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whir (Structural) (OP)
11 Aug 08 8:53
I have a one story building in the Houston area where the geotechnical engineer is telling me that they don't want void forms under the grade beams supporting the building because they don't want water to get in the voids.  He says to instead to design the grade beams for 1" of uplift.  
I have not seen grade beams designed without void forms underneath.  That means that the peirs will have to be designed for some uplift load from the swelling soil.  Has anyone had any experience designing grade beams without void forms?
dougantholz (Structural)
11 Aug 08 9:07
For your design I would design it for gravity considering that the void forms are there (just in case there is settlement of the soil) and then check it for a reverse load condition that would create 1" of camber in the beam.  Grade beams without void boxes are pretty common in the rest of the country without swelling soils.  Its almost like a trench footing, except it hits a pier every 30 ft or so.
And, yes you should check the pier for D+H load combination that could create a tension, although it doesn't seem likely you'll be lifted out of the ground unless you have a very light building.
lkjh345 (Structural)
11 Aug 08 9:40
Any chance you could use closed-cell compressible joint material below the grade beam?

Closed-cell material will not absorb or transmit water, and will not transmit soil heave pressures to the bottom of the grade beam.

Just a thought.  
dik (Structural)
11 Aug 08 10:02
I've done reports on gradebeams that have heaved due to ice forming in the void; it's not common.

I ususally spec a material called 'Frost Cushion'; it's a closed cell foam material that is cut to minimise the uplift.  Foam materials often have a crushing strength of 5-10 psi which adds up...

Dik
osubae (Structural)
11 Aug 08 12:10
lkjh345...Do you have a spec for a closed-cell compressible material to use in this situation?  Have you had any adverse effects?
lkjh345 (Structural)
11 Aug 08 12:49
pb2ae:

As I think about this problem. I realize that expansion joint material would not work at all. It would not support the weight of the wet concrete.

A bad idea from me. Glad others have good ideas.  
JAE (Structural)
11 Aug 08 12:51
whir,

I think your geotech may be a bit off base here.  Tell me...how in the world does the presence, or non-presence, of void forms allow water to get to the underlying soils?  I see no difference.  If there is water draining into the area, then the voids won't add to the presence of the water in the surrounding soils.  They simply provide a reservoir for the water...but the water would have been in the soil materials regardless.


I've used void forms under grade beams in the San Antonio area and never had a problem.


 
JStephen (Mechanical)
11 Aug 08 13:35
Frost heave is not an issue in the Houston area.
Mike1144 (Structural)
11 Aug 08 14:10
It could have nothing to do with frost heave. Water will wash the surrounding soil into the void. Our details usually show some type of soil retainer on either side of the void forms. If you picture a 14" wide trench and a 10" wide void form, you get 2" of concrete on each side. Contractors would then pour a  lean concrete just deep enough to bury the void form. That way any rain won't destroy the void forms. The grade beams are then reinforced and the structural concrete is placed.

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