Grade Beam Foundation: Standard Dimensio
freematt
| Posted in Construction Techniques on
Is a grade beam foundation for a single story dwellling something that can be built of off standard code specified dimensions?
For example; 24″w x 12″t, with (4), #4 rebars; on top of piers every 8′ (or something like that)
Or is it something that needs to be engineered for each case as they arise? The structure is in Wisconsin which has its own Uniform Dwelling Code, but I would like to hear from builders in other juridictions too.
Also, methods of protecting the beam from frost heave between the piers? — Matt
There are a thousand hacking at the branches of evil to one who is striking at the root. — Henry David Thoreau
Replies
around here the minimum is 18" wide x 12 " thick at a debt of 18 Inches with 3 #5
Well, here are the reaons that I'm considering a grade beam foundation:Yesterday I took advantage of the warm weather, it was about 50 here in S.E Wisconsin and the 10" of previously frozen ground have become unfrozen. I dug down to 48" for a footer and stem wall foundation for a 96 sf addition (12' x 8'). The site is about 500 ft from a lake and maybe 10' above the high water line. The ground in adjacent low spots is pretty soft and it looks like the house is on a pad created from fill that was placed for that purpose. In the course of my excavation I got into a layer of peat at about 42". So now the bottom of the trench is so spongy that it gives way to my feet when I stand on it. A contractor working on a neighboring property had to bring in a soil engineer who determined that the peat layer went down to 9', under which is a layer of blue marl (a type of clay soil). He is building a two story addition of 700 sf. The prescribed solution was to excavate down to the clay layer and build back up to 48" with 1-1/2" gravel and then proceed with a normal footer and stem wall. I thought of grade beam for my situation to avoid enlarging the excavation. Taking my excavation down to 9' on such a small footprint adds a lot of complications. I'm worried about undermining the existing structure and the need to slope back the walls of the dig to prevent the possibility of them falling in on a worker. which means enlarging the area of disturbance on the property and a bunch more soil to stockpile and haul away, all things I'd like to avoid if possible. So, everything hinges on the the depth of the peat layer. If the peat layer is 4' or less I'm hoping I can find a solution that would let me dig down at each corner for a pier to carry a grade beam and backfill the rest of the excavation with gravel. Before I call in the engineers I'm going to do a test hole with a posthole digger to see if I can find the bottom of the peat layer.10 miles to the west of here, on the shores of the same lake, I recently watched builders drive timber pilings down 40' to support grade beams for a condo complex that consisted mainly of a block of three story structures. There are lots of older residences nearby that were built many years ago on normal foundations that seem to be doing fine, which makes me wonder if we don't sometimes go overboard on the engineering factor these days. ;-) -- Matt There are a thousand hacking at the branches of evil to one who is striking at the root. -- Henry David Thoreau
You've got some pretty serious subgrade conditions there, don't you? I do not envy your position.Another way to look at the situation is to consider the bearing competence of the soil in psf. I don't know if you could get an engineer to assign a value to what you have there, it sounds pretty bad, but it's worth a try.If you can get a value (bear in mind that this will change if the height of the water table changes), you might be able to cut the bearing pressure of your building to match.Basically, float the building in a manner of speaking.In general (I mean REALLY general, you need an engineer to check this) the resolution of forces under a footing is a function of the square root of two, so every time you double the depth and width of suitable soil under your footing you cut your bearing pressure in half.It doesn't take a real lot of overex and replace under and around footings to get a lot of bearing.BUt because of the site conditions you've described, I wouldn't touch that on a bet. You could beat dirt into that hole forever.How about digging a wider trench for the footing (designed by an engineer) and using CLSM aka lean fill, then conventional footings and stems on top of that?Here are a couple of links;http://www.nrmca.org/aboutconcrete/cips/17p.pdfhttp://www.prmconcrete.com/flowablefill.htm
Success is not the key to happiness. Happiness is the key to success.
