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Re: Location of footing reinforcement

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In the case of a "T" configuration (upside down that is), the longitudinal bar
serves two purposes:  1) to keep the footing portion together if there is any
differential settlement; and 2) to proved some support for the vertical bars in the
stem wall.

Personally, I have always used a second longtitudinal bar at the top of the stem
wall to anchor the sill bolts as well as to provide additional support for vertical
bars.  I believe this is now a requirement in the code.  DSA has required it for
schools for a long time and we required it in the residential retrofit standard
found in UCBC for new foundations.

Sleiman Serhal wrote:

> 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
>