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RE: Void forms under Grade Beams

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Gerard,
We use rectangular "void carton forms" all of the time. They are supposed to collapse under the upheave stresses (the path of least resistance). They are designed to support the wet concrete of the formed "grade beams" during construction (these are structural beams... soil forming is not allowed ) . The "grade" beams are designed to span between drilled shafts. Soil retainers are places on either side of the "grade beam" to protect the "void space" (some folks use trapezoidal shaped void cartons where legs are formed when the beam concrete is placed...we do not allow this type of carton form to be used...). Void cartons are also used beneath the slab, which also is designed to span from support to support.
 
TD
-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: Monday, August 05, 2002 8:17 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Void forms under Grade Beams

Although I have little experience on this particular subject, I have occasionly seen buildings with grade beams detailed with "frost points". These are hand dug points on the bottom of a grade beam that supposedly will deflect heaving soil (due to frost) when it tries to push up on the bottom of the grade beam. I don't know if this really works but I would assume the same concept could possibly work with swelling soils.
 
Jim K.
-----Original Message-----
From: Gerard Madden, PE [mailto:gmadden(--nospam--at)attbi.com]
Sent: Sunday, August 04, 2002 12:32 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