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RE: Void forms under Grade Beams

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I'm talking about pier & grade beams with a crawlspace, not a slab
(sorry if I wasn't clear).

If I was doing a slab, I would make it a mat slab or PT rather than
trying to elevate it on grade beams. Unless of course it's on a landfill
or something, then it's piles and structural slab.

-gerard

-----Original Message-----
From: Davis Parsons [mailto:dparsons(--nospam--at)msc-engineers.com] 
Sent: Monday, August 05, 2002 10:55 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Void forms under Grade Beams

At 1000 PSF, the path of least resistance is the grade beam.

The expansive soils here in Texas are extremely cohesive and do not have
any capability to "flow" around something. When the soil decides to
heave, large areas move and shift anything sitting on them. This
includes foundations and all plumbing lines buried in the ground.

We get soils with PVRs (Potential Vertical Rise) that range from  2" to
12" and PI's above 30 with LL's above 50. Uplift pressures can range up
to 5000 PSF. As a geotech stated in a meeting years ago, "You can't put
enough building on that site to hold that soil down."

We typically put all floor construction off grade and design drilled
piers to resist uplift caused by a depth of the soil lifting on the pier
due to uplift skin friction. Sometimes, piers will have as much as 2%
reinforcing steel area. Depending on the site, grade level floor can be
suspended above the grade by a few inches to several feet. With large
PVRs, we typically have a crawl space below the floor so that all
plumbing lines are suspended above the soil.

I am currently working on a project in Arlington, Texas, in the Woodbine
formation, which requires 12" void forms under the floor slab and 6 to
8" void forms under the beams.

--
Davis G. Parsons II, PE RA
Fort Worth, Texas



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