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Re: horizontal bar distance below top of wall?

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Scott:

I am not aware of anything in either the ACI 318 code or the 2000 IBC that
would give a maximum permitted distance for the first layer of rebar from
the top of a wall.  The only thing that would "come close" would be cover
requirements (but that gives what the minimum distance would be rather
than what the maximum distance would be) or what would be required to
satisfy shear provisions (i.e. section 11.5.4.1 of ACI 318 which states
"spacing of shear reinforcement placed perpendicular to axis of member
shall not exceed d/2 in nonprestressed members or .75h in prestressed
members, nor 24 in....thus, if shear is really an issue, it would seem
that a maximum limit for the top layer of the wall steel [assuming it is
needed to resist shear] would be at d/2 [which would likely be a large
number] or 24 in.).

Now, presumably as well, it would still be the engineer's "responsibility"
to ensure that the bar locations could still achieve the minimum required
ratio.  So, to some degree, there could be an arguement made that if
spacing the first layer of bars at the full spacing causes the overall
ratio to be slightly below the minimum code required ratio, then the
engineer would need to adjust the spacing of that first layer to meet the
ratio requirements.  OTOH, the response to the arguement is that if you
are missing the ratio by such a small amount (i.e. a couple percent off),
that you are still meeting the intent of the code.

The only other thing that give some "ammunition" would be section 1910.4.3
of the 2000 IBC that gives tie requirements for the top of columns with
anchors in them for Seismic Design Catergory C or above.  One could
logically extend such a need for walls that might have anchor bolts in
them (although, truth be told, the code does not require that).  One could
further extend the logic beyond that, but again it would be required by
the code.

As to my own practice, I generally have the first layer of wall
reinforcement at about the cover distance from the top of the wall.  This
is assuming that the wall stops at that level (i.e. like a basement wall
or retaining wall).  A wall that continues is a WHOLE other matter.

HTH,

Scott
Ypsilanti, MI


On Thu, 4 Sep 2003, Haan, Scott M. wrote:

> Hello:
>
> How far can the first horizontal bar be below the top of a poured concrete
> wall?  Code reference?  It seems you would need one in the first half of a
> spacing to get your reinforcement ratio or if it was required for shear.  Is
> there something that says you need to have reinforcement within a certain
> definite distance?  Thanks.
>
> Respectfully,
>
> Scott M. Haan P.E.
> Chief of Building Inspections
> Municipality of Anchorage
> Development Services  Department
> phone: 343-8330
> fax: 343-8200
> cell: 317-6934
>
>
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