Shelf strength & thickness for 4ft adjustable bookcase span

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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)
2. 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)
3. 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?
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blueman wrote:

...
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.
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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.
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blueman wrote:

....
I'd assumed since you were building a sizable project you had stuff w/ which to do it...
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My wife already complains about my tool budget...
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Why don't you mock up a shelf with some scrap, load it up, and test various designs?

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[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
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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. (http://www.woodbin.com/calcs/s agulator.htm )
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adb wrote:

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.
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See http://www.woodbin.com/calcs/sagulator.htm
Gut tells me that two 1/2" sheets of ply glued together would be much weaker than 1" thick ply.
Mark
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Excuse my ignorance, but why would it be weaker?
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Mark Wells wrote:

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.
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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.
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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
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On Wed, 13 Jul 2005 15:49:08 -0400, Bill Leonhardt wrote:

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
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Bill Leonhardt wrote:

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.
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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...
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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
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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.
--
Rumpty

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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)
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