Structural Slab, Grade Beam Alternatives?

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Old 08-22-17, 08:49 AM
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Structural Slab, Grade Beam Alternatives?

My builder informed me this morning that due to weaker than expected soil conditions, they will need to make my garage floor slab structural using grade beams. The soil is 2,000 psf clay, but there is a lot of backfill from original construction in 1983. They had to dig down 8 feet as opposed to 4 feet in one location to hit stable soil. I have no clue how much the grade beam install is going to cost, but I imagine it is pretty significant. Are there alternatives to using grade beams? The existing home has a 10" thick foundation wall and has a finished basement, and the new garage slab will be about 12' wide by 25' deep. Could they drill and epoxy rebar into the existing wall on the left side and have the slab sit on the new wall on the right side (on a 6" shelf for example)? Perhaps use extra rebar throughout and make the slab thicker?
 

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Old 08-22-17, 09:39 AM
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Man, you just can't catch a break, can you?!

Generally, you don't want to try to tie new construction into existing unless you have a very detailed structural analysis. What happens many times is that the existing is not designed to take the additional loads of new construction.

Hard to say what the best way is to handle the poor soil conditions. If you or the contractor have a licensed structural engineer on board, I would rely on the structural engineer to come up with the most efficient system. Just ask questions and express concern about cost. If it's any consolation, at least it appears the contractor is trying to do things right and not throwing something together, taking your money and driving off into the sunset.
 
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Old 08-22-17, 09:53 AM
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Doubt this is an option, but around here, a soil analysis sometimes leads them to pile tons and tons of soil onto a lot temporarily (for many months) to compact the soil... then when the engineer says so, the fill is removed, and the the soil is retested.

I imagine they have come to the conclusion that the beams are the most timely and cost effective method. But I find it hard to believe that the soil can't be compacted, and the pad reinforced with rebar. The footing is one thing, the pad is another... After all you aren't driving a tank on it.
 
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Old 08-22-17, 09:55 AM
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Things have not been going well, needless to say. While I am glad they want to do things properly, I can't help but be concerned that they can execute this properly based on the way things have been going. Hopefully they will split any additional cost associated with this unforeseen complication due to the amount of stress, aggravation, and lost work I have experienced thus far. I'm almost at a point where I just want them to fill everything back in and plant some sod so I can have my home back to normal. Unfortunately, half of my porch and driveway have already been demolished, so that isn't really a viable option at this point.
 
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Old 08-22-17, 10:00 AM
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Doubt this is an option, but around here, a soil analysis sometimes leads them to pile tons and tons of soil onto a lot temporarily (for many months) to compact the soil... then when the engineer says so, the fill is removed, and the the soil is retested.

I imagine they have come to the conclusion that the beams are the most timely and cost effective method. But I find it hard to believe that the soil can't be compacted, and the pad reinforced with rebar. The footing is one thing, the pad is another... After all you aren't driving a tank on it.
By compacting, do you mean backfilling a foot or so at a time then tamping the soil with a tamper rammer? Couldn't they mix gravel and sand in with the clay backfill, tamp it down, and end up with a pretty solid base?
 
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Old 08-22-17, 10:05 AM
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I had a similar though much smaller problem when I added on my toy car garage. Where it met the existing construction I had to go slightly deeper than the old footer. Then I removed all the back fill and replaced it with crushed stone. Because of your volume of fill it may be more economical to go with grade beam or a structural floor but in my case removing and replacing with crushed stone was less expensive.
 
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Old 08-22-17, 10:21 AM
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Yes, but like I said, maybe they figure it will be less expensive and/or time consuming to use the beams... soil testing and engineers are very costly, so they might just be taking an easier route.
 
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Old 08-22-17, 10:25 AM
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You're likely right. Grade beams require an engineer don't they? Or at least require an engineer to stamp the drawings?
 
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Old 08-22-17, 10:46 AM
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Yes, probably, but they probably don't require any on site visits or testing. ($$$)
 
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Old 08-26-17, 10:59 AM
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If this slab in your garage is not going to be supporting anything except the vehicle parked on it, it certainly sounds like a grade beam is a major overkill. Grade beams are designed to be used to support structural elements of a building, not just a floor slab. 2000 psi soil should be very sufficient to support a floor slab. If the area they excavated is just filled in with a self compacting gravel I don't see any reason that a floor with rebar could be poured on top of it and hold up very well.
 
