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Location of footing reinforcement

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Let me ask an engineering judgement question which I hope sparks a lot of
controversy and causes many different opinions to be brought forth.

A week or so back someone had posted a question regarding minimum thickness
of a concrete footing.  It reminded my of a engineering philosophy question
of where to locate the longitudinal reinforcing in a continuous strip
footing.  I think traditionally we locate longitudinal reinforcing in a strip
footing at the bottom.  My question is WHY?  What is the purpose of
longitudinal reinforcing?  Is it to:
  -span across local settlement of the soil?
  -tie the wall footing together to prevent 
     it from pulling apart, somewhat similar
     to a bond beam at the top of a wall?
  -somehow provide strength to the footing element?

Why does it have to be at the bottom to provide these functions?  Wouldn't it
make more sense to put it at the top?  For strength, a larger load can be
supported over a slight settlement using negative moment at ends then a
positive moment at the middle.  It would be easier to place the reinforcing
and not worry about bottom cover if the contractor were able to pour the
concrete to say 2 inches from the top of footing, lay the reinforcing in
place and then pour the rest of the concrete.  What difference does it make
if the reinforcing is top or bottom in regards to shrinkage?

I look forward to reading many different opinions on this topic.


Richard Lewis, P.E.
Missionary TECH Team

The service mission like-minded Christian organizations
may turn to for technical assistance and know-how.