Garage Build Question

[JStructsteel]

Just want to get some thoughts from you guys.

For a garage addition, would you spec a slab on grade over some compacted granular fill with a thickened edge, or provide a design for a footing down below frost and then a separate slab pour?

 

[jayrod12]

Attached garage?

I would have the perimeter walls supported on a foundation that matched the existing house to minimize differential movement at the roof level. The floor could be slab-on-grade dowelled at the man door.

Unattached, then do whatever floats your boat. Likely a thickened edge slab is sufficient.

 

[msquared48]

The garage slab should be sloped slightly for drainage. The stem wall will allow you to do this and also serve as a level base for the stud wall construction. The stem wall also serves to better keep out moisture.

Mike McCann, PE, SE (WA)

 

[BUGGAR]

Do you have thin walls at the garage door that will require special foundations for lateral loads?

 

[DETstru]

Both can be used and have their own advantages. A couple questions that might help us respond:
[ol 1]
[li]What is the frost depth in your area?[/li]
[li]What type of facade are you using (brick, siding, etc)?[/li]
[li]Is this attached to your house or a standalone garage?[/li]
[/ol]

 

[JStructsteel]

Frost depth is 30", it would be a addition to slab on grade garage. Will just be wood or vinyl siding.

At the garage door, could increase thickness to take the load, or some individual footings under the slab.

 

[RareBugTX]

I have dealt with this situation before. While a thickened slab is typically sufficient from structural standpoint, code establishes that footings have to go below frost line. So yes, I would advise you to specify a footing to go to that depth otherwise plan examiner will likely reject it and worst of all, you will be liable for any future settlements. I have seen bad ones in the midwest up to 18". Concrete is cheap and lawsuits are expensive.
Best,
RareBug

 

[RHTPE]

I am of the opinion that a concrete garage floor should always be independent of the foundation that supports the garage walls & roof. This allows for a variety of necessary conditions that should be met.
[ul][li]It keeps the bottom of the wall framing dry (or at least dryer).[/li][/ul]
[ul][li]It allows for the future replacement of the garage floor should the slab not prove to be durable.[/li][/ul]
[ul][li]It easily permits the slope that should be established so that water drains out of the garage (or at least to a floor drain).[/li][/ul]
[ul][li]While some may think it extreme, I would always recommend a minimum slope of 2% (1/4" per foot). Slab cannot be accurately finished such that positive drainage is provided at a shallower slope.[/li][/ul]

Given a frost depth of 30", chances are you're in an area that gets snow. Road deicers will be transported into the garage. With a positive slope the floor can be hosed down as needed to help minimize the impact of deicing chemicals on the concrete.

While there are construction practices that would permit a slab only approach that is above the frost line (ASCE 32), the cost to construct to meet that standard (so that the building official is happy) is likely to be comparable to a basic perimeter frost wall sitting on a footing.

Ralph
Structures Consulting
Northeast USA

 

[jayrod12]

All of the above advice is the best you're going to get.

In the end, I usually default to matching the structure you're tying into. No point in putting a deep foundation if the existing portion is supported on a thickened edge.

 

[moltenmetal]

Not a structural, but curious about this.

Why would anyone suggest that an addition to an existing slab on grade garage be made with anything other than a slab on grade foundation? Not saying it's wrong, just wondering what the reasoning would be, and how it would work? Doesn't a slab on grade move with the frost heave, at least a little? And if so, in either case, how would the tendency for the two slabs (original and addition) be encouraged to move together rather than independently, distorting the building they're supporting?

Based on some research I did for a neighbour, I do know that thickened perimeter slabs on grade with suitable reinforcement are acceptable for detatched garages in my locale, which has significant frost depth. Built without a stem wall, this is very much more economical to construct than a perimeter wall and slab construction would be.

Whether or not you will get any benefit out of a stem wall around the perimeter on top of that slab on grade depends on the grading around the structure, whether or not you want any slope on your garage slab, and what you want the finished building to look like. If the grading is good, you don't need it, though you may want to specify a pressure treated sill plate. If the grading is poor, your framing won't survive long term without it. But I can say from experience that framers hate having to stand stud walls up on top of stem walls, especially when the anchors are already set into the concrete- it's a royal pain. Framers here far prefer to platform frame a structure rather than to stick build it in place on top of a stem wall, toenailing all the studs to the bottom plate and then sheathing it when it's standing, because platform framing (framing, squaring and sheathing a wall on the floor and then standing it up) is way faster and safer and easier.

 

[RHTPE]

moltenmetal: jrisebo (OP) did not originally specify much about the original structure that a garage was being added to. It wasn't until later in the thread that we learned it was a slab-on-grade garage that was being added to. Nor does he mention where (what jurisdiction) this garage is located, which may have an impact on what the building official would expect.

Not knowing the 'lay of the land' surrounding this structure makes it difficult to provide a response tailored to the specific conditions that exist. My comments were more general in nature.

You would be correct in presuming that it is best to match the existing bottom of foundation in most circumstances. However, to assume that the new, plus the existing, would move together is debatable. All of the soil consolidation and settlement under the existing has reached equilibrium. The soil underneath the addition will be disturbed during construction, leaving subsequent settlement after completion a best guess, and future movement due to frost will be an unknown. As Jayrod12 mentioned, defaulting to matching the existing may be appropriate, though I would recommend that potential future independent move of the addition be considered in the details of the framing. One can certainly dowel the new slan to the existing, but that only ties one side of the new to the old. Subsequent settlement of the far side (away from the existing) could cause the roof framing to pull away from the existing.

I had mentioned ASCE 32 (Design & Constructed of Frost-Protected Shallow Foundations) - depending on the local building official's interpretation of the applicable building code (if any), this Standard of design and construction might be acceptable.

Framing the wood structure above a frost (stem) wall need not be exceptionally different than on a slab foundation. The foundation walls are usually (in my area) completely back-filled before any wood framing starts. Ideally, the garage floor slab could be completed in advance of the wood framing. Having the frost wall extend a few inches above the interior floor should not be a major obstacle for the framer. As far as the anchor bolts are concerned, I don't disagree that they're a pain, but they are a necessary item - slab foundation not excluded.

If this is simply what I would consider a "throw away" building, then ignore all of the advice offered and do what you can get away with. It won't be my license on the line. OP asked for our thoughts, and that's what he got.

Ralph
Structures Consulting
Northeast USA

 

[medeek]

With that frost depth how did they eve get away with a slab on grade in the first place? I'm going to assume this was built some time ago.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[JStructsteel]

Thanks for the replies. What if the addition was just a pole barn type structure (but a smaller scale) and the slab then poured?

If the footing is 12" wide, its 2.5 cubic feet per foot. thats about 8 yards of concrete on a 20 x 20 structure plus excavation and labor. I am just seeing if its feasible and wise to try and save that money for a garage.

 

[jayrod12]

You aren't answering the question about the existing building construction. If you want to minimize problems you should be matching that construction. A pole barn type structure is acceptable as a garage on it's own, but it is subjected to significant movement (vertically and laterally) as an acceptable outcome of the economics. I would not be attaching a new pole barn to an existing non-pole barn style structure.