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#46059 - 12/11/04 12:53 AM Membrane under slab  
dmattox  Offline
Member
Joined: Oct 2004
Posts: 265
Anaheim, CA
I am doing a project where they have an environmental membrane under the slab and footings of several tilt-up commercial office buildings. This membrane as a side effect will act as an insulator between the building and the earth.

The E.E. has requested I install 2 ground rods outside the building, which I will do. I will also use the cold-water as a grounding electrode and bond building steel and the ufer.

One last thing, the cold water is just barely over 10' copper in the ground before switching to plastic.

Doing what I described seems to meet the code minimum, but I am just concerned this is not sufficient in practicality. Are there any other issues you guys can see?


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#46060 - 12/11/04 11:36 PM Re: Membrane under slab  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
Dont see why it would not be sufficiant? Many buildings out there without Uffer...


Mark Heller
"Well - I oughta....." -Jackie Gleason

#46061 - 12/12/04 07:24 AM Re: Membrane under slab  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
dmatt,
Are you talking about a layer of Alkathene sheeting as a Damp-proof Membrane?.
Personally, as far as you've explained your intended course of work, I can't fault it.
The only thing I would suggest is, that you make sure that the outer cladding (if metallic) is adequately bonded in all places and then Grounded. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#46062 - 12/12/04 08:17 AM Re: Membrane under slab  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,259
Fullerton, CA USA
Is this one of the methane membranes like they're used in SFS?
I did one up in that area, and poured a little "dedicated" footing outside of the building/membrane for the Ufer.

Sounds to me like you should be fine as you've got it planned.


#46063 - 12/12/04 09:17 AM Re: Membrane under slab  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
I'd suggest being aware of any bonding _between_ buildings, in particular if there are any 'metallic structures' that extend between the buildings.

I presume that each building will have its own service, with its own ground/neutral bonding. Any metallic connection between the buildings will be a parallel path for the neutral return.

I don't know the scale of these buildings; if they are all fed from a common transformer, or if they have their own transformers.

If the metallic path is not continuous, you might see high localized potential differences (eg. a non-continuous hand rail, with one side attached to one building, and one side attached to the other building, with a small gap).

If the metallic path is continuous, you might see high currents in the event of a fault.

I don't believe that this is an issue addressed by the NEC; multiple houses are permitted to be fed from a single transformer, each service with water pipe bond, even when the water pipes are continuous underground. Just something to keep in mind when you are looking at the plans.

-Jon


#46064 - 12/12/04 11:32 AM Re: Membrane under slab  
dmattox  Offline
Member
Joined: Oct 2004
Posts: 265
Anaheim, CA
I am not sure of the name of the material being used, I just know its an insulator. I'll ask when I go back.

Electure, SFS?

I wish I had thought of pouring a footing just for ufers earlier. I don't know if the owner would now go for 1 day of backhoe, 20 yards of concrete, and 1 man's day labor + 200' of 1/0. Anyone think this additional expense is warrented?

Winnie, all the buildings are seperated by at least 50' of parking lot, with no metalic connections, so we should be good there.

Thanks for the tips guys, guess I was just being a bit timid dealing with something new.


#46065 - 12/15/04 06:27 PM Re: Membrane under slab  
Bert66  Offline
Member
Joined: Nov 2004
Posts: 80
Belle Chasse, La. USA
I'm not familiar with the term UFFER, what does it refer too?


#46066 - 12/15/04 07:42 PM Re: Membrane under slab  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,259
Fullerton, CA USA
Dave, Santa Fe Springs. Actually the "dedicated" idea came on another job, not as any stroke of genius, but a catch up when somebody swiped the copper electrode overnight, and the concrete crew poured the footing the next morn.

Bert66
A Ufer is a type of concrete-encased electrode. See NEC 250.52(A)(3). The #4 or larger Cu was H.G. Ufer's suggestion after he ran an 18 year test on some from 1942 to 1960.



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