Quote:
Originally Posted by Fvn
Rebar mats like this are actually pretty "hollow" since all rebar structures are generally massive cages. The reinforcement cage for a mat foundation consists of a thicker layer of rebar on the bottom and the top with supporting bars to hold the top layer up/tie the layers together. The ends of the top and bottom bars are also bent at 90 and then tied together with additional bars which makes it even more like a cage. A mat like this could only have 10 actual layers of rebar.
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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).