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RE: PT SOG

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This is one of those innocent sounding "little" problems that is actually
more complicated then it sounds. I think you could probably go ahead and
analyze this for shear and flexural loads as a slab/plate on elastic
foundation, assuming you've got some software to do the heavy lifting. Since
your modulus of subgrade reaction is probably also low, you may need a
fairly stiff slab (thick)to distribute the load. 

If you have load reversals, then a typical PT slab with stiffener webs may
not be suitable. And if you try using a thick slab of uniform thickness to
get the stiffness you want, you will get penalized by needing to provide
more PT. Maybe a conventional RC slab is the way to go here, with some
partial prestress for crack control. If you do use PT, you need to account
for losses due to soil friction on the slab. I would also be sure to
prestress in both directions if you want to avoid cracking problems parallel
to the tendons.

Sounds like pretty lousy soil, probably close to the bay. Are you sure you
don't have some liquefaction to be concerned about here? 

-----Original Message-----
From: Gautam Manandhar [mailto:Gautam_Manandhar(--nospam--at)ci.richmond.ca.us]
Sent: Monday, August 01, 2005 2:39 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: PT SOG


Gm:

I agree it is a mat ( tho'  the designer calls it a SOG).  The designer is
currently neglecting the overturning forces as the PTI program does not seem
to deal with this (I do not have a program manual; I guess it does not based
on the input data printout which does not have an input requirement for
moments).

Gautam

>>> gmadden(--nospam--at)maddengine.com 08/01/05 01:39PM >>>
Sounds like a mat foundation rather than a S.O.G.

-gm

-----Original Message-----
From: Gautam Manandhar [mailto:Gautam_Manandhar(--nospam--at)ci.richmond.ca.us] 
Sent: Monday, August 01, 2005 12:08 PM
To: seaint(--nospam--at)seaint.org 
Subject: RE: PT SOG

Bill:

The project site I am reviewing is in the San Francisco Bay area about 4
kilometers from the Hayward fault.  Foundation consists of fill.
Allowable bearing capacity is 500 psf per soils report.

I have not used PTI slab.  My understanding is that the PTI slab program
is based on  PTI method outlined in Chap 18, Div  III of CBC.   The code
method is basically for gravity load and forces resulting from expansive
soils and does not deal with overturning forces from the superstructure.
The problem is critical when you use the 0.9 DL + - E load combination
(largest eccentricity).

Gautam


>>> bill(--nospam--at)polhemus.cc 07/31/05 06:18PM >>>
> To all the PT gurus:
>
> I have seen many PT SOG designed for gravtiy load.  If a slab supports
a
> three story building, as this thread suggest, how is the overturning
> forces from seismic forces   on the three story building accounted for
in
> the footing design - especially if the bearing soil is very poor (say
500
> psf).  The overturning forces will create eccentric loading on the
slab
> and therefore unequal soil pressure across the width of the SOG.

Don't forget that there are many areas of the world--and the
U.S.A.--where
seismic forces are not a concern for design.

That may be a simplistic answer, but it is part of the answer anyway.

One area I know of where moderate seismic forces may come into play
(depending on the building type) when considering such a foundation
would
be in the vicinity of Oklahoma City, OK, where expansive clay soils are
also a problem. However, keep in mind that PT SOGs are really more
appropriate when you have a lightly-loaded foundation (residential,
multi-family residential, warehouse, e.g.) on expansive soils. Such
structures will typically not be governed by seismic design
considerations
(except perhaps in consideration of frame detailing per the IBC), even
in
moderate seismic zones.

I do know that there are situations where "compressive clay soils" can
indicate the need for PT SOG foundation design, but I'm not familiar
with
the particulars.


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