Designing a Half-Suspended Shelf

Question

I'd like to make a shelf from 17mm (2/3") thick plywood, whose back will be affixed to a brick wall and its front will be suspended by metal cables from a concrete ceiling (e.g. something with the same principle of the construction in this image).

I'm aiming for a shelf about 2m (6ft 6.7in) wide and 80cm (2ft 7.5in) deep.

Naturally, I'd like the shelf to be as stable and robust as possible. I tried consulting the Sagulator, but I think my case requires 2D load analysis, whereas the Sagulator is apparently only 1D. So my questions are:

1. For affixing the back of the shelf to the wall, are 3-4 metal "L" brackets (distributed along the shelf's length) preferable to having a wooden support beam (as in the previously linked image)? What length of brackets should I get?

2. Will 3-4 metal cables in the front be enough? Will adding a metal support strip in the front of the shelf (through which the cables will pass) provide a significant improvement of the shelf's robustness?

3. What mass could I safely load on such a shelf?

Many thanks in advance for any help.

Answer 1

1. The wooden support beam ("ledger" around here) is nice because it supports the shelf weight evenly. It also gives you the opportunity to put a lot of fasteners in the wall if you feel like a few isn't going to be good enough. The bad thing is that there's relatively little to anchor against a horizontal pull forward. So, if it was me, I'd combine the ledger with a smallish (10cm/leg) L bracket on each end to prevent that pull. (For aesthetics, I'd put the bracket on the top side of the shelf.

2. Sag-wise, in all likelihood, 4 support points will be adequate. If your load causes sagging, simply add a metal or wooden strip. If you see sagging on the short edge, add wood or metal in that direction.

3. If your wires are anchored well into the ceiling (note, this is tough, as a straight pull out -- "tension", as opposed to "shear" -- is the weakest direction for a concrete anchor), this should hold anything short of a book and bullion collection. Be sure to use turnbuckles or equivalent to ensure that your cables are carrying the load equally. I'd suggest when you shop for anchors that you aim for something like a 10mm x 75mm in an expansion sleeve design. For reference, this would be something like an ITW Red Head Trubolt Wedge Anchor, which offers around 3000 pounds resistance to pull out. To attach wire to a bolt in the ceiling, use a "bolt hanger".

If you set the anchors and test load them and discover that they aren't holding well enough, you might put a (roughly) 100mmx50mm wooden ledger on the ceiling. This will allow more fasteners into the ceiling and permit screw-in eyebolts for the cables.

One last note, if you're drilling into a ceiling that's not entirely owned by you, check to see that you're not going to accidentally drill into water, power, or (worst case!) a post-tensioned slab.