Shelf strength & thickness for 4ft adjustable bookcase span

[blueman]

I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks.

- The sides and back of the bookcase are 3/4" cabinet-grade birch plywood.

- Plan is to create 48"x12"x1" shelves by gluing together two sheets of 1/2" birch plywood.

- Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes.

I would like to avoid adding shelf-pins along the back because I don't want visible holes. Also, I would prefer not to split the span since I like the open look and don't want to have 2 narrow 2ft spans.

Questions:

1. Is this construction strong enough to support a 4 ft span loaded with textbooks? (I have had 3 foot span bookcases constructed from 3/4" material and they seemed to hold)

1. Does anything else need to be done to stiffen the span? e.g., Thicker shelves? (I could use 3/4" to replace one or both of the 1/2" pieces but then the shelves get heavier and bulkier looking Torsion box construction? (I could sandwich a 1/2" layer between two 1/4" layers but this is added work and added opportunity to mess up :) Shelf pins along middle of back wall? (I really would like to avoid the visible line of holes down the middle)

2. Will the 1/4" shelf pins with sleeves be strong enough to hold the load? (e.g., should I use stronger or more shelf pins?)

Also, do the sleeves do much to strengthen the holes or am I just as well off without them?

[Duane Bozarth]

...

I personally think it will be marginal w/ ply. I'd make every attempt to find solid lumber to make the shelves from. It would take some clearance away, but you could use an edge on the shelves to gain some extra stiffness if you could live w/o having close margins between tops of books and next shelf. Overall, for a span over 3-ft, I'd really try to put in a vertical center support if I could.

The 1/4" end pins are probably marginal and w/o the inserts I definitely think they will tend to round if a 4-ft shelf is loaded to the maximum possible. There are library systems that are more sturdy. Seems like Woodworker Supply had some??? Or was it Woodworkers' Hardware in MN??? I can't recall just now.... :(

If it were me, I'd split it I think w/ single carcase, center support.

[bob]

Why don't you mock up a shelf with some scrap, load it up, and test various designs?

[Mark Wells]

See

tells me that two 1/2" sheets of ply glued together would be much weaker than 1" thick ply.

Mark

[Knothead]

You can offset the shelf sag and use 3/4 ply for the shelves if you wrap them with a verticle grain face. Make a "frame" for the shelf out of 3/4" x

1 1/4" hardwood and rabbet in for the 3/4" plywood shelf. The shelves will appear more "massive" but will actually be lighter and more ridgid. Even if you stick to your 1/2" laminated idea any sort of verticle grain facing is going to add to rigidity.

Knothead

[Rumpty]

IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves

1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag.

[firstjois]

[snip]

[snip]

Are the shelved attached to the case? If not, you could finish both sides and flip the shelves occasionally. What bends one way will bend the other - that sort of thing. I do this with closet top shelves of 1" pine that sag over time. When flipped they straighten out and eventually sag the other way.

Josie

[Dave W]

Shelf design is a matter of how much deflection is acceptable (some deflection is inevitable). The previous suggestion of making a simple mockup is very sensible. If I had to guess, I would say that the two half inch plywood shelves will have deflection noticable from across the room......but that is only a guess. Dave

[blueman]

I like this idea... Curious though why you pick 1/8" wide by 1/2" deep rather than the other way -- i.e. rather than a wider narrow strip.

Also, where is a good place to get steel strips like that (I have seen steel rod at the borgs but not strip)

[blueman]

Excuse my ignorance, but why would it be weaker?

[blueman]

What type of solid wood (without knotw) would you recommend that I could buy in 12" width, 1" thickness, 4 (or 8) ft length without breaking the bank?

I don't have a jointer so I would prefer not to have glue up a panel using narrower stock.

[Duane Bozarth]

....

I'd assumed since you were building a sizable project you had stuff w/ which to do it...

[Duane Bozarth]

He's putting it on edge--the strong dimension. For this to work well you will have to have the depth of the grooves at pretty close tolerances.

Most any hardware store will have 3-ft lengths, some may have longer. Look in your phone book for steel suppliers in your area--some will deal w/ individuals (for a price, but not any more than the exorbitant piece-price at most retail outlets)....

[Duane Bozarth]

Perhaps, but I don't think I'd count on it for long term...I suppose you could fill the groove sufficiently, but my experience is that epoxy and smooth metal surfaces tend to separate w/ age--or maybe I'm just unlucky.... :)

[alexy]

Actually, using epoxy to seat the steel should eliminate this concern, shouldn't it?

[bf]

I've read that a sheet of 3/4 ply is stronger than two 1/2 sheets glued together. I can't prove it (lost the references), but it makes sense.

To the original poster. If it was me and I was dead set against having a center support and it had to be 48" long.. I would use 3/4 ply, and put a 1.5" strip of hardwood on the front. If you attach the shelves to your 3/4 back, that will help too.

Another option is to just make the bookcase 32" wide. 3/4 ply (particularly with a 1.5" edge) would definitely handle that span.

[bf]

Just to clarify on my other post.. when I say to add a 1.5 inch of hardwood to the front of the 3/4" shelf, I mean that the top 3/4" of the edging will be fastened to the shelf, and the bottom 3/4" of the edging will hang down. This helps resist sagging.

After you glue the hardwood to the ply, you can put a 1/4" roundover on the on the hardwood edging. I do this because I think it looks nice.

[RVH]

Andy Rae's book, The Complete Illustrated Guide to Furniture & Cabinet Construction (from Taunton) has a good listing of span limits for various materials. I doube your 1 inch plywood sandwich will be enough. You might add hardwood edges that will not only cover the rough plywood but add some stiffening.

RVH

[alexy]

Probably strong enough, but probably not stiff enough. I.e., the shelves probably will bend (objectionably, I'd guess) but not break.

I like Rumpty's idea of the imbedded steel stiffener. But other possibilities might be worth mentioning.

To add stiffness, You'd have to make it thicker, which adds to the visual "weight", even if not to the mass.

A lip on the back, maybe both above and below the shelf to allow for reversing as longer term sag occurs, would stiffen the shelf (though not as much as front and back lips).

Ask yourself (then answer realistically) how important the movability of shelves is. Most are never moved, once set up to the owner's liking. If that is the case, work with the side pins until you have the shelves spaced like you want them, then attach cleats to the back and sides under each shelf, and attach the shelves to the cleats.

[Duane Bozarth]

alexy wrote: ...

Actually, your idea suggests to me that if the shelves are to be heavily loaded, the visibility of support pin holes in the back may not be such a big issue, particularly if only the ones very close to the actual height are drilled out instead of all possible. Then, a ledger strip could be made which fit into those holes which would add quite a bit of rigidity...