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Re: Slab connection to Exterior Walls

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The base connection to the slab is necessary for tilt-up walls, but not if there are dowel bars up from the footing into the wall like in CMU. The situation description more than slightly confusing (because of the number of conditions) ... when you say ties, I would say dowels if I'm understanding right, ties to me are shear reinforcement. :-)

Closure Pours around the perimeter are always a good idea for slab on-grades for shrinkage. It's the best way for New Tilt-up wall panels to be locked in at the foundation for out of plane loads as well.

-gm

On 8/30/07, Donald Bruckman <bruckmandesign(--nospam--at)verizon.net> wrote:
We do a lot of big box retail work. In the course of that work, we get
involved in remodeling existing big boxes into new, squeaky clean big boxes.

Here's the question.

We run into a myriad of various conditions at the perimeter wall.  We see
rebar ties coming out of the wall into concrete with wire mesh or no slab
reinforcement at all;  We find cold joints at the slab perimeter with no
rebar wall ties at all, we find full rebar slabs tied to rebar wall ties,
etc. We've seen just about every combination you can come up with.

My questions are these:

1.      Is this kind of base connection ALWAYS necessary?

2.      Is it dependent on any particular structural design which would
allow it NOT to occur, such as a grade beam array?

3.      If I find no connection at all at an existing wall, should I worry?

4.      Why do engineers sometimes show turned down footing to bear the slab
on the footing below at this connection?

5.      If I cut an existing tie, would there be some way of knowing if it
was a necessary connection or not?  (I ask this because I sometimes find a
lot of structural details that are habitual and not necessarily borne of
necessity, because if it becomes a problem, the engineer will say something
like, "Oh, forget it then, that's just our typical detail, you don't really
need it.")


After we resolve these questions, I run into the second half of the problem.
If I pour a new slab on grade that is tied to the entire perimeter wall
structure via these ties, I have to pay a lot of attention to shrinkage
cracking.

A fully reinforced slab tied to the four sides of a big box introduces all
kinds of (I assume) rebar distributed stress into the slab as it cures and
shrinks.  My empirical conclusion to this is that the ties to the walls
induce tension stresses that sort of "prestress" the slab and make all other
additions in stress to it a potential cracking problem.

I find that nearly any rigid element passing through the slab or butted up
against the slab (pipes, etc.) in these situations can induce enough
concentrated stress to create a crack.

I've gone so far as to introduce pourstrips around the perimeter to allow
the main slab to "float" while it cures, and that seems to help a lot.

The bottom line of this, I guess is crack control.  The more I understand of
the reasoning behind this critical joint in a building system, the better
armed I am to control the finished product.

Thanks.


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--
-gm