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I am trying to build a climbing wall out of 4 plywood panels 4'x4' each in a grid pattern. I'm comfortable building such a wall with either 6 pieces of 2x6 joists or 8 pieces of 2x4 joists running up the back vertically.

- Joists: 2 foot spacing, 2 in the middle
- 2 cross beams on the top and bottom. 4x2 foot cross beams in the middle  
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<--- 8 ft --->

This time around I'm trying to build it with a steel metal backing to reduce the footprint behind the wall. I'm looking at using 1x1x1/8 steel angles to replace the 2x6 beams. This will also make my saftey calculations easier when I route metal cable through the back to adjust the wall angle. Some other background information

  • I have no experience welding, but if it needs to be done I can probably have a friend help do some welds. If the joins are do-able with just screws would prefer that.
  • Does this seem reasonable? I'm not as familiar with working with metal. Are there cheaper load-bearing joist materials out there (like structs or channels)?
  • Any recommendations for braces or screws or connectors

Update: Here are some hand wavey steel rigidity calculations:

2x4 bend strength: 2,200 psi Steel bend strength: 21,000 psi

  • 2x4 section modulus : 3.0625
  • Steel angle 1x1x1/8 section modulus: 0.02108 vertical leg 0.02409 horizontal leg
  • Steel angle 1x1x1/8 section modulus: 0.08362 vertical leg 0.08919 horizontal leg
  • Steel square tube 1.5x1.5x1/4" section modulus: 0.45139
  • Steel square tube 1.25.1.25x3/16" section modulus: 0.24735
  • Steel square tube 1.5.1.5x3/16" section modulus: 0.3845

So seems like if I were to go steel I need something between 1.25 and 1.5 size and 3/16" and 1/4" thickness square tubes. or 2x2x3/16 steel angle

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    Pretty sure a 2x6 outdoes a 1x1x1/8" angle iron as structure. Square or rectangular tubing might get you more of a benefit, but 1" is still pretty wimpy. Steel is stronger than wood, but skinny steel is very bendy, so you can't just handwave that steel is stronger than wood and ignore the depth of the member. Angle also likes to twist...
    – Ecnerwal
    Nov 10, 2023 at 2:42
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    Is weight a concern? Criss crossing OSB panels with glue between layers is my favorite option if thickness is the only concern. Steel angle sucks for about everything except tension. An I-beam shape is the most efficient for what you're doing. You won't find anything small enough from a mill, though.
    – popham
    Nov 10, 2023 at 3:21
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    Throwing quick numbers at it, a 2x4 oriented the thin direction is stronger in bending than a L1x1x1/8 by about 150%. And that's with the bottom of the barrel lumber grade (stud grade). For deflection, the same wood section is about 150% stiffer.
    – popham
    Nov 10, 2023 at 3:42
  • Thanks! Weight is not really a concern. @popham, do you have any reading links or rough numbers for working with OSB for structures? i.e How many layers of how much thickness, if other supporting structures are needed etc.
    – James
    Nov 10, 2023 at 4:40
  • How do you intend to support this? Is the "metal cable through the back" for positioning it while more rigid supports are adjusted? Such supports seems well suited to some steel fabrication.
    – popham
    Nov 10, 2023 at 6:28

3 Answers 3

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Well, there is a stock product (which you an hand-make if you really want to go "custom") that would give maximum rigidity in minimum thickness, By The Power Of Physics...

Stressed Skin Panels. Rather than gluing layers of OSB or plywood together, you glue foam (or another filler material, such as honeycomb) to the back of your face panel, and then glue another sheet to the back of that foam or filler. The sheets provide strength, the spacing provides stiffness, the whole is very structually efficient. Typical real-world applications inlet framing around the edges, but no framing is needed other than that.

Stock off the shelf structural insulated panels with 3.5" core (SIPs) can simply be purchased, and you'd bore holes to access your T-nuts. A thinner panel with 1-1/2" core could be field-fabricated with T-Nuts already installed, and the foam or honeycomb core won't interfere with the bolts in the T-Nuts. A third core alternative from "the same thing but called a torsion box" would be wood spacers, probably on the same 8x8 spacing as your T-nuts, but offset 4" both ways, and with a solid rim all the way around. That would be highly amenable to just using your 3/4" birchface as the front layer, with spacers under the joints.

