On Thu, 05 Oct 2006 23:10:46 GMT, blueman wrote:
snip

Even if I assume that the plywood has only 50% of the MOE of pure
hardwood, then using the equation for deflection of a shelf with
uniformly distributed load and supported ends (like adjustable
shelving), I get that the shelf could support about 280lbs and still
deflect only 1/32 inch per foot which is reportedly the approximate
limit for being noticeable to the average human eye. If I assume that
the MOE is more like 2/3 of the hardwood equivalent, then I get a
maximum support without visible deflection of about 380 lbs.

Given that I need to only generously support 200 lbs (and probably
more like 150), I should be OK, even taking the worst case and
allowing for additional sagging with age.

Don't confuse initial deflection with sag over time.

I have a whole wall of bookcases with 3/4 oak plywood shelves. They
didn't deflect much when I first loaded 'em up; barely visible, I
would say. Over time they eventually sag about 1/2 inch (they are
30" wide) at which point it bothers me enough to take the books off
and flip them over (time to dust them anyway!). It probably takes 6
months or so to sag that far.

The shelves with paperbacks don't sag at all; but the ones with
hardbacks all do eventually.

I'd add a stiffener along the edge if I could afford to give up the
extra space, but I don't have enough shelf space as it is.

Paul