Wide shelving advice needed

[Markem]

They make good shelves as long as you do not hit hard with the edge of a can.

[Clare Snyder]

All of my garage/shop shelves are doors - in use fie well over 10 yeaers - holding tools, supplies, car parts, etc and I have yet to damage one. I'm not the bull in a chinashop type, but I do USE my stuff. As $2 each from the local ReStore they were the cheapest I could come up with. I used them to build the base for the building table for buildingthe plane too. It was EXTREMELY rigid and light weight at the same time We used 2 layers of 3/4" MDF glued together for the top surface - 4X16 feet

[dpb]

...[snip for brevity]...

Just to confirm what sagulator does; went to the "Engineering Toolbox" beam calculator for rectangular, uniformly loaded, pinned joint beams and entered the necessary info for a 3/4" x 36" shelf with no edge material included in the calculation of the moment of inertia and an assumed modulus of elasticity of 1.2E6 to guess what it uses for "Plywood, fir".

And indeed, voila! it returned maximum beam deflection of 0.108" while sagulator only reports two digits of 0.11". It's a simple beam deflection calculation.

Also confirms the plywood E appears to match that from the FPL handbook (which is what I thought I recalled from doing the exercise in the past).

If one doubles E in the beam deflection calculation, the maximum deflection is then 0.054", precisely half as the maximum deflection for the assumed geometry and loading is proportional to L^4 but inversely proportional to both E and I.

Let's see if sagulator is more sophisticated in the edging calculation than just adding the extra I, not counting for the induced asymmetry...

Adding a 3/4 x 1 edge also of the same material, the beam calculator returns max deflection of 0.0941" where as sagulator is again limited in its output precision so returns only 0.09".

Again they match so that's all it's doing -- the total moment of inertia is simply the sum of the two composite pieces.

The only additional sophistication in the sagulator is that it can select a different material for the edge so the overall effective modulus of elasticity is a little different whereas the beam calculator is primarily intended for structural beams, etc. so is single material.

[krw]

Who would have expected differently? It even says it in the text, or it did at one point. This doesn't change its utility.

[dpb]

No, nor was intended to say anything less...only to confirm precisely what it does do and how and to illustrate more specifically that the comment earlier that the edge negates the spec's is false.

[dpb]

Also, I at least was curious to see whether the deflection calculation in the sagulator reported anything different based on more sophisticated incorporation of the asymmetry of the geometry. As suspected, it doesn't.

[Leon]

The sagulator determines sag, not strength. Yes this discussion is mostly about what is best for shelving and it went in two different directions. What is stronger and what sags less.

The statement I responded to, near the top of this response, was that plywood was weaker than solid wood, that is not necessarily true regardless of what the sagulator says.

Considering shelving intended to carry a lot of weight both solid wood and plywood will bend to some extent and solid wood will likely bow less along its length, but it will bend.

So how do you get rid of bow along the length when using plywood or solid wood? You add support. Typically along the front edge and often along the back edge. Lets say you use equal steel beams to reinforce the length of the shelves so that there is no bow at all along the front and back edge

Add weight to the center until one fails. My money is on the plywood coming out the winner. The hardwood is likely to split/fail along the length, with the grain.

Have I seen solid wood shelving split along its length? Yes, especially as it get older. And that was not necessarily with a lot of weight. And the deeper the shelving the more likely the solid wood will fail over plywood.

Again, I'm not saying plywood will sag less than solid wood when used for shelving, I'm saying that plywood is stronger than like sized wood.

[dpb]

Well, the poster of the above comment was used the word strength in an imprecise manner and really was referring to the ability to resist bending moment which is, for a fixed geometry, directly proportional to the modulus of elasticity.

What you're describing also is not really "strength" per se, but the effect of the very high anisotropic nature of wood with and across grain. Certainly if you don't load it or constrain/support it across grain, solid wood will failure much sooner in that direction. Plywood definitely has the advantage in that regards and in dimensional stability, yes.

[Leon]

Actually the addition of adhesives between ply's, a necessary component, makes plywood stronger.

[Clare Snyder]

Even a unidirectional laminated wood is stronger than "solid" wood if properly designed and implemented (with the layers alternated "cup up" to "cup down") and is also more dimensionaly stable than "solid wood" as it resists warpage pretty effectively.

[dpb]

On 2/13/2020 3:33 PM, Leon wrote: ...

More rigid...

[whit3rd]

But for a pantry shelf (18 x 30 inches) dimensional stability and warping/cupping aren't a big problem. If you trap the edges in dadoes, warping is nil. If the face of the shelf is pinned (or hidden by a face frame) the shelf can change depth; who'd notice?

[Leon]

Agreed

[dpb]

On 2/13/2020 9:36 AM, Leon wrote: ...

It's far more stable and the likelihood of splitting along the grain (since there isn't any grain throughout) is insignificant in comparison, yes. That's not "strength" per se, though.

Doug fir ply has MOR (modulus of rupture) of about 5-6 kpsi whereas clear Doug-fir (12% moisture) is more like 12 kpsi.

As noted in the text for MOR the value is the accepted measurement but the computation from measurements is based on limits of elastic region and specimens will be/are deformed beyond such before they actually fail.

All not to deny that ply is a most suitable alternative for shelving; particularly wide shelving than single wide board solid material, only to be more precise in what it is that is measured/calculated...