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Re: Stair landing pans and rebar cover

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Dave,

My response/opinion is a conditional "no" (as in no, it need not be fully
encased).  For traditional rebar, then I would say that it should be
encased.  But, you situation is not what I would consider as traditional
rebar.  I liken it to composite metal floor deck.  Composite metal floor
deck does not have proper cover, yet is it used as reinforcement for a
slab deck.  The little divots in the metal deck create the ability to bond
with the concrete and then act as the positive reinforcement.  I would
argue that the weld rebar that you are talking about will act in a similar
manner.  Thus, I would argue that the metal pan in combination with the
rebar acts as the reinforcment.

Now, if the metal pan is not enough in terms of needed reinforcement for
the span (i.e. rebars cause the bonding of the concrete to pan but the
pan is the reinforcment), then you would have to rely on the rebar.  In
that case, then I would say that without full cover of some sort, you
might not get full development of the bars (you might not even then due to
the short span).  If so, then you should take some sort of reduction for
the capacity/effectiveness of the bars.  What that reduction would be is
debatable.  You could consider something the percentage of perimeter of
bar actually encased in decent amounts of concrete (i.e. something like
50 to 60% of the perimeter of the bar).

HTH,

Regards,

Scott
Adrian, MI


On Tue, 5 Jun 2007, David Watt wrote:

> Does rebar need to be fully encased in concrete with clear cover on all
> sides to be considered effective in resisting flexural stress in a
> concrete supported slab?
>
>
>
> I am checking calculations for a concrete filled pan stair landing and
> the detail has #3's@9 welded to 1/8" thick plate in the direction of
> span. The 1/8" plate is considered as form deck and ignored in the
> capacity calculation but the rebar is considered as longitudinal
> reinforcing for a 2" thick concrete slab spanning 40". The rebar is
> welded directly to the topside of the landing pan so concrete cannot get
> all around to fully develop the reinforcing. In ACI the information I
> can find about cover relates to protection of the steel from corrosion.
> I cannot find information relating to cover or lack of cover to
> strength.
>
>
>
> I am used to stiffening the pan with angles and considering it as the
> structural support of the landing (dead and live) and the concrete fill
> as just a walking surface. From my perspective the condition noted above
> is somewhere between a reinforced concrete slab landing and a concrete
> filled pan landing. I just don't know if it is an acceptable way of
> reinforcing a landing pan.
>
>
>
> Any thoughts
>
>
>
> David M. Watt, PE
>
>
>
>
>
>

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