I want to build a bookcase as part of a large built-in, and due to the geometry of the situation, it would be desirable to have one column of shelves be 66" wide. The problem is that even using hardwood boards, a shelf 66" will sag under the weight of heavy books.
So, the natural solution is to use a a steel flat or angle to reinforce the front edge of the shelf and then hide it somehow so it cannot be seen.
How can I do this?
You can put a 1/2"-thick wooden board over the metal piece to camouflage it. And if the shelf is really that long, you'll need something strong for the metal piece. A structural engineer could probably tell you the minimum dimensions, but I would guess a 1.5" angle, or square tube, would do it. And that means you're limiting access to the shelf below that.
If this was my shelf, I'd focus on finding shelf supports that take up as little space as possible. Something like this:
A good way to add stiffness to a book shelf is to add an edge wise board along the underside edge of the shelf. If you are able to support the long shelf in one or two places along its length at the rear of the shelf then you can get by just adding the stiffener along under the front edge of the shelf. Appearance can be improved by setting the stiffener back some from the front shelf edge.
If the stiffener is made of the same material as the shelf then it is easy to use the same finishing on both pieces.
The stiffener can be attached to the shelf in various ways but all should include a glue joint all along the top edge of the stiffener board. Methods can include:
I recommend trying out the Sagulator.
It suggests that a 66" run in 1" Beech should have an acceptable sag. If you try to reduce the thickness to ½", it will be very saggy. An edging strip will have to be quite deep to strengthen the shelf.
Those calculations assume the back edge is unsupported. If you support the back edge, and add brackets, things will get a lot better (but the Sagulator won't tell you how much better).
If a steel flat is secured to the back edge rather than the front it'll hide better. If the flat bar must be taller than the thickness of the shelf it could be concealed by installing a thin false back sized to fit neatly between adjacent shelves.
There's probably little benefit in choosing an angle rather than a flat bar. The vertical section is what gives strength to resist sagging; the second leg of an angle would serve mainly to prevent the vertical leg from twisting and failing. A flat bar can be prevented from twisting simply by securing it well to the wood shelf.
For that matter, a very thick false back (plywood, for instance) fitted between the shelves would accomplish the same thing as a front edging strip or a rear support stringer without standing out as an obvious support structure.
Use Unistrut, such as A1001A or P6001A. Flat surface up.
Several rows of it. Tie them together somehow - either tackweld them or tie them with rigid steel straps on the bottom which bolt into the Unistrut to hold it parallel and surfaced. Have 3 of these at the 1/4, middle and 3/4 of the span. They could be pretty conformal, say 3/16" thick if you use 3/16" strap and flat-head bolts into countersunk holes.
You could replace some of the Unistrut in the middle with a wood plank of same thickness.
Instead of just using a plank, have you thought of making the shelf more like a floating shelf instead? If you do that, you could mount the shelf directly to your studs and provide significant support in a hidden manner.

