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Re: Location of footing reinforcement

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Richard Lewis wrote:
> 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.

Well I think the role of the longitudinal bars is:
1) to tie the transverse bars so that they can be properly spaced and
that they won't twist under normal handling.
2) to provide support for the vertical dowels of the wall.

Putting the layer on the bottom is much more practical than on the top,
but my favorite detail for strip footings is to use closed hoops as
transverse reinforcement with longitudinal bars on top and bottom this
would look like a neat long thick beam with very little reinforcement.
(Unless the contractor and owner object to this little cost increase)

As far as bridging soft spots, I think that the RC wall which is
infinitely more rigid than the footing will take all the stresses caused
by these soft spots !! 

My 2 Piasters worth,

Moni Serhal