Concrete pier and grade beam in Northeast

[jstoddard]

Hello Building Scientists
I am trying to dig up (pardon pun) some acceptable details for a poured pier and grade beam foundation that would work in a in northern climate. Frost heave under the beam is the concern as the beam has to extend to ground level for aesthetic purposes (it can't be truly suspended).

In the location where this is going there is actually very little chance of frost heave, but I want a detail that will work anywhere. I was thinking a few inches of low-density foam under the beam - but I want to be able to use off-the-shelf stuff, not expensive (and hard to get) geofoam etc.

Any ideas ?

Thanks in advance
Joe Stoddard

[Dick Seibert]

Joe:

Everything we build is on pier and grade beam and has been for 30 years. Because of expansive soils, adobe in the flat lands but the engineers call the same for sandstone hillsides, we have to keep the grade beams 2" above ground. We install 2" Styrofoam and knock it out after the pour, or put 2" cardboard under the beams and let it rot out. My construction supplier who supplies all of my nails, rebar etc. delivers it along with the rebar, foundation bolts etc.

The hardest thing about this system is trying to explain to the customers that they can't dig out and put storage under the house, even if built on a steep hillside. The soils engineers want the water to be able to flow under the house unimpeded, but the customers always do dig out and we have a ton of problems with that. A good friend recently died and I've tried to explain it to his widow and she won't believe me, she believes everybody should be able to have a dry basement if they want to dig one out under their house, try telling that to a soils engineer!

[Dancing Dan]

I'm not sure what the question is - will off-the-shelf rigid hold up with ground contact? I think that rigid 3-4 around the perimeter of the beam will certainly keep frost heave down, if that's what you're wondering - we do it around piers frequently up here in the frozen north and it seems to work well - haven't dug them up in January yet.

[James Eggert]

Joe
What is is that will be on this "exposed" grade beam?
You could use a detail similar to the FPSF, Frost Protected Shallow Footing/foundation, which is now in some of the IRC Codebooks. Not knowing the site details, let's see... what operating system...I mean what are the dimensions of this grade beam, how much is actually buried? What spy software are you using....err, how many bosses on the site each day? What about the aesthetics? What is the code req frost depth?

The FPSF details typically use 2" foam under AND horizontally to each side, for a determined distance. This 2" eps can be buried an inch down or 10" down, depending on circumstances. Just placing foam under and not to the sides will not stop any potential frost movement! However, the rigid eps will handle most compressive loads off-the-shelf. One of the things I would do for this grade beam, and any beam of footing I have possibly exposed to frost action, is to place 2-3 runs of rebar T&B. Cheap insurance. A few #5 or #6(pretty big, #6s) will help hold the beam in place vertically assuming the frost action varies along the length of the beam and helps the beam stay intact. Even #4s are better than nothing, and increasing the cement content can help with limiting water intrusion and subsequent popping of the beam. Just in case you want to leave this beam natural and exposed, 5000 psi concrete is typically considered waterproof!

Did you reboot before you asked this question?

[jstoddard]

Ok - touche... I first created a long post with all the details but then pruned it because the structural issues are not really the issues - but here they are.

There is nothing much going on this -it's a porch. Any vertical roof load is a straight shot down from porch column to pier to footing. The only thing the grade beam is carrying is a concrete deck 8'-10' wide (Contributory load 5' - so 300#/LF dead load tops - live load typical residential floor loads....

- Piers will be 8' OC or so - so the clear span of this beam will never be more than 7'4" or so. It would be difficult to under-size it, in fact beam will be *way* over-sized just because it needs to be to form the skirt of the porch. It can be any width I want - probably make it 6-8" just because it's easy to form and pour that way... but will be 16" or more in depth just because it needs to be to make it from under the deck to the ground - more like a stemwall. The a beam that size is properly reinforced and cured it could hold up a lot more than this porch slab .

- It'll be properly reinforced . Couple of tension bars in the bottom, compression bars in the top, lateral ties, etc. I always over-do that.

I'm hoping that both the bottom of the beam, and the bottom of the deck slab can initially be touching *something* because I don't want to suspend the forms or pour it over steel decking. My plan was to use something granular and loose enough to support the deck that could collapse if there was any frost movement before it pushed the slab up. Under the beam, my plan was like Dick suggested - 2" of low-density foam. But I don't want to knock it out. I want to be able to cosmetically grade up to this enough to prevent critters from running under it.

My question is: "What can I use" to provide the frost cushion. Will 2" low-density EPS do it?. I don't want to purchase special geofoam or cardboard meant for this purpose because I have no source of supply for it, other than buying a truckload of it. It's seldom used around here other than for bridges and the like.

We don't have termites here - too far north (so far) - but carpenter ants and other unwanted critters will dig through foam.

What about this idea of pouring the slab over collapsible fill instead of steel decking ? (remember it's only 8' wide - and I can "form" in some thickening to make it a 2-way slab. We've done suspended garage slabs without much more than that under them - seems like this would work no problem.

I'm also thinking that the flare footings of the "bigfoot" forms resist a lot of frost lift - as the ground freezes around them it it actually contains them. I'm not worried about the piers moving up -I'm worried about earth pressure on the middle of the beam on Feb 15. doing something bad. The sheer depth of the beam (16" ?) for cosmetic purposes should resist a lot of pressure - but I don't know how to actually figure out how much loading frost might actually provide.

I could also use some horizontal foam to discourage frost altogether,but that would require a lot of extra site disturbance I'm trying to avoid. I want to drill a few piers.... form/pour the beam, then cap it like any on-grade project and be done.

The beams will be fully tied to the piers with re-bar BTW.

Also - This whole thing faces SW - frost doesn't penetrate much on that elevation anyway in a typical winter, but I'm just trying to cover my bases in case the global warming slows down.... or a nuclear winter blocks the sun sometime soon.... or in case I want to repeat it on the north side...

JLS

[jstoddard]

Here's a pic of what I'm thinking of doing - make sense?
JLS

[Sweep8]

It seems to me that it would be cheaper and easier to provide a conventional perimeter foundation to frost depth.