I took civil engineering a long ago in college but diverged on my career. This is what I recall. In reinforce concrete beams the bending causes compression on

I took civil engineering a long ago in college but diverged on my career. This is what I recall. In reinforce concrete beams the bending causes compression on the top and tension on the bottom. Since concrete is weak in tension (maybe 1/10 of compression strength), the rebar at the bottom will carry the tension. When you do the design calculations, some approximations are made like concrete carries no tension on bottom half of beam and how the stress is distributed on the top portion. Then with iterative analysis you can converge on an solution. Then once you got the basic design, there will other rules like displacement, buckling conditions. Here is an example design calculation I could find: https://civilengineeringbible.com/subtopics.php?i=32 https://civilengineeringbible.com/subtopics.php?i=32