Shelf strength & thickness for 4ft adjustable bookcase span

[bridger]

why 3/4" for the back? that will ad a lot of weight to the cabinet without any real benefit. the back is mostly there to resist racking forces (keeping it from becoming a parallelogram). 1/4" ply is more than sufficient for that, although it will feel a bit hollow at 48" wide. 1/2" backs will take care of that and make the case a lot easier to handle.

consider preloading the shelves when you glue them up. that is, glue them so that they have a crown or upward curve equal to the amount that they will sag once they are loaded.

4' is a pretty long span for plywood shelves. given that you are laminating 2 layers you will likely want to cover the edge anyway, so make the edges wider than the shelf thickness by another inch or so. it'll help avoid sag.

some testing is in order.

if you determine that a preloaded shelf laminated from 2 layers of 1/2" BB is insufficient, a torsion box shelf or center pins may be in order. if you go torsion box, consider 1/2" for the top layer, a 3/4" web and

1/4" for the bottom.

another approach would be to make the shelves fixed. then you could rabbet the shelves into the sides and pin it solidly to the back. you lose the adjustability, but you gain a lot of strength.

the pins are plenty strong. where you may have failure is the wood carcase that the pins are supported by. it's a lot of load on a 1/4" wide section of wood. once the load reaches the crush point of the wood fibers the hole elongates, the pin starts to shift, reducing the area it has in contact with the wood, increasing the point load and failure quickly follows.

the sleeves increase the load area, and if you glue them in they help resist shifting.

[SonomaProducts.com]

Yes, use the sagulator to check this out. It can tell you if 1" ply deflects more or less than various solid materials.

Other ideas.

1. You could use a stronger attachment at the ends. Have you ever seen the old cabinet technique where you have a notched piece of molding at the front and backs of the sides (kind of looks like dentil) and you have a loose piece of wood say 3/4x3/4x12 that spans across a selected set of notches and the shelf sits on that? Of course this assumes you have a face frame to hide the ends of the shelves.

1. You can add a beam under the shelf. Say a 1" wide piece of 4/4 hard maple turned on edge half way back. Pretty un-noticable but adds a huge amount of strength.

2. If you use solid material you can go thicker but make it "look" thinner by using an edge treatment. I like to use a panel raising bit (without a back cutter) or a table edge bit (like a thumbnail). If I use the pane bit I then round over the underside front edge too.

[Bill Leonhardt]

I like the suggestion to add a 1x2 (3/4x1-1/2) piece on edge to the shelf front and I think it's a great way to go because: 1. It covers the plwood edge of the shelf. 2. It adds quite a bit of stiffening. The part I don't like is that it stiffens the shelf in an asymmetrical way, therefore as the shelf is loaded, you may encounter twist, etc.

It seems to me that if the books can clear a 1x2 on the shelf front (assuming you don't insert them horizontally and rotate), they can clear a

1x2 on the shelf back as well. The second 1x2 will increase the stiffness even more and the "C channel-like" shape will be loaded more uniformly.

Opinions???

Bill Leonhardt

[adb]

Or even better yet...weigh a shelf full's worth of books. (ie, stack textbooks until they're 48" high on the bathroom scale) That'll give you an accurate weight to plug into the sagulator.

agulator.htm )

[Duane Bozarth]

But, as was discussed in a thread here not long ago, the "sagulator" is based on homogenous material and (to my surprise) the only plies that add significantly in ply are those w/ longitudinal grain--the others only add roughly 20% or so. So, one would have to look at the plywood construction in order to estimate the proper dimensions to plug in. Those would be only roughly 1/2 of the actual thickness so that the two-1/2" sheets would be equivalent to only roughly 3/4", maybe...

I posted a link to the US Forestry Research site w/ a detailed paper -- a Google over the last week or so should find it.

[Roland]

another option is to rebate the back of the shelves into the backing of the bookcase, you will lose the adjustability but you say you will be using the bookcase for textbooks - a known height. rebate 1/8 of an inch

1nto say 1/2 inch ply backing? you should gain a lot of strength and stiffness

[dgadams]

Bill is right on the money here. SWMBO bought a particle board shelf with 40 inch wide shelves 5/8 inch thick. It was my job to make it work. I added a 1.5 inch front rail to all the shelves, then added a 1.5 inch stiffener both front and back on each shelf. With a 1.5 inch face frame all around the case painted up nice, the shelves all look good and you don't really notice the stiffeners. She loads this thing up with heavy notebooks and it deflects just a bit (maybe 1/4 inch).