-Albert Schweitzer, philosopher, physician, musician, Nobel laureate
(1875-1965)
I dug a test hole down into the peat layer today. I got down another 16 " (a total of 64" below grade) and struck water! The speed with which the hole filled makes me think I'm into the water table. As a crude test I drove a 6' 2 x 4 with a blunt point cut on the end (like a stake) into the bottom of that hole about an additional 3' where I met substantially more resistance. That would put the bottom of the peat layer at roughly 8-1/2 feet below grade and perhaps 3 feet below the water table. Time to call in the engineers. -- Matt There are a thousand hacking at the branches of evil to one who is striking at the root. -- Henry David Thoreau
Yes, definitely time to call the engineers.Please post here and let us know what you end up doing. I would be most interested. Thanks.
Success is not the key to happiness. Happiness is the key to success.
-Albert Schweitzer, philosopher, physician, musician, Nobel laureate
(1875-1965)
My understanding of model codes is that these prescriptive mnimums are not intended to discourage alternate methods, but if you depart from them you will need permission from the building official in your jurisdiction, which usually means an architect or engineer's stamp.
I've been thinking about grade beam foundations for years, here is what I came up with after consulting an engineer.
Single-story homes don't weigh that much and aren't typically that tall, so compressive strength, tensile strength, moment connection, and overturn are not problems, but frost is.
The approach that my engineer liked was 18" piers to below frost depth every 20', a trench to below frost line with crushed stone under the grade beam (so no frost heaves) and a suitable grade beam with rebar turned into the piers. We were looking at some thick walls, so the beam was substantial. For a frame structure I'd ask an engineer what was the least that was safe. My guess is that you won't need much.
The reason why is that the piers give you your overturn and moment, particularly if you incorporate posts at the piers. With gravel under the grade beam, the beam is uniformly supported.
From the perspective of cost savings, the part about "below frost depth" is the concern for me, especially in cold climates. One of these days I'll take the NAHB guidelines for shallow frost protected foundations to the engineer and see what he thinks. Unless you do it first. <G>
http://www.nahb.org/reference_list.aspx?sectionID=235
http://www.nahbrc.org/Docs/SubsystemNav/Foundations/3808_NAHB_fpsf.pdf?TrackID=&CategoryID=1802&DocumentID=3808
What do you think?
We will bankrupt ourselves in the vain search for absolute security.
-Dwight David Eisenhower, U.S. general and 34th president (1890-1969)
IIRC for shallow frost protected foundation that the IRC now has standardized tables for the design of the insulation. It does not need to be engineered.If you had that and then if you have good bearing soil then the you could use the default footing in the codes, I would think.
Cool -- thanks!We will bankrupt ourselves in the vain search for absolute security.
-Dwight David Eisenhower, U.S. general and 34th president (1890-1969)
Catskinner wrote:
From the perspective of cost savings, the part about "below frost depth" is the concern for me, especially in cold climates. One of these days I'll take the NAHB guidelines for shallow frost protected foundations to the engineer and see what he thinks. Unless you do it first.<!----> <!---->
Wisconsin has it's own "Dwelling Code". I initially tried to do this on a shallow frost protected foundation, but the Wisconsin requirements were for horizontal insulation something like 60" out from the perimeter! -- Matt There are a thousand hacking at the branches of evil to one who is striking at the root. -- Henry David Thoreau
Well, that would not be any fun, would it?
Success is not the key to happiness. Happiness is the key to success.
-Albert Schweitzer, philosopher, physician, musician, Nobel laureate
(1875-1965)
Wisconsin requirements were for horizontal insulation something like 60" out from the perimeter!
Matt, you've got some very interesting ground there. I'll also be curious as to what you come up with, but is the above a big deal? What're we talking about, 200 sq ft of insulation? 2" xps isn't really that much money if that's thick enough. There's a very good reason for running it out that far. Mine goes considerably farther, and gives you an opportunity to divert surface water farther from the building. PAHS Designer/Builder- Bury it!
