Snader's second example is great. If you need to, you can reduce the tris even more by getting rid of the center edge and rotating the vertex normals outwards to make the planes look rounded.
Okay, I might be completely wrong on this, but unless the game is a swing pushing game I'm thinking you would just use simple planes on that. Like if this was a first person shooter game and you walked up to the swing set, those chanins are so small,I don't think it would warrent modeling them out. So if this is just a…