[taliaegh] wrote in message
...

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.

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Horizontal 3/8" rods won't prevent sagging for 10 seconds, even if you could
magically drill 48" deep without breaking out through the face of the shelf.
I just went down to the secret laboratory, clamped one end of a 3/8" x 3'
steel rod in the milling machine vise and deflected the other end 4" with my
little finger. All types of steel deflect about the same distance under the
same load, regardless of their hardness.

What you described works in concrete bridges because the steel rods are in
the bottom edges of beams of considerable height, and they are tightened to
resist a pure tension load, not bending.

My heavily loaded wall of book shelves are of 1-1/16" thick x 9" wide red
oak with supports 30" apart, and no sag is visible when sighting down them.
I sawed the logs into 1-1/2" planks hoping for 5/4" finished thickness, but
they cupped during seasoning and 1-1/16" is where the planer cleaned up both
sides of most of them.

https://www.woodbin.com/calcs/sagulator/

The simple fix is to insert upright supports in the middle of the spans.
Mine are simply a friction fit and none have shifted.

What braces the bookcase against tipping sideways?