Saturday, July 30, 2011


Hilary and I got Brynna a pair of Big Girl Swings for her birthday (shhh, it's a surprise!). These would replace the utterly decrepit and downright dangerous swings that were attached to the swing set when we bought the house. For some time now, Brynna has been content to swing in the baby swing, after I restrung it properly with new hardware. But she's more or less outgrown that and is ready to move up to the kind of swings she likes at the park. We decided to get two so that there's one for when friends come over.

That's all well and good except for one thing: the structure of the swing set itself is definitely not up for the increased load. It may have been designed for it, and it may have been capable of it when it was first built, but at this point it is decidedly not up to snuff. The main structure has a very noticeable 10-degree list. When Brynna swings with just the little swing, the crossbar it hangs from bows back and forth, rocking the whole structure along with it. I tightened up numerous bolts a few months back, but hardware can always loosen up. I had to add reinforcement to one end just to be able to string the closeline this summer without greatly distorting the shape of the thing. In general, I've found the choice of construction to be greatly lacking as well. 4x4s for vertical posts, fine, but 1x4s for crossbracing? The framing is square all around. Have these people never seen a proper truss? See all the triangles? Squares and rectangles can, as demonstrated here, get pulled out into parallelograms. Suffice it to say: if I were to design and build a platform/swingset, I would have made very different decisions.

In any event, I decided that I wouldn't put up B's new swings until I had done some reinforcement work. I started by loosening many of the bolts. Then I took off the ladder. These would make it easier for me to pull the thing back into square. This I did with a sort of block-and-tackle arrangement.

Into the ground I drove two 12" garden spikes. On each was a pulley. The rope got strung from one stake, through a pulley on the structure, through a pulley at the first stake, through another pulley on the structure, through a pulley on a second stake, then tied off to the structure.

In all, there were the equivalent of five ropes strung between the ground and the structure. When I pulled on the free end, my pull was multiplied 5x into force pulling the structure back into plumb. By my guess, I had 50-75 lbs of tension in the line, or about 250-375 lbs of force tugging on the structure up high. This was enough to bring the posts back to vertical.

With the pulley system still in place, I applied Loctite to all the bolts and tightened them back down. The red loctite should ensure that these bolts don't ever loosen again.

I also added a diagonal brace between the posts to help keep it square (triangles, people!).

The cross piece that the swings hang from is basically a ladder. It's missing several rungs, and two were pulled out from there holes and dangling. So when Brynna was swinging before, most of the load was going through one single 1x6. Lovely. I put the rungs back into place and tightened the hardware at either end. Then, for good measure, I also added diagonal bracing across the top of the "ladder" to tie it all together.

This done, I feel like I will be able to put up the swings tomorrow without being deathly afraid of something breaking. And, gosh, it only took four hours of hot work on a Saturday.

If I'd designed and built it, it would have been a freakin' tank!

1 comment:

Clara said...

Ah engineers! Every one of you convinced that there is no such thing as over-engineering....

When my dad put an addition on our house while I was growing up he decided that the fixtures for bracing the roof were "not good enough". He had the local black smith fabricate some 1/4in thick steel angle pieces to brace the roof trusses. The code compliance guy came by, took one look at that, and then never showed up again.