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In brief - I want to use a 2x4 to resist the tension put on trellis posts ~16ft apart, rather than the other two options.

I'm building a grape trellis with upright 4x4s connected by high tensile wire. As with any wire setup, like a long wire fence, you usually need to brace the ends to avoid leaning. I'm thinking I can just run a 16ft 2x4 between the posts at the top, using brace pins to attach to the posts. This 2x4 wont really hold any load, as that will be held by the high tensile wire. The 2x4 is only there to resist the tension force that the wire puts on the main 4x4 posts, turning it into a compressive force on the 2x4.

Can a 2x4x16 even make that span given it's own load? And if so, is that compressive force (it should be straight and level) going to force it to sag?

FWIW this is really to avoid the other two options - 1) angle the posts at 60deg so the tension forces the posts into the ground, or 2) build large H braces on the ends.

  • It’s common to anchor the trellis wires into the ground at both ends. This should keep the end posts from swaying. Were you trying to avoid doing this? – Stanwood Nov 13 '17 at 13:17
  • Anchoring high tension trellis wires into the ground for a vertical post is never a long term solution. Tension from trellis wires will always force a perfectly vertical post to lean, eventually collapsing the trellis. You usually prevent this by installing the posts at ~60 degrees to the ground, outward. At that point the tension force is directly along the post axis, so it will hold in place. But I'd like to put the posts vertical for aesthetic reasons. So in that setup, you need to use a bracing option to resist the tension. – The Shoe Shiner Nov 13 '17 at 13:31
  • an unsupported 2x4x16 is quite likely to warp in a noticable way even with no load. A piece of 5/4x6 deck lumber (edgewise) might do, although there is a warping issue there too especially if you are buying treated lumber from a box store. – agentp Nov 13 '17 at 15:03
  • I like the notation about 60 degrees on the end brace. I’ve never noticed that before. I’ve got to go find a vineyard and see if that’s true. (It’d be much better than a 45 degree support.) – Lee Sam Nov 17 '17 at 3:27
  • And there are definitely options for bracing vertical posts, so you'll see vineyards using vertical with other bracing, but the simplest, and only stable, non braced wire trellis is one where the tension wants to drive the post further into the ground, which the ground can essentially always resist. If it's done correctly it's a very elegant setup. As long as you're asthetically okay with the outward leaning end posts :) – The Shoe Shiner Nov 17 '17 at 3:47
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Yes, it will sag. I'd guess anywhere from 1-3 inches over time, depending on the moisture content and grain.

Whether it's still able to resist compressive forces isn't dependent on that, though. It'll fail laterally (on the thin dimension) before it'll fail vertically due to sag. You should use a T configuration or a heavier (or metal) rail.

  • Which force is causing it to fail though? The compression between the posts? Or the vertical load of gravity? And I assume by T you mean the beam should be T shaped? To give it more strength in both directions? – The Shoe Shiner Nov 13 '17 at 14:22
  • Yes, the compression. I'm guessing that the weight of the plants could result in several hundred lbs. of force on the beam. Eventually it'll begin to bow sideways and become quite weak. This is why steel I-beams have dual flanges. – isherwood Nov 13 '17 at 15:55
  • Gotcha. Yea the weight of the plants will get transferred to the post and then forced onto the ends of the beam. Hadn't really considered how that force may affect it laterally. So If I end up doing this then I'll set it up like a T or I beam. – The Shoe Shiner Nov 13 '17 at 16:09
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Wow, this is really interesting. I’m not sure I understand the problem, (and I’m suppose to know this stuff.) I hope it’s just terminology...

Seldom is a member only in “tension” or “compression “. (I can only think of some chords in a truss...maybe.) So, when your horizontal 2x4 member sags (bends), it’s in tension on the bottom and compression on top. Then, when you tighten the wires, you’ll induce more compression into the 2x4. This is what we call “double bending” and often causes buckling.

We can calculate this buckling by knowing the load (compression load) and the span...we call this the “slenderness ratio”. Every material, (wood, concrete,steel, jello, etc.) has an allowable slenderness ratio based on the strain (not stress) of the load on the material and the span. (This is how they calculate allowable loads on columns to keep them from bending.)

So, the more you tighten the wires, the more the 2x4 will “tend” to bend and must resist the load or it will buckle. Because the slenderness ratio is based on the skinniest side of the 2x4, it helps to add a 2x4 in the opposite direction. (It has nothing to do with the flanges on a steel beam, like @isherwood says.)

So, to answer your question, “yes, the 2x4 will sag over a 16’ span AND the compression added by the wire will not save it.”

Whew...that was hard to explain...but the more compression you add to keep it from sagging, the more you’ll cause it to buckle...until you add another member perpendicular to the 2x4.

By the way, if you try to use the 2x4 member, make sure you tighten the wires evenly or you’ll introduce “twist” to the 2x4, and that is way too difficult to calculate...but will cause failure even quicker.

I don’t know how many 16’ spans you have, but after about 2 spans, you’ll use a lot less lumber by using either of your 2 options.

  • Yea I think after looking at all the available options, I'm just going to brace the end posts with inward diagonal beams towards the ground, pinned in place. These beams will be 4x4 and all the tension from the wires will be transferred to them. And Using inward diagonal braces helps me avoid the additional row length of an H brace. – The Shoe Shiner Nov 17 '17 at 3:14

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