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Re: New Masonry on Existing Slabs on Grade

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Title: RE: New Masonry on Existing Slabs on Grade
It really depends on the load on the wall and the capacity of the slab to act as a footing.  The slab capacity depends on the strength of the concrete, thickness of the slab, and the capacity of the subgrade.  Of these, the subgrade capacity is the most difficult to check.
You could check what soil bearing pressure you would have based on the capacity of the slab to cantilever past the edge of the new wall.  (I've used wl^2/2 for the slab moment (assuming the slab will crack), and treated the slab as a plain concrete section fb=6*M/(b*t^2).  Be conservative with t because the tension side of your section was cast against soil.  Use the structural plain concrete section in chapter 19 of the UBC.  Fb=5sqrt(f'c) IIRC.)  If the calculated bearing pressure is low, the slab may function adequately.
You could sawcut a crack control joint at the calculated crack location based on the maximum cantilever and then have a nice straight crack instead of a random one.  OTOH, if you're going to this expense, just sawcut the full thickness and put in a footing while you've got the saw there.  Plus, you can cast dowels for your wall steel in the footing instead of epoxying bars into a slab.
Do you have any jamb forces and in-plane shear on this wall that might require a footing anyway?
Just a few ideas
Conrad Guymon, P.E.
----- Original Message -----
Sent: Wednesday, February 20, 2002 3:58 PM
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?