools.