I have a job that I will be building a set of built ins next to a fireplace. There is 50" on each side. Question is when I do my shelving if I double layer & glue my plywood will it still be to wide for no center support? Once I get it built the shelves will be approx. 47" to 48" wide. I hate to put a center support in the middle. Thanks

Do they have to be stiff enough to support books?

The one thing that is rarely brought up is a solid wood shelf. You can put a ridiculous amount of weight on a 1" x 10-12" x 48" piece of solid maple or 1 1/2" pine(try standing on 2 x12 x 4' framing lumber)

We typically glue two pieces of maple plywood together and then glue on a solid piece of maple front or front & back. The timber on edge increases the stiffness big time.

Now that ply is so expensive, I would seriously consider solid. By the time you chop up plywood glue it together, mill up some 3/4 x 1 1/2 glue that on. Spend a ton of time filling glue joints and nail holes. You might as well glue two pieces of wood together and mill them to size.

Yes Adam I see your point. but then I would have the seam of the 2 pieces of 3/4" glued together showing. I just watched a video on Utube a guy cut a groove in his 3/4" ply and inserted a piece of 1/8" x 1/2" bar stock between the sheets that looks like that would add a lot of strength along with the piece of 3/4"white oak glued on edge. But it maybe strong enough with out the bar stock. And the question about the books its a remodel who knows what they will put on the shelves over the years.

Sagulator -- google

If you add a strip of solid wood up the back and have the same adjustable hole pattern in that strip, you greatly increase the load carrying capacity. Personally I would spilt the shelves in half. I can't see a case where a 50" clear span is required.

I'm feeling pretty confident that by gluing 2 3/4" plywood with a piece of 3/4" white oak on the front edge and also a piece of 3/4" white oak on the back bottom that i won't have any sag on a 47" span.

Did you run it through the sagulator? 1.5" x 12" x 47" with 3/4" 1 1/2" white oak edge comes in at .05" sag with a 40lb/sq' uniform load. 40lbs is massive heavily loaded. Not sure what the factor would be for both edges. But it come in as "acceptable". If they are able to be flipped its even better.

If you glue 2 pieces of plywood directly to each other at that length they will warp. Building a torsion box will not only prevent that, but will be stronger.

We've laminated 2 pieces of 3/4" ply for a 1.5" thick shelf faced both sides numerous times. Same with 1/2" for a 1" thick shelf and no issues with warpage. Always best if you can bring in true 1" ply but we often dont have that option.

The 47" span is going to sag no matter what which is why we always try to make longer shelves flippable so the customer can flip/reload as needed though it likely never happens.

Sounds good Mark I seen on Utube a guy cut a groove on front and back and placed a piece of 1/8” x 3/4” bar stock between them that would help for sure.

Avoid the tube. No small metal bar in there is going to appreciably increase the capacity of the shelf. Buy your bar and bend it over your knee. Even bonded in its not going to gain you anything.

Good to hear appreciate the input

There are zero problems with warpage when applying various width/thickness pieces of wood to fronts/backs of plywood shelves. Absolute worse case is a little bit of bow. This can be a good thing in long spans. Just thick of the shelves as beams/joists. Crown up.

Putting the center shelf pin thru the back of the cabinet(1/2" back is required) can cause visual problems if the customer doesn't fill the shelves. You will always see this long row of shelf pin holes. The midspan pin is fine for kitchen cabinets.

We've done all of the variations over the years. As well as some unusual Architect/Designer designed ones. In retrospect many of those should have been done in solid maple. The cost is actually lower when you factor in all of the various steps to get to paint. 20/20 hindsight. Obviously, this observation is project dependent. I think we all see it as a "waste" of wood. Perhaps its a waste of time & cabinet ply to build shelves.

I'll have to disagree with Mark, a trapped/glued 1/8x1" steel bar on edge will add significant stiffness to any shelf. Making the slots 9/16" deep and using 3pcs of 1/8" dowel (top/cntr, bott/ends) during glue-up will add a bit of camber (think tractor trailer flat bed).

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Good ideal about the 1/8” dowels took me a minute to figure out what you was saying but I did get it.

