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Re: New Masonry on Existing Slabs on Grade

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Dear Zachary
 
How high is the load?  I have done this few times in the past.
 
If the laod is high, I do not see a way around cutting the slab.  I have done it both ways; keeping the top of footing = top of slab or placing the footing below the slab.  In both ways the saw cut was larger than the footing width.  45 degrees from the top of footing if it is below the slab, or 18" larger if t.o.f.= t.o.s.
 
I always placed dowels between the new slab and the old slab to avoid any setlement and then carcks.
 
I hope this is helpfull.
 
Ibrahim
 
----- Original Message -----
From: Zachary Goswick
Sent: Wednesday, February 20, 2002 5:46 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: New Masonry on Existing Slabs on Grade
 

        I should probably clarify some more.  It is an interior slab on grade with
a new masonry bearing wall.  I haven't seen any requirements for this condition
in any code, so I have talked with a couple of other engineers.  Others have
said that they have done it two ways.  Either put the masonry walls directly on
the slab or cut the slab and put in a new footing flush with the slab.  Both
ways have worked in the past for load bearing walls.
        If I do not sawcut, then the masonry wall bears directly on the slab.  The slab
does 2 jobs, it serves as the slab and as the footing (an integral footing).
The problem that I see with this is that the existing slab will crack possibly on both
sides of the wall, depending on a couple of factors such as slab thickness, concrete
strength, slab reinforcement, soil conditions, wall load, etc.  
        One of the problems I see with putting in a footing is that it costs more than doing
nothing and just resting on the slab.  The other thing about putting in the footing is that
it then puts a construction joint on each side of your wall, out from the face of the wall. 
In an industrial environment this may be ok, but when you are trying to reduce the number
of slab joints under tile or some other brittle material in a finished space, it is not ideal.
        I asked these questions because I was trying to get an idea of what criteria other
people use to decide when to sawcut and when not to sawcut.  Is this a judgment call?  For
the responses that said to put a footing in, what did you base this on?  Was it just because
the wall is a bearing wall, or was there another reason?



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