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RE: Void forms under Grade Beams

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Robin,

In this particular scenario (of which I am not fond), the void forms are
placed under the slab but NOT under the grade beams.  Again, you still have
to design for uplift on the grade beams.  The concept is that as the soil
expands you get local bearing failure (in reverse) of the soil under the
grade beams.  The soil is free to expand in the void under the slab.  That
is why there is supposed to be a limit on grade beam width, and relatively
high uplift soil pressures under the grade beam.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Eric Green [SMTP:EGreen(--nospam--at)walterpmoore.com]
> Sent:	Monday, August 05, 2002 2:29 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	RE: Void forms under Grade Beams
> 
> Riddle me this batman:
> 
> If there are void forms under the grade beams but not under the slab, and
> the soil heaves 6 inches, what differnce do the void cartons under the
> grade beams make? Isn't the pressure on the slab going to raise the
> foundation? Or does the soil magically expand only under the grade beams
> and ignores the larger area under the slab? Please bring clarity to my
> thinking.
> 
> eric green
> -speaking for me, not my employer.
> 
> -----Original Message-----
> From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)bv.com]
> Sent: Monday, August 05, 2002 1:47 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: Void forms under Grade Beams
> 
> 
> Gerard,
> 
> The concept is that the grade beams take up very little plan area as
> compared to the slab.  When the soil expands, it will push upward
> (obviously).  If there is a marginal area of soil contact, the soil will
> be
> restrained in the small areas and the soil will expand around the grade
> beams.  This assumes that your grade beams have significant uplift
> resistance.  1000 psf does not appear to be that great.
> 
> The geotech engineers should also give you a maximum width of the grade
> beams.  I had a similar problem in Colorado, but the geotech gave me much
> higher uplift pressures on the grade beam with a limit on the width of 12
> inches.
> 
> I gave up, made it all structural, and did not try to form grade beams.
> 
> It should be noted that even if you use the cardboard void forms, you
> should
> still provide uplift capacity.  The soil might expand before your forms
> have
> degraded significantly.
> 
> Another concept that is worth looking into is the use of precast composite
> concrete.  You form precast slabs that can support the weight of the
> precast
> and composite plastic concrete unshored from pier to pier.  The precast
> contains all of the bottom steel.  You place the top steel and temp steel
> in
> the field, and cast the topping.  It forms a crawl space of any height
> that
> you want.
> 
> Regards,
> Harold O. Sprague
> 
> > -----Original Message-----
> > From:	Gerard Madden, PE [SMTP:gmadden(--nospam--at)attbi.com]
> > Sent:	Sunday, August 04, 2002 1:32 PM
> > To:	seaint(--nospam--at)seaint.org
> > Subject:	Void forms under Grade Beams
> > 
> > This has been debated before, I think Bill P brought it up a year or so.
> >  
> > On pier and grade beam foundations, soils engineers commonly call out
> for
> > void forms under the grade beams or you have to design for an upward
> force
> > of something life 1000 psf on the grade beam.
> >  
> > I guess I have a little trouble picturing why this is required. In my
> > mind, if soil wants to expand, wouldn't it just expand around the grade
> > beam taking the least resistive path?  
> >  
> > Any comments/ education is appreciated
> >  
> > -gerard
> >  
> 
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