"blueman" wrote in message
...
I'm building a bookcase with adjustable shelves with a 46" span that
will likely carry some heavy books.

That's pretty big span

To cover the load, I am making the shelves 1.5" thick (double 3/4"
plywood).

Plywood is not the most deflection-resiting material as half of the plys are
going the wrong way to help. Solid wood will perform better. If I were you I
would try it out an see how much deflection wou get applying hand pressure
to the center of a test-shelf.


I now want to make sure that the shelf pin setup I am using is as
strong as possible.

- Can I assume that all other things being equal, 1/4" pins will support
a heavier load than 5mm pins (since the diameter is larger)?

I think that is a fair assumption


- Will using a "sleeve" in the shelf pin hole add strength and prevent
tearout from too heavy a load?

Yes it would resiste the tendency for the hole to deform


- Would adding a third parallel column of pins on each side support
*materially* improve the strength?


By 50%.... 3 pins carying the weigh previously carried by 2.

- Any other suggestions to maximize strength of a shelf pin setup?
(note: I would prefer not to add any intermediate supports)

You may not need to take any of these measures. The pins are not going to
sheer off. Try a test to see how much weight it takes to significantly
deform a hole. Just mock up something with a single pin in the proposed
material and put most of your weight on it. I bet it will hold up pretty
well.

-Steve