In article , says...
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?


another option is to rebate the back of the shelves into the backing of
the bookcase, you will lose the adjustability but you say you will be
using the bookcase for textbooks - a known height. rebate 1/8 of an inch
1nto say 1/2 inch ply backing? you should gain a lot of strength and
stiffness