See also: https://en.wikipedia.org/wiki/Sandwich_theory

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  • did some light searching, had trouble finding easy options to purchase some structural stressed skin panels. any recs?
    – James
    Nov 11, 2023 at 2:56
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Gluing two layers of 3/4" OSB together is as strong and stiff as 2×4s spaced at 16" centers. It will weight 320#, but you've said that weight isn't an issue.

For stud grade Douglas Fir, the bending strength of a 16" wide band of wall under the National Design Specification (chapter 4) and its Supplement (chapter 4) is

Mₙ = (1.15)(1.5)(700psi)(1/6)(1.5in)(3.5in)² = 3700#-in

and its stiffness is proportional to

EI = (1000000psi)(1/12)(1.5in)(3.5in)³ = 5360000#-in².

From the AWC's Manual for Engineered Wood Construction, the bending strength of a 16" wide band of 48/24 OSB (3/4" OSB) is

Mₙ = (1000#-in/ft)(16in)(1ft/12in) = 1330#-in

and its stiffness is proportional to

EI = (400000#-in²/ft)(16in)(1ft/12in) = 533000#-in².

The single sheet of OSB, then, has 36% of the 2×4's strength and 10.% of its stiffness. Strength increases with the square of thickness, so doubling the thickness implies 36% ↦ 140%. Stiffness increases with the cube of thickness, so doubling thickness implies 10.% ↦ 80.%.

The two layers seem thick enough. OSB isn't as strong or stiff in the other direction, so use 2 layers and orient them both vertically. To stagger joints, cut one of the 3/4" OSB sheets in half to get two 2 ft by 8 ft pieces. One piece will have a tongue and the other will have a groove, where each can interface with the edges of a full sheet of OSB. That new configuration will put the joints at 2 ft and 6 ft so that the next layer's joint at 4 ft doesn't overlap.

To glue everything together, a V notch trowel will come in handy. I would buy a gallon of Titebond II Extend and go nuts. The purpose of screws is primarily to achieve pressure during glue up. Just use any old 1-1/4" screws. And you might have a quart of squeeze out (never short yourself in a land of plenty), so be sure to work over some plastic sheeting or something.

Oh, and before you install the screws you have an opportunity to build a camber into the wall if you want a bow in it. If you want it flat, then do the work on a nice flat substrate.

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  • Thanks! This is a really good start for me! I think there is a minor caveat specific to climbing walls around how the face is normally mounted with T-nuts for the climbing holds. There needs to be a little bit of space behind every T-nut when a bolt threads through to pull a hold to the wall. I think if I go this route I'll need to Swiss-Cheese some holes in the OSB every 8 inches. Maybe I'll stack an extra layer of OSB for stiffness.
    – James
    Nov 10, 2023 at 7:04
  • @James, why can't the T-nut flanges rest against the back of the 1-1/2" wall? From your "adjust the wall angle" I intuited that there will be space back there. You just need longer bolts to engage them, right?
    – popham
    Nov 10, 2023 at 7:12
  • @James, I think you're right on the third layer. Your loads aren't distributed across 16". It's more like 5", where that probably would have been a more appropriate width for comparing the OSB to the 2x4. The 5" is about 1/3 of 16" and the percentages are linear for width, so that 88% is more like 30%. And (2³)30% = 240%. But if a 2x4 wall sheathed with 5/8" OSB is sufficient, then the softest part of that wall follows from a springs in series calculation--it's not just the 2x4's stiffness. If not for expensive bolts, I would experiment with 2 layers and then maybe upgrade later.
    – popham
    Nov 10, 2023 at 7:25
  • @James, is there some standard climbing wall to calibrate the stiffness against more realistically? 16" centers sheathed with 5/8" plywood? And as an aside, is there some veneer that's typically applied to avoid abrasion and slivers? OSB is pretty abrasive and sliver-rific.
    – popham
    Nov 10, 2023 at 7:46
  • I'm still planning for the front face to be 4 pieces of 4'x4'x3/4" birch plywood. Because of your glue+OSB suggestion I'm also looking at composite lumber options too
    – James
    Nov 10, 2023 at 18:52
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Metolius has an extensive article on building a home bouldering wall. One design difference is their plan has the joists braced by the house joists.

In other words, it does not have an adjustable angle. See: https://www.metoliusclimbing.com/pdf/How-to-Build-a-Home-Bouldering-Wall.pdf

If you opted for a hinged approach, it would be significantly more complicated. Maybe use a pair of “come along” winches.

Best of luck with your project

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