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RE: Foundation on Expansive Clay

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I have read the PCA book and the example you referenced.  Essentially their
design methodology was to design the foundation based on the PTI method and
then size a conventionally reinforced grade beam with a cracked moment of
inertia equivalent to the uncracked PT grade beam.  I think this method is
somewhat conservative, because grade beam deflection may not be the
controlling criteria for a metal building.  Especially if you are using
standard metal panel cladding with little or no interior gyp board walls.
If I remember correctly, you are in East Texas and your soil conditions are
very similar to those here in Northwest Louisiana (WOW, imagine that).  I
have successfully designed a hundred or better of these types of structures
with a "standard" 12x24 grade beam with 2#5's or #6's top and bottom and #3
stirrups @ 12".  I normally place grade beams at the rigid frames and midway
between if the frames are greater than 15' on center.  Many times I design
the grade beams light on the lighter side and install a spread footing
directly under each column, this also helps with counteracting the uplift
forces.  Regarding the soil properties, there are some geotech firms that
provide em and ym and some that do not.  If you don't get all of the
properties you think you need, PTI has provided a method to estimate them in
their manual.

Regarding the BRAB reports, I had a copy along time ago, but they have been
lost.  From memory, they gave minimum beam sizes and beam spacing for
differing PI's.  You may can get a copy of them from your local HUD guy or

Brian K. Smith, P.E.

> -----Original Message-----
> From: Richard Lewis [mailto:rlewis(--nospam--at)]
> Sent: Wednesday, June 14, 2000 9:36 AM
> To: seaint(--nospam--at)
> Subject: Foundation on Expansive Clay
> I am designing a 50'x100' foundation for a metal building on
> expansive clay.
> The Pi is about 25 and the PVR is 1/2 inch.  The geotech is calling for a
> stiffened slab on ground.  He tells me that most foundations in
> his area are
> conventionally reinforced (not post tension) stiffened slab.  He said most
> engineers use the Building Research Advisory Board (BRAB) method
> of design.
> I am not familiar with this method.  Can someone give me
> information of where
> I can find literature on it?
> The PCA text "Designing Floor Slab on Grade" gives an example of a post
> tension design (PTI Method), and then a modification of the design to
> conventional reinforcing.  Can anybody give me recommendations
> one way or the
> other as th whether this is a good design procedure for a conventionally
> reinforced stiffened slab?
> Also, what information should I get from the Geotech for the PCI method of
> design.  The text seems to indicate the geotech should provide values for
> 'em' and 'ym', and obviously the soil bearing value.  Are there
> other items
> of information I should be asking for?
> Rich