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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.

enter image description here

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:

enter image description here

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. – Matt May 4 '17 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 '17 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. – Jared Rogers May 4 '17 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. – Matt Sep 13 '17 at 23:44
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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.

  • 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. – Jared Rogers May 4 '17 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 '17 at 14:56

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