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I'm about to buy hardware for a treehouse project, and am wondering about the best way to attach 3/8' galvanized cables to a 4x10 PT beam. The cables need to be attached near the ends of the beam to keep them level, since it is anchored to the tree only in the middle. Basically it's an inverted tri-beam. See the image below.

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Initially I was thinking just 5/8" galvanized lag eyes would work, but now I'm wondering if just slinging the whole beam with the cable as a loop would be cheaper and stronger. If I chose the latter, I would probably fasten 2x4s to sides to keep the loop from creeping toward the fulcrum. Here's a crude diagram of that idea:

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The other ideas I've been kicking around are 1)drilling through the entire beam at an angle and using a standard eye bolt, since this could eliminate the need for turnbuckles all together, or 2) Using a heavy duty D-Ring surface mounted to the top of the beam. But these seem like they would be weak, or worse weaken the whole beam.

My favorite so far is the loop idea, but I have a feeling I'm missing something.

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    Instead of cable, have you thought about using chain? then you could put a bolt through chain and the beam. Problem with bending the wire around the beam is it could create a weak point and rust a little more there and over time snap.
    – hookenz
    May 4, 2017 at 0:22
  • Is this beam going to be one kids play on? If kids will be around it I would to run the cable through the beam through a washer then use a wire rope clamp to keep the cable from pulling through, after that I would make a cap out of a scrap piece of 2x4 to cover the end cable or wire rope by carving / drilling a pocket to cover the clamp / wire end. Cable can cause nasty scratches or cuts on the ends if not protected.
    – Ed Beal
    May 4, 2017 at 4:18
  • Yeah, the bend is a concern. Just not sure if it's worse than drilling into the beam. No kids yet, and these cables aren't really near the main structure. May 4, 2017 at 14:34
  • What is a PT Beam? is that wood. You could hold the wire with a galvanised staple from below. That would stop it slipping back. And as well cut into the beam a little underneath to round it off and remove any stress points on the wire from the sharp angles.
    – hookenz
    Sep 13, 2017 at 23:44

3 Answers 3

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I'd do a hybrid of your other ideas: Drill through the beam as you would for an eye bolt, but simply feed the cable through and terminate it with a stop of some sort. You could loop back on itself and use a clamp. Back it with a washer to prevent pull-through of any sort.

I did almost exactly this with a circular (ring-shaped) suspended treehouse in a huge white pine at my old place. It held solid for years.

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  • I think if I was going to drill through the beam I would use an eye bolt, because I could skip the turnbuckle and just tighten the cable by tightening the nut on the underside. My concern is drilling 10" + through the beam will be tough to do straight, and wouldn't it weaken it structurally? I should also mention I would Iike my connections to be solid up to or beyond the safe working load of the cable, which is around 3000lbs. May 4, 2017 at 14:24
  • Fair enough. It won't weaken your beam. The main stress is at the middle third or so. Drilling from both sides of the beam allows precise entry/exit location.
    – isherwood
    May 4, 2017 at 14:56
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I’d say the basket or cradle is ideal except for one serious problem that’s not so much a structural issue. The problem that I would be more concerned about would be dynamic and shock loads created by wether. Another concern would be pressure treated lumber used as structural members as I’m not sure they would carry the load your putting on the beam. I’d consider looking at laminating 2x10’s and putting aluminum fascia on top and around as a flashing. Silicone where screwing fasteners through. I’d consider using a tree house tab to secure to tree. This advice is based on years of craning experience and carpentry experience but I’m not an engineer and would suggest checking into an engineer to verify for your safety.

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If you're in a climate that gets much rain (and why are you using PT, if not?) I don't like drilling into your beam, for an eye bolt or otherwise. Seems like a place for water, dirt, leaves, etc to collect and get to work attacking the wood and metal. Even PT will get soft and weaken after a while if you keep it wet and dirty.

If you loop the 3/8" cable under the 4x10, keep in mind that the steel cable will definitely not play nice with the corners on the bottom of that 4x10.

Maybe use a PT 2x4 sawzalled into a semicircle under the 4x10 to protect your beam from the cable, and relieve the bend on the cable too. Then use your "stoppers" to hold the cable in place.

You could also cut a notch in the bottom of the 4x10, to hold the loop of cable in place.

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