Frost Protected Shallow Foundation... Excavation Costs?

[platinum_jello]

So, recently had my post-frame building put up and now want to give it the FPSF treatment of foam board down the posts (heated slab going in next). Problem is, while this design is easily located on the internet, I'm having a difficult time finding contractors who know anything about it. As such, I'm calling around now for simple excavation costs to cut a 2'x2' trench in around the 36'x48' building, and I'll do the work to put the foam in and backfill. The only quote I've been able to secure thus far has come in at just under $3,000. Can I get a sanity check if this is high, or if I'm at least in the ballpark? Given what I paid to have the site prepped (which included several trucks of road bond for fill), it seems a bit much.

I know location plays into things, so I'll say I'm in the southeastern corner of Wisconsin. And to be clear, I'm not looking for the steal of the century, I just don't want to later find out I foolishly paid a guy who was giving me his "I don't want to do it" price.

Any input would be appreciated.

 

[Firebrick43]

If your willing to do that much work, why not rent a mini excavator and have at it yourself?

 

[PCustoms]

I've never heard of doing a FPSF after a pole building is up

Seems like a lot of work to dig around the structure. $3k seems reasonable given the risk.

Maybe get a quote in a "fresh" site for comparison?

 

[billconner]

I've been looking at and researching SFPF for a few months and not a lot of folks really know much about them, especially for unheated buildings which require an engineered design, not prescriptive. ASCE 32 seems to be the best and accepted standard, the HUD Revised Builder's Guide to SFPF close to it.

I'm a little confused by your goal. Insulating to save energy I get, but it seems generally slabs in post frame buildings are not required to be frost protected. And do you plan continuous insulation under slab? I plan to put it in but see many claims it's not worth it.

 

[jack stand]

I think that you can get away with a modified insulation design where you simply excavate around the perimeter down 18"-20" (below the finished exterior grade) from the inside of the posts outward 4' and lay a full sheet cut around the posts. Then on the interior you can go back to slab depth.
The thickened edge slab that is often illustrated online details a foundation. Your posts are the foundation and presumably are below your frost