Quote:
Originally Posted by ardecila
Yeah the greatest forces on a horizontal element (like a beam or a foundation) are always concentrated at the top and bottom so that's where you put the reinforcement.
It's also why I-beams have heavy top and bottom plates and a pretty thin web connecting them, and you can even cut holes in the web without affecting the strength (note: don't try this at home, kids).
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My understanding is that the mat is supporting a load - both compression and tension - but principally compression.
While Steel is best for both as far as capability, a thicker concrete section will take as much load at a fraction of the cost of steel. The concrete sucks at tension, really sucks, so where the load will be compression only they use concrete, where you expect tension you use steel. In most foundation slabs the tension is transferred to the outside, thus lots of steel on the top and bottom.
( corrections and clarifications welcome and appreciated )