system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the presence of steel bars in concrete slabs used in construction help in strengthening buildings constructed using them ?

Modern buildings are often constructed with slabs of concrete within which steel bars are enclosed. 
How does just the presence of these steel bars within the concrete slabs increase their strength as compared to the concrete slabs without these steel bars enclosed within them ?
gpt
Concrete is good in compression, but terrible in tension- it’s relatively weak and when it fails it normally fails by snapping without any warning.

Steel is good in tension - it has a high tensile strength and when it fails it stretches for a bit first, so you see things bending and sagging in a bad way before it falls down, giving you time to GTFO.

So you can take a concrete beam/slab and stick steel bars in the areas that are going to be in tension. When you load up the beams/floors the concrete cracks a little and then the steel carries the tension.

This lets you have a structure with the concrete working in compression, which it is good at, and the steel working in tension, which it is good at.