The issue is what is the gain for the investment. I would guess in reality is a ton of investment for very little gain. One could have a shelf done and out the door in a fraction of the time it takes to implement some half measure that only gains a very slight advantage and moreso to the maker more than the customer.

Sagulator has proven invaluable to me.and been very accurate. If your trying to defy the laws of gravity... sure carbon fiber, whatever it takes. But the bulk of it is met with traditional convention. The mass of a unit with 50" wide shelves is usually complimented by thicker shelves anyways so it works.

If I had the machinery and the budget, I'd use solid hardwood shelves. With my equipment, a stacked and banded or painted edge is the easiest and most aesthetic. Adding steel reinforcement on long spans is easier, more aesthetic and effective than an applied 1x2 hardwood edge.

The problem with the Sagulator is that it doesn't account for time. Heavily loaded shelves will gradually sag more and more over time (particleboard and MDF in particular). It has taken over seven years for my heavily loaded and unsupported 48 x 12 x 1-3/4" MDF torsion box shelf to sag 3.5mm. A similar (1.5" stacked edge, lighter load) steel reinforced particleboard shelf that is at least twice as old hasn't gone past 2mm for over seven years. The later was probably before I started pretensioning the steel and was a lot easier to make. The labor costs on the former were probably the equivalent of using plain hardwood.

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Dave
So I don’t have any 6/4 white oak but you think I would be better off laminating 2 pieces of 3/4” white oak for my 47” span ?

How many times have torsion boxes been suggested as fixes for these types of problems?

Or, the Sagulator?

Steel? Double layers of ply? Why complicate things?

When we have to build longer shelves that present a possible future problem we inset .625 x .625 x.125 angle iron into the bottom of the shelves. We have found this to be quick and an easy fix.

David can you build a torsion box 1-1/2” thick?
Tom that would make it stronger

Yes - 1/8" "door skin", 1-1/4 x 1/2", and another 1/8" skin.
Use 1-1/2" with a 1/8" rabbet and frame the shelf four sides. Make some 1-1/4" x 1/2" stock and run it the length, down the center.
Then a few short pieces from edge to center. Glue it all, and press it to set.

You can vary it if you can't find 1/8" - then you can use the so-called 1/4" ply. And 1"+ spacers.

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The cabinets in the photos are both torsion boxes of 1/4" MDF, ply 2 sides, with a solid rabbeted nose of solid wood and a solid spine down the center and one along the back. I think they were 1-1/2" or 1-3/4" thick. The back edge tongues out thru the backs and catch the 1/2" back panels, giving 100% support along the back. When I sat on one of the long shelves, my svelte 280 lbs deflected the shelf less than 1/8".

You can also use honeycomb door core and skin it. Think of the strength of an interior door. Those are 1 3/8 thick. We made cored ones about 20 years ago. I believe they were 6' long.

4' is not a big deal if you want regular load shelves. If the Customer wants to pay for some exotic construction method for 4' shelves that will support David then go for it.

I couldn't figure out how to input David into the Sagulator.

Exotic? One problem with torsion box construction is it is not visible since you cannot see how those shelves are made. Surely, when the lawyer talking head is on the box, we can all see how the law books he never uses have bowed the solid wood shelves. Torsion box would not have sagged.

Input my posterior into the Sagulator? Try Fatwood.

The only advantage of a torsion box shelf over a solid wood shelf of the same thickness is weight.

How well a torsion box shelf holds up over time depends on the materials and construction used.

A simple steel reinforced shelf sags less over time than a veneered all MDF torsion box (see above).

The major limiting factor in torsion box construction are the skin material/thickness and the total thickness of the panel. Ideally you want a unidirectional material(wood) as the skins. The further apart they are regardless of the internal webbing the stiffer it will be.

If you made what you are calling a torsion box out of one 1mm unidirectional fibreglass 1" thick with a foam/balsa core it would out perform all of your torsion boxes by a long shot.

The mdf skin is a poor choice. There is no fibre strength. You would be better off using 1/4" ply.

The way it works is one of the skins is in tension the other is in compression. Similar to an i-beam

People tend to over build the framing in a torsion box because they don't understand the composite panel theory.