A good rail both front and back on each shelf and perhaps some kind of stiffener would work. There are also shelf pins that are inset into the shelf so they aren't seen. These could be used along the back.

DGA

[Rumpty]

Usually a 1/8 grove and a 1/8 strip of steel have a pretty good fit....if you can find cold rolled strip (McMaster Carr) it's exactly 1/8, drop in your epoxy and go.

We use this method for making the tables on the Radial arm saw. You could also use a sheet of 3/4 ply and rout a 1/2 inch deep slot in the bottom and glue in your strip, but you'll see the slots on the under side of the shelf.

[Duane Bozarth]

...

It's the depth that's significant...

[bf]

Bill, since he was making the back out of 3/4 ply, I don't think a second stiffener on the back is necessary, if he attaches the shelves to the back. Of course, if he prefers adjustable shelves, I agree with your idea.

[Duane Bozarth]

bf wrote: ....

But, OP did specifical want adjustable shelves and was looking to avoid the holes in the back being obvious if he used pins there as well as the ends...

[alexy]

The HD near me has them, with rather unique pricing: 3' for $3.27 or

4' for $2.17! Both seem high to me, but for only a couple of pieces, so what?

[Mike O.]

We built a library for an attorney who wanted his bookcases as large as possible with adjustable shelves. Our limit, because of sheet goods, was 4' wide with shelves basically 46 1/2". We told him that there was a possibility the shelves could sag but that we would try to make them as strong as we could. We ended up gluing 2 layers of 3/4" oak veneer core plywood together then put a 1 1/2" x 3/4" routed oak face on both the front and the back of the shelves. We told him that if the shelves did sag he could just turn them over. The bookcases are full of big ass law books, been there for ten years and he's never turned them over.

Mike O.

[blueman]

My wife already complains about my tool budget...

[blueman]

Well, it is a built-in, so I don't care much about moving it (other than the first time). Plus, since cost of 3/4" is not very different from a 1/4" sheet, I thought that the added stiffness to the back and lack of hollowness would be worth it.

Assuming that you are talking about 3 sheets of plywood sandwiched as above, does a torsion box like that really work in the sense of being stronger than just two 3/4" sandwiched together (both have the same total width of 1.5")

Agreed, but I like the adjustability :)

thanks

[blueman]

Well, I was using the term "textbook" pretty loosely plus suprisingly my textbooks do vary a fair bit in height :)

[blueman]

I think I will go with this idea since it seems to meet my criteria best though it does entail additional work (but at least I get to play with my new DEWALT DW618B3 router :)

My plan is to do the following.

1. Use two strips 1/8" x 1/2" with each strip aligned with the depth of the shelf pin I assume that "two is better than one" and that aligning with the plane of the shelf pins should help transfer the weight directly from the shelf pins to the steel supported part of the shelf (Is this correct?)

Also, I assume that two strips spread out that way is better than one larger central strip of the same total mass.

1. I plan to build a template for routing the groove out of a similar 4ft x 12" piece of plywood with slots routed out. Then I will use a 1/8" straight cutting bit (is this right? I am new to the world of routers)

1. One other thought I had. Would I be better off adding an additional 3rd row of shelf pins down the center. i.e. even thought they are 1/4" and have glued-in sleeves is the support itself marginal?

Please let me know whether I am barking up the right tree now. Thanks

[Patriarch]

blueman wrote in news: [e-mail]:

Seems to me that 1/8" is exactly the width of the kerf of a regular kerf table saw blade. Big table. Nice fence. Excellent depth control. Good chip collection.

Use your new router. You told you wife you needed it, right? ;-)

Patriarch

[blueman]

Absolutely -- how else do you save money by buying toys (I mean tools)?

[bridger]

the solid shelf will be slightly stronger. the torsion box shelf I was picturing involves dimensional 1x as the interior webbing. the advantages of this approach are weight, cost, ease of assembling a prestressed panel.