Shelves
Most built-ins look much better when the design isn't constrained by the load limits of stock materials. Applying 1x2" solid wood to the edge of the shelf is a common solution. For me, its easier to build up the edge. Adding steel to the stacked edge is relatively painless and allows for significantly longer spans.
Library Shelves
Tension Rod Shelves
Shelf Spans
Library Shelves
After experimenting with some rather complex shelf reinforcement (Tension Rod Shelves, below) I've settled on a 3/4" build-down with 1/8 x 9/16" matching slots - in the bottom of the shelf and the top of the build-down strip - that accommodate 1/8 x 1" steel. The steel/shelf is pretensioned with 1/8" dowel, an inch or two on each end and, on the opposite/bottom edge, an inch or two in the center.
I use spring clamps to hold the pieces together while the glue cures. With MDF a piece of angle-iron between the clamps and MDF is needed to create some tension, minimize MDF flex and keep the slight camber a fair curve. Viewed from a different angle you can see that I use small bar clamps where the dowels are, I add two intermediate ones on longer spans.
With stacked edges, getting a flush joint easy because the shelf gets ripped to size after the glue dries. Stacked edges also don't have unlike material (e.g. MDF edge VS face density) or grain/color not matching issues.
With MDF, I prep the edge with OneTime. With plywood, or particleboard, I use edgebanding that's as thick as the radius that I plan to put on the shelf edge (e.g. 1.5mm banding w/ 1/16" radius). The seam is hardly noticeable because its where the radius breaks.
The holes on the ends are for Recessed Shelf Clips which I have to drill before glue-up (undermount head). I use a 1/2" flush trim router bit to flush out the slightly longer buildup strips.
Since there is very little material left above and below the steel, good adhesion to the sidewalls is important. I thoroughly wet the slots and coat the steel with glue. I use a large bore needle on a squeeze bottle to spread the glue in the slots for 1/8" steel and a Minicol biscuit glue bottle (tongue depressor for spreading) for 3/16" steel. The primary strength is the vertical 1" and I probably don't gain much with the extra 1/16" thickness.
The first time I used this method, the longest shelves were 42" and loaded with books. When I grabbed a picture a year or so later, the shelves still had a very slight crown.
edited Sept '08
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Tension Rod Shelves
My earlier aluminum reinforced shelves hadn't worked very well. I figured that threaded rod would allow me to counter the self load by adjusting tension on the rod using recessed nuts on each end. I cannot remember everything I tried, but applying tension to a straight rod isn't enough. I tried straight slots with spacers so that the rod would be lower at the center of the shelf. This worked but required way too much tension (the self started to delaminate). I then made a router template so that I could route mirrored curved slots. While this was a definite improvement, it still required a lot of tension. The only way I could get this design to work reasonably well was to shift the slots so that all of the rod was below the shelf proper (pictured). The problem was that the labor involved had increased exponentially. It wasn't a dead end as it gave me the idea of pre-cambered steel reinforcement (Libray Shelves, above).
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Shelf Spans
WoodBin's Shelf Design Guidelines seem a tad generous for book shelves. The examples below are for a 3/4" thick x 10" deep shelf w/ 20lbs (books are 20-25) per foot. The numbers in parenthesis are the Sagulator (below) calculated deflection.
Particleboard - 26" (.21")
Plywood - 32" (.05")
Yellow pine - 36" (.05")
Red oak - 44" (.13)
1-1/2" edging on plywood - 42"
I agree with WoodNet's suggested Shelving Spans for books on a 10" deep x 3/4" thick shelf.
Particle Board - 24" (.08)
Plywood - 30" (.04)
Solid (13/16") - 36" (.04)
1-1/4" edging on plywood - 36"
The The Sagulator is a shelf deflection calculator. The examples are based on a 10" deep by 3/4" thick shelf spanning 36" and evenly loaded with 60lbs (20lbs per foot).
Plywood - .09"
MDF - .30"
Melamine - .41"
Solid wood - .06-9"
The melamine/MDF numbers seem a bit high... I should probably redo with a 30" span (common in lighter duty applications). I also would have thought that there would have been a bigger difference between solid wood and plywood.
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