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Here is my general plan.

2x4 framing diagram

These will be built in the corner of a room and each bay will have a lag into the wall's 2x4 including one in the adjacent wall. The idea is to build these 2x4 frames and wrap them with 1/4" plywood.

I have a few questions.

  1. How can a calculate what kind of load the wood itself can take? I'd probably use #2 Hem-fir.

  2. Being 2x4's, I'd have 3.5" for the lag to hold, eg. I could use a big bolt. How can I calc the right bolt and does that affect the load rating? Is there a point it causes the wall's stud to fail?

  3. I was looking at 5mm birch ply to wrap with, do I get more strength by using 1/4" ply or does the added weight counteract it?

  4. If I built the whole thing with 2x2's (1.5"x1.5" actual), how much would it change things?

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    The back 2x4 will likely never fall off the wall, however, you have weak points at every butt joint. What type of fasteners are you using to connect the 2x4s together? – Tester101 Sep 14 '16 at 12:29
  • As mentioned, the joints are weak... I'd suggest brackets to tie them together. And to help prevent sag, I'd put a long and substantial bracket at the outside corner. You're unlikely to find a stud within 15" on the adjacent wall, so consider a couple of substantial toggle bolts. – Aloysius Defenestrate Sep 14 '16 at 12:45
  • @Tester101 I was planning to use 16D nails or 3" deck screws, which is better? – Will Sep 15 '16 at 4:53
  • @AloysiusDefenestrate By brackets do you mean something like simpson ties? Or did you mean a bracket under the shelf? I was hoping to make this a floating shelf and avoiding any kinds of bracket under/over or having a vertical support in the front. – Will Sep 15 '16 at 4:53
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    @Tester101 Considering this will be boxed in with 1/8 or 1/4 inch plywood, do you think that will help give it structural integrity? If I added simpson 90 plates to each joint and then added the plywood (glued and nailed) does that up the box strength from sagging, or what I'm more worried about, the bolts fastening that back 2x4 to the wall, having that 2x4 break. I expect these to hold books, so a good amount of weight. I also have to imagine a scenario where a kid might lean or hold onto the edge furthest away from the wall – Will Sep 15 '16 at 16:38
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Updated: I see a lot of questions referring to spans, strengths etc. Here is a very useful link for calculators regarding all things mentioned. http://www.awc.org/codes-standards/calculators-software

Edit: Based on your diagram now showing.

I do not what type of finish look you are going for but as mentioned by others, the chance of hitting studs where you need them and lack of support are problematic. Either rethink your material or consider either metal bracing for a rustic look or a cable system if you want something that doesn't stand out.

enter image description here

enter image description here enter image description here Since your diagram is not showing up and you have a shelving tag, I will give it a go.

For a shelf you can use a sagulator calculator to give you the info you need. Which can be found here. http://woodbin.com/calcs/sagulator.htm

Strength values for lag bolts into 2x4s.

  • A 3/8" lag bolt 2" deep has a pull force of 1900lbs.
  • At 1/2" at 3.5" deep and you have a pull force of 3500lbs.

If you are looking for a box frame dead load, let me know.

  • Thank you for the numbers on the lag bolts, that's really helpful. Can you tell me what the pull force of a 2x4? Also, I don't know why I would be hard to hit the studs considering I"ve spaced the bays at 16" and my studs are 16 on center. I agree the adjacent wall probably isn't going to have something in the 15" available but maybe I'm missing something? – Will Sep 15 '16 at 4:55
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I don't know how you'd make such calculations, but I have enough experience with exactly this structure that I'll weigh in.

  1. The others are right--the weak point is where the cross members attach to the rear ledger. You can resolve that by using at least two 4" construction screws or light lag screws, properly piloted and countersunk, at each joint. Also put plenty of fasteners through the decking plywood into the ledger, and also into the face board. The ledger connection is most critical. Construction glue may help but isn't ideal in an end-grain situation like the butt joints. It would help immensely with the decking joint.

  2. As far as what it'll carry, you'll see substantial rotational sag long before it tears apart (or off the wall) due to the soft nature of the materials involved and the inherent imprecision of wooden structures. You may also see indentation of the substrate (assuming drywall). That said, it'll probably carry 50 lbs. per linear foot, evenly distributed front-to-back, reliably and without a problem.

Note: With any such structure, keep fasteners arranged near the edges to reduce torsional load. In your drawing, for example, you show lags centered vertically on the ledger. You'd want to stagger them to be placed near the top and bottom edges, weighting quantities toward the top, where the most load is located.

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