You put your numbers in. You get an answer you can build to.

Here is the kind of question the book answers. On the Winterholm slab pours, the heavy rebar cages doubled as walkways, so the supports had to carry a person's weight. The question: what size angle iron do those supports need? You take one formula, put in the span, the load, and the allowable bend, and it tells you a one inch by one inch bar is enough. No guesswork, no engineer's fee, no folklore. Every structural member in the book is solved the same plain way.