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?
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.
Re: Membrane under slab#46062 12/12/0408:17 AM12/12/0408:17 AM
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.
Re: Membrane under slab#46064 12/12/0411:32 AM12/12/0411:32 AM
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.
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.
Re: Membrane under slab#46065 12/15/0406:27 PM12/15/0406:27 PM
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.