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Old 08-26-17, 12:27 PM
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Remember that the difference between a 8" thick poured concrete foundation wall that was originally planned and a grade beam is just a (very) little extra reinforcing. If I am remembering correctly, wasn't there a structural engineer that came up with grade beams and a structural slab?
 
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Old 09-11-17, 09:17 AM
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If this slab in your garage is not going to be supporting anything except the vehicle parked on it, it certainly sounds like a grade beam is a major overkill. Grade beams are designed to be used to support structural elements of a building, not just a floor slab. 2000 psi soil should be very sufficient to support a floor slab. If the area they excavated is just filled in with a self compacting gravel I don't see any reason that a floor with rebar could be poured on top of it and hold up very well.

I'm still not sure what their plan is (don't think they know either), but they had the concrete contractor form a slab ledge on the perimeter wall. My best guess is they are going pour a structural slab with extra reinforcement (rebar, ww mesh) and let it rest on top of the foundation wall. That way if there is any heaving of the soil below, the slab can rise and fall if need be. Seems to me they could do away with any fill underneath (by using void forms). That way the slab would never experience any vertical movement. Question I have about this approach is what would support the slab on the left side where the existing home's foundation is? Bolt a steel support/ledge along the entire wall so the slab has something to rest on?
 
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Old 09-11-17, 09:52 AM
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This is what I'm dealing with. What are my options for a structural slab? I'm hoping they didn't screw up by putting the ledge on the new wall. Considering it is wood framing on the house side at that level, supporting the slab on a ledge on the right side and an alternate method on the left doesn't seem wise. And if the ledge on the right side isn't used, then how is that gap going to be filled? I suppose they could fill it in with CMU block and cap it with a 2x6 sill plate.
 
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Old 09-11-17, 10:03 AM
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Perhaps something like this would work...
 
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Old 09-11-17, 10:07 AM
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Here is an actual photo of the site prior to the walls being poured...
 
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Old 09-11-17, 11:02 AM
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Just throwing ideas out there guys. I'm not experienced in this area. I think one thing is for certain, the slab should not be anchored to the walls, and one side should not rest on a ledge while the other side is unsupported.
 
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Old 09-13-17, 11:27 AM
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Slab on Expansive Soil

The builder of my garage addition has indicated they will be pouring a structural slab because of my expansive soil (clay). It has been three weeks and they have not given me any details with regards to their approach, despite asking them repeatedly. Anyway, my question is, should I have them go ahead and backfill, do the framing and roofing, then wait a couple weeks prior to pouring the slab? It has been raining quite a bit and it seems to me that giving the ground a few weeks to dry out would be a good thing to minimize the chance of settling. There's a huge pile of dirt in my front yard (has been for four weeks) and there has been no rain for at least a week, and you can clearly see cracks in the soil from it shrinking.
 
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Old 09-13-17, 11:49 AM
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Foundation drain necessary for garage?

I'm having a garage addition built (with poured foundation walls) and am wondering if I should install a foundation drain, or if drains are only needed if there is livable space on the other side of the wall. The foundation wall is about 2 feet below grade, and my concern is moisture making it's way up under the slab and contributing to frost heave. Also, should I apply damp proofing to the exterior walls?
 
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Old 09-13-17, 12:38 PM
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I assume they intend to use extra rebar or rebar instead of wire mesh. I painted some houses that were built in a drained swamp. They had to pour monolithic slabs with a lot of steel, as far as I know those houses did ok.
 
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Old 09-13-17, 01:02 PM
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Yes extra rebar, but I also planned on having them use ww mesh as well. Regardless, would it be a good idea to let the soil dry out first before doing anything?
 
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Old 09-13-17, 02:36 PM
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If the soil/fill is wet it will slow the set up time of the concrete placed over it, not necessarily a bad thing - a lot depends on how wet it is.
 
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Old 09-13-17, 02:47 PM
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There are no drain tiles around garages but its not going to hurt anything if you install, same for sealing!
 
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Old 09-13-17, 03:15 PM
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Did they install a moisture barrier beneath the forms and rebar . . . . just as you would for a building intended for living quarters ?

I've run into a few recently where no moisture barrier was installed in damp areas and the occupants had a perpetual mold problem.
 
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Old 09-13-17, 05:28 PM
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A whole lot depends on what the plans are for the garage.
"Poured foundation walls" do you mean stem walls?
If I build a garage that's going to be worked in It's going to have have foam insulation and a vapor barrier under the slab.
 
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Old 09-13-17, 06:40 PM
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Only the stem walls have been poured at this point. No backfilling has been done. Their plan is to backfill the exterior with the native clay soil, as well as the perimeter of the interior, then putting a layer of compactable gravel on top, followed by a 6 mil moisture barrier, then a structural slab. For some reason they had the concrete contractor put a slab ledge on the wall, but I think they screwed up because there would be no ledge on the left side where the existing home is (slab will butt up against the rim board). So the slab would be supported unevenly, which doesn't sit well with me.
 
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Old 09-13-17, 06:44 PM
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My concern is the soil they removed has been sitting in a huge pile in my front yard soaking up rain water. And the area where the slab will eventually be poured is saturated as well. If they dump all of that heavy wet clay back that area, they won't be able to compact it properly and when it evenlltually dries out, it will shrink. If there is a slab on top, it will settle and possibly crack of it settles unevenly. This is why I'm thinking the garage should be framed first to allow the soil to dry out for a couple weeks prior to pouring the slab.
 
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Old 09-13-17, 07:23 PM
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Apparently it's pretty common to do the framing first. I'm going to go ahead and ask them to get started.
 
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Old 09-14-17, 12:52 AM
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All posts merged. All related to same project. Makes for better throughput for members replying and also us moderators.

Thank you ....
 
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Old 09-14-17, 04:46 AM
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All posts merged. All related to same project. Makes for better throughput for members replying and also us moderators.

Thank you ....
Fine with me. Thanks. Would be nice if there were a way to consolidate questions regarding a single project so we don't feel like we have to create new threads for each question.

My primary questions from this thread:

1) Is it okay to float a structural slab on grade with foundation walls on both sides?

2) Will resting a structural slab on a slab ledge on three sides but not on the fourth cause issues (stress fractures)? The fourth side is 17 feet long.

3) Will leaving the short stem walls (perpendicular to existing home) that supported the old porch slab and that will be underneath the new garage slab cause issues with uneven support?

4) Should I have the garage framed/roofed prior to having the slab poured to allow the clay soil to dry out for a few weeks?

5) Is it okay to backfill with the native clay soil to a certain height (12" lifts?) then put a gravel base on top, or is it better to mix the gravel with the clay for the entire fill? I'm guessing mixing it is the best, unless the soil is undisturbed. Only the perimeter of the garage has been disturbed and the entire mudroom has been excavated and needs to be filled back in (about 8 feet of fill).

6) Should I insulate under the garage slab even though the garage will be unconditioned? I'm in northern VA, climate zone 4 (gets down to single digits for a week or two during February, but mostly lows in the 20's during the winter months). I may have a space heater in there on occasion during the winter when working on my cars, but that's about it. Maybe I just do a 2' perimeter?

7) How thick should the gravel base be considering the soil condition (<2,000 psf)?

8) What's the minimum thickness the slab should be considering it will be structural and taking the 2,000 psf soil into account?

9) How many control joints should I have for a 35' x 13' structural slab?

10) Should the perimeter of the slab be thickened or is this technique only for slab on grade that supports walls?

A lot of questions, I know, but the builder hasn't offered any information whatsoever.

Here's a plan view of the site:
 
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Old 09-14-17, 05:37 AM
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A whole lot depends on what the plans are for the garage.
Parking my car, working on my car/truck, motorcycle, house my tools, a workbench, drinking beer

"Poured foundation walls" do you mean stem walls?
Yes, stem walls. I said poured to distinguish between concrete and masonry
If I build a garage that's going to be worked in It's going to have have foam insulation and a vapor barrier under the slab.
Vapor barrier definitely, but will insulation do anything if the space isn't conditioned? I suppose it couldn't hurt, and would likely keep the slab from getting as cold. I was thinking of insulating vertically against the interior stem walls as opposed to under the slab. The mudroom area (back of addition) will be insulated horizontally under the slab.
 
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Old 09-14-17, 06:02 AM
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Would be nice if there were a way to consolidate questions regarding a single project so we don't feel like we have to create new threads for each question.
Well Mike just did that! OR you can just add a reply to a thread if the new question pertains to an existing question, instead of starting a new thread.
 
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Old 09-14-17, 06:06 AM
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I meant in a way that segregates different subjects/questions/comments related to the same project. Maybe not possible in a typical forum format.
 
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Old 09-14-17, 06:23 AM
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I get the impression that when you talk about a ďstructural slabĒ, you donít really mean a true structural slab. A structural slab is one that is supported on two or more side and requires nothing between supports. In other words, with a structural slab, you wouldnít even need to backfill the interior at all because the slab provides all the support. Having said that, I have a hard time imagining youíre really getting a structural slab. If the span is north/south, your maximum span is roughly 33í; thatís a heck of a span, even for a residential garage. If it spans east/west, thereís no indication of how the slab would bear on the existing foundation; assuming the existing foundation was analyzed for the new loads.

Not being familiar with the soils and type of clay you have, Iím afraid I canít comment intelligently about the interior backfill and base. If you have a typical slab on grade, a typical thickness is 4Ē. Control joints generally like to be as close to square as possible, with additional joints where there are corners, such as at the north and south ends.

Suit yourself as far as insulation under the slab. My garage slab is not insulated and itís not a problem to warm it up a couple times a winter enough to take the chill off; and thatís here in MN where it gets well below zero several times a winter, sometimes in the -20ís!

Assuming a slab on grade, there should be no problem working on a fresh slab if itís been cured 5 to 7 days; just no vehicle traffic of any kind.

Thatís as much as I know.
 
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Old 09-14-17, 07:01 AM
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I get the impression that when you talk about a ďstructural slabĒ, you donít really mean a true structural slab. A structural slab is one that is supported on two or more side and requires nothing between supports.
The builder mentioned structural slab on more than one occasion, but also mentioned a gravel base. I don't know what their plan is, and I'm not sure they know either. Considering they had a slab ledge formed into the new walls, I guess they are planning on supporting the slab that way. I didn't realize this was a possibility. What is going to support the slab along the existing home considering the existing foundation wall is 14" lower? Could they dig down, bolt a metal ledge to the foundation, and pour down to the ledge on the left side? Does a structural slab need control/expansion joints since there should be no settling? Like so...
 
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Old 09-14-17, 07:47 AM
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All excellent questions. That's why I sound confused. It is feasible to support a structural slab on a steel angle (I've seen it done), but there's a lot of engineering required: capacity of the steel angle and the fasteners, capacity of the existing foundation wall, bearing length, etc. A structural engineer would be a MUST for this.
 
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Old 09-14-17, 07:55 AM
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All excellent questions. That's why I sound confused. It is feasible to support a structural slab on a steel angle (I've seen it done), but there's a lot of engineering required: capacity of the steel angle and the fasteners, capacity of the existing foundation wall, bearing length, etc. A structural engineer would be a MUST for this.
They said a structural engineer is involved, but they aren't providing any details. And it's been over three weeks. I'm sure they're not used to dealing with a homeowner such as myself that wants details and constant updates, but sorry for them.
 
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Old 09-14-17, 05:16 PM
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I merged your posts because IMO many here have lost interest in your questions.. What happens as I see it as the lost interest gathers no replies. Therefore it seems you start another to regain interest..

I dont think it would be that hard to get the structural engineers name and talk to him about your issues.. Or talk to the town about their code requirements.

You probably should of hired your own structural engineer.. I can only guess why you did not...

So its like beating a dead horse and slowly becomes a nuisance as you do not heed advice given..

Last this is really out of the scope here of a DIY project and these forums.



With that said Im going to close this thread on that basis.
 

Last edited by lawrosa; 09-14-17 at 06:27 PM.
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