Yes, then; dowels. Picture the
base of an 8” masonry wall sitting on a footing. At the slab level,
‘L’ shaped #4 dowels @ 24” o.c. are installed so that they
poke out through holes in the masonry, (or aren’t installed). That
is the connection to which I refer.
Now, I fundamentally understand the base
connection need for tilt-up, but what purpose does the dowel serve in a masonry
structure? Is it stiffening to laterally reinforce the footing? Are
we worried about the footing bending normal to the wall? Do I need to think of
this as a base level diaphragm connection or something?
From: Gerard Madden,
Sent: Thursday, August 30, 2007
Subject: Re: Slab connection to
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
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.
On 8/30/07, Donald
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
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
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
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
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
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
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
induce tension stresses that sort of "prestress" the slab and make
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
The bottom line of this, I guess is crack control. The more I
the reasoning behind this critical joint in a building system, the better
armed I am to control the finished product.
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