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.

why 3/4" for the back? that will ad a lot of weight to the cabinet
without any real benefit. the back is mostly there to resist racking
forces (keeping it from becoming a parallelogram). 1/4" ply is more
than sufficient for that, although it will feel a bit hollow at 48"
wide. 1/2" backs will take care of that and make the case a lot easier
to handle.



- Plan is to create 48"x12"x1" shelves by gluing together two sheets of
1/2" birch plywood.

consider preloading the shelves when you glue them up. that is, glue
them so that they have a crown or upward curve equal to the amount that
they will sag once they are loaded.




- 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.

4' is a pretty long span for plywood shelves. given that you are
laminating 2 layers you will likely want to cover the edge anyway, so
make the edges wider than the shelf thickness by another inch or so.
it'll help avoid sag.





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)

some testing is in order.





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)

if you determine that a preloaded shelf laminated from 2 layers of 1/2"
BB is insufficient, a torsion box shelf or center pins may be in order.
if you go torsion box, consider 1/2" for the top layer, a 3/4" web and
1/4" for the bottom.

another approach would be to make the shelves fixed. then you could
rabbet the shelves into the sides and pin it solidly to the back. you
lose the adjustability, but you gain a lot of strength.



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?)

the pins are plenty strong. where you may have failure is the wood
carcase that the pins are supported by. it's a lot of load on a 1/4"
wide section of wood. once the load reaches the crush point of the wood
fibers the hole elongates, the pin starts to shift, reducing the area
it has in contact with the wood, increasing the point load and failure
quickly follows.





Also, do the sleeves do much to strengthen the holes or am I just
as well off without them?

the sleeves increase the load area, and if you glue them in they help
resist shifting.