blueman wrote:

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

2. Does anything else need to be done to stiffen the span?
I like Rumpty's idea of the imbedded steel stiffener. But other
possibilities might be worth mentioning.
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
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).

Shelf pins along middle of back wall? (I really would like to
avoid the visible line of holes down the middle)


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.
--
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