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Re: 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.
> 
Actually, I specify equal amounts of reinforcement in the top and 
bottom, unless I am dealing with moments caused by points loads. In 
this case, I assume, for convenience, that the load is carried by 
simple bending of a section whose length is sufficient so as not to
exceed the soil bearing capacity, when all loads are considered. 
Comparing the cost of reinforcement to the cost of analysis, I doubt 
a more precise analysis can be justified, especially in view of the 
fact that the owner would  get more from $100 spent on reinforcement 
than $100 spent on analysis.

 Robert