The problem in this case for the frost protected shallow foundation is that there is an enclosed concrete slab patio next to the addition I'm building. The 60" insulation perimiter would have required tearing up the patio and making temporary supports for the roof, which at the time looked like an unnecessary expense. The building inspector was also somewhat hostile to the idea of the frost protected foundation (he called it experimental), steering me towards the footer and stem wall solution instead. If I had gone with the shallow frost protected foundation I'd be done with the job by now.Over the weekend I made a rig to do my own core sampling out of some pipe fittings and sticks of black gas pipe. I sampled in a different area of the excavation than my last digging experiment and found that the peat layer, which starts at 40" below grade runs to 106" below grade, where a layer of hard clay begins. At first i thought that tthere was less water in the new test site but after a couple of hours it was filled with water up to the -60" point just like my first digging test. This is somewhat consistent with a soil engineers findings on a property a block and a half away in the neighborhood. Except that the peat layer is twice as thick and down far enough to get into the water table (or what I believe is the water table). The rate of water infiltration into the test areas is slow enough that I am thinking that I could dig down to the clay for pier footings and keep it dry enough to pour concrete with a pretty small sump pump. I'd like to build a pier at each corner of the 8' x 12' building footprint and then fill the whole excavation in with gravel up to grade level where I would build grade beams to carry the single story structure. -- Matt There are a thousand hacking at the branches of evil to one who is striking at the root. -- Henry David Thoreau
Wow, Matt, you're sure having some fun...
I built on a mountain, nothing but rock down there, and not very far down. Had never heard of frost protected shallow foundations when I asked for a variance on footing depth. One corner of the house was about at grade and I simply didn't want to buy the extra concrete.
Head guy took a look at my plans, which included an insulation umbrella extending 20' out past the perimeter. I offered to explain why, but he wasn't interested, only pointed out that it'd never freeze down there. Pretty sure he'd never heard of the concept either, but it was intuitively obvious to him.
Having absolutely no suggestions for you, let me tell you about some apartments in Denver. 11 acres, expansive soils (bentonite), piers under the stem walls which had minimum 4" air space under them between the piers. About every fourth pier hole we hit water. Anybody mentioning it to the (mostly absent) engineer wasn't gonna be working there any longer. We dumped as much concrete into the standing water as we could. Sometimes more than a few yds.
Seemed to set up OK and the stem walls were cast over the piers. That was 1969. Apartments were still standing last I looked. And I'm sure glad I didn't have those problems, or yours, here. Wanna buy some solid land? <G>
Good luck. Keep us posted.PAHS Designer/Builder- Bury it!
My garage grade beams are 8" wide x 16" deep with four 5/8" bars and 1/2" stirrups 6" on centre for 3' on either side of the piers. Piers were 10' on centre, 24" dia. Concrete was high strength with 6% entrained air. The surface gravel layer is overlain by 2" of foam underneath the grade beams to provide a crush zone to protect them against frost heave. Piers have 6 x 5/8 bars with four bars hooked and extending into the grade beams. Construction is single storey wood frame, no masonry and no living space above the garage.
Edited 1/12/2006 9:26 am ET by moltenmetal
Whenever i've run across the use of grade beams, it was usually as a last resort (lack of virgin/stable soil, high bedrock, etc.). It was also pricey and required an engineer since it was still a beam. But then again, these were for larger projects like hospitals and university buildings.
Are you doing this to save digging to frost depth (would imagine it to be pretty deep in Wisconsin . . . it's 36" here in central Ohio)? Am thinking there's too many variables for a one-size-fits-all solution.
As far as frost heave between piers, most (at least in Colorado) use the cardboard void forms between piers to make sure that all the weight of the beam sits on the piers and not the ground. Any backfill won't be compacted, so frost shouldn't cause any uplift on the beam.
do they not compact the soil on purpose or just be lazy. 2+3=7
If the backfill was compacted under the beam, some of the weight would transfer to it rather than the piers - then frost heave or expansive soils (clay) could push it up and cause problems.