Building an 8'x8' metal climbing wall frame

Question

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  
______________
|  |  ||  |  | 
|  |  ||  |  |
|  |  ||  |  |
|__|__||__|__|
|  |  ||  |  |
|  |  ||  |  |
|  |  ||  |  |
|  |  ||  |  |
-------------- 
<--- 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

Answer 1

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

Answer 2

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.

Answer 3

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