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Hey guys and gals, 1st question here. I’m having my wedding in my backyard and my fiancé insisted on having a light canopy of bistro lights in my back yard. Of course I obliged, but the quotes I got for temporary lights were 4 -5k, so I decided to set up permanent ones myself.

I've only cemented in the posts, which are the blues dots, and the green and red lines were my plan until my neighbor said that it won’t work. I believe my design is sufficient, but your input would be appreciated.

Design:

  • Blue dots: 13 ft 4" galvanized steel square posts, 3ft deep and 18 inches wide concreted in
  • Green lines: guy wires for lights
  • Red lines: lights with guy wire

The photo has the measurements of the yard, the light run is over 55', probably around 65' since it’s running at a diagonal.

I'm considering putting posts in the middle of the green line if necessary, but I'd prefer not.

Thoughts?

Update: I do have two extra posts, I didn’t want to add them, but would be a more feasible set up? The yellow dots are the additional posts. enter image description here

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    I don't see a problem with your plan. Be more specific about the concern your neighbor raised. What's the question? – isherwood Oct 30 '18 at 14:57
  • He said he believed the poles wouldn’t support the load. This isn’t his field, he was just stating his opinion but it planted the seed of doubt in my mind. – JwaddPHx Oct 30 '18 at 16:23
  • Please take the tour to learn what's expected of you when you ask a question. Hint: "Thank you" comments aren't appropriate on SE networks. – isherwood Oct 30 '18 at 20:34
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This will work fine, with these caveats:

  1. The posts will certainly be strong enough. The question is whether the soil surrounding their concrete bases will give, allowing the posts to tilt. Worst-case scenario you add guy wires down to the ground as support.

  2. Your primary (perimeter) wires should be 1/8" steel wire rope, at least, but light enough to not add to the challenge. You'll need as much as several hundred lbs. of tension on them (with the cross wires in play) to reduce sag far enough. Synthetic winch rope could also work well, but won't be cheap.

  3. Your cross wires will also need to be able to handle 50 lbs. tension or more. Picture wire might do, or 1/16" steel wire rope.

If you can accomplish all that I think you'll have a fantastic wedding scene. Good luck, and GOOD LUCK. ;)

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I think doing really solid engineering calculations on this, which is always the right thing to do, would be pretty difficult. It's easy enough when you just have a span between two poles but when that span is supporting a number of other span at angles to another span ... ugh.

I might just try to take a very conservative common sense approach and see if it works, erring towards caution. Your mileage may - just suggestions here.

I don't really know how much tension you can put on a 13' 4x4 steel post - it would depend upon the gauge of the metal etc. With a 3' x 18" base in the ground ... I don't see it being an issue for this fairly lightweight application.

Before I go any further - strongly recommend you use low voltage 12V lights - much safer and simpler to install in a code-compliant manner. The electrical code requires a messenger wire for spans over 40' and requires strain insulators for the attachment of the messenger wires. Although with GFCI, 120V is much safer than it is without, I'd stick to low voltage.

Also keep in mind that wires, chains, ropes, etc. under tension can be dangerous if they snap, or come loose - the tighter they are, the greater the tension, the more energy they store, and the more dangerous if they come loose.

I am going to call the wires in green in your drawing "messengers" instead of "guy wires." I usually call guy wires wires that go from the post to the ground to help keep the post straight. I'd really try to avoid using guy wires, people are always tripping on them or walking into them. You want posts that are strong enough without them.

The main thing you have to look at here is tension in the messengers - that's what is pulling on the posts. The tension on the messengers depends on how tight you stretch them, and the weight they're supporting. If you want a really laser straight messenger, that will generate a lot of tension, I would NOT attempt that, I'd allow quite a bit of sag.

The string lights are also under tension. In fact, I am skeptical you'll be able to find string lights that span over 50' unsupported. I believe you'll need to string messengers between the messengers to support the string lights.

You'll want to use wire for the messengers that's sufficiently strong, but I wouldn't go overboard, since it's just more weight to support.

I think I'd string the pole to pole messengers with a foot or so of sag, then string the messenger-to-messenger messengers with a couple feet of sag. You could attach the lights with zip ties or maybe wrap the light wire around the messenger as you go, and tape it at the ends. See if the tension is something you can tug on and make it move a little with one hand at the poles. You know that if you can move it a little with one hand, it's nothing that's going to make your posts lean.

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    The tension concern isn't with the post, but the concrete base. I can see them tilting under tension, depending on how soft the soil is. – isherwood Oct 30 '18 at 15:03
  • And the original design doesn't specify lights under tension. Wires are indicated in all spans. – isherwood Oct 30 '18 at 15:04
  • The lights would be under slight tension, expecting a 2’ to 3’ drop from the 13’ poles – JwaddPHx Oct 30 '18 at 16:31
  • The more sag you can tolerate, the less tension there will be ... 2'-3' should be pretty manageable. – batsplatsterson Oct 30 '18 at 16:37
  • @batsplatsterson - Add guy-wires to stop the poles tilting under load. This could be added later if maintaining tension is a problem. I suspect it may be ok. Depends how deep your posts are and how soft the ground is. – Matt Nov 4 '18 at 22:57
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I'd be really skeptical that the green wires will be sufficient over such a distance. I'd go with a post at the end of each red light string, which would be 6 posts instead of 4. But even then I'd worry that the 40'+ length of each light string would sag too much.

My suggestion would be to forget the concrete (a lot of work) and just build the 6 temporary light poles on 2x4s or similar lumber. Nothing fancy, but inexpensive, easy to put together and the lumber can be used for another project later on as long as it is stored well after the big event. You can either figure out a way to anchor them to the ground or screw them into a wooden base and weigh them down with sandbags. Cheap, easy and nothing 13' tall left afterwards to annoy the neighbors.

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    The concrete is done, according to the OP. – isherwood Oct 30 '18 at 14:58
  • It is, so I’m trying to salvage what appears to have been a bad idea – JwaddPHx Oct 30 '18 at 16:40
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As it sounds like you already have the posts cemented in why not just route the lights between the posts as shown below. For the longest cross runs you could put another post of some sort right next to that long table where they cross over. This approach still has to be based upon finding the sag over the distances involved to be acceptable. I would still be skeptical of sag being too great as mentioned in the other answer.

enter image description here

  • Thank you for taking the time to do this, I really appreciate it. – JwaddPHx Oct 30 '18 at 16:26
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Since no one mentioned yet, if the wires are too slack, you can tighten the steel wires with a wire tightener like these: https://am.gallagher.com/global/product/2754/in-line-wire-tightener . Do beware of the tension and the breaking risk, don"'t over tension the wires so they don't snap.

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Sure it'll work. But you need to use appropriately gauged wire to hold it. And you may need to add guy-wires to make it stay taught. Use turnbuckles to add tension.

You'd be surprised how much tension can be added onto wire. It can be very strong. But use appropriate wire. LED lights don't weight much. You'll probably over engineer it. And if it doesn't work out as well as you expected then add more poles. What have you got to loose? Just try it.

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