On Wed, 20 May 2020 15:33:51 -0700 (PDT), wrote:

Will this prevent any sagging at least for 10 years, if not 20+?

Basic idea.
I am building a bookcase primarily of hickory for my reference library. The library consists of the Oxford English Dictionary, Great Books of Western Civilization, various books on programming languages, ...

Calculations of load.
Many of the shelves will be loaded with books for full 4' length and books of 12.5 inches tall by 10 inches wide. That gives a total volume of 6000 cubic inches or 98322.384 cubic centimeters. Based on the specific gravity of paper of 1.2 grams/cubic centimeter, that yields a load of just over 260 pounds. Allowing for 10% safety margin, that is about 290 pounds load per shelf.

Design:
The shelves will be .75" x 11.5" x 48" hickory boards. Under the back for support will be a .5" x 1" x 48" (w x h x l) board. The sides will use standard metal shelf braces. I wish to drill two 3/8 inch holes the full length of the shelves, one in the middle and one an inch back from the front and insert 3/8" diameter stainless steel rods for further bracing.

You may have some problems with drilling 3/8" holes 4 feet deep and
round rod is not particularly stiff. I wonder whether a different
solution might not be simpler and, perhaps, stronger. For example 3/4"
angle inlayed on the front edge of the shelves. Or even 3/4 inch flat
stock fastened to the front edge with multiple fasteners.
One assumes multiple fasteners along the rear of each shelf to the 1"
rear of the case.
--
cheers,

John B.