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RE: Void forms under Grade Beams

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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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