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Re: FNDT: Shearwall and slab foundation question

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Dennis,

You might consider the following rational, albeit simple method for design
of a thickened edge & slab "T-grade-beam":

1)  Use 4x the thickness of the slab as the effective width of the flange on
either side of the thickened edge, or on one side only as an "L-beam"
(reference ACI 8.10.4).

2)  Calculate the controlling moment of inertia for the T-beam and use this
as the input value for calculating grade beam shear, moment and soil
pressure distribution along the length of the grade beam.  NAVFAC DM7.2 has
a good treatment of grade beam calculations that lends itself to a MathCAD
template...
Check the grade beam for both the point load at the chords and the
couple-moment to get the controlling condition.

3)  Design the T-beam appropriately using the forces calculated in #2 above
for both concrete design and allowable soil bearing.  Transverse
reinforcement connecting the thickened edge to the slab flange(s) is
required to support the respective soil pressure on the flange as a
cantelever beam  projecting from the thickened edge, which may limit your
flange width below the 4x suggested (ACI 8.10.5.1).
    
Respectfully,
Joe McCormick, PE


At 01:06 PM 2/4/98 -0800, you wrote:
>A recent plan check correction stated "For holddowns HD8A or above, please
provide a gross stability calculation of the holddown and bearing pressure
calculation under the shearwall footing."
>
>If the wall is supported independently on a grade beam, I would have not
problems with this. However, the walls are supported by either a continuous
thickened edge around the slab and the slab itself which contributes
resistance by nature of the tensile connection through the wire mesh. A wall
in the center of the slab which rests upon a foundation monolithically
poured should also be able to consider the effect of the slab connection to
the grade-beam to resist sliding and uplift.
>
>I would like some opinions on this issue. If anyone has done an analysis
that they can share with me that takes the slab and friction between soil
and slab into consideration for both resistance to uplift and protection
from over-bearing soil pressures, I would be appreciative.
>
>Finally, I do not mind increasing the foundation size as though it were
independent of the slab, however, I would still question this in order to
save the client some money.
>
>Dennis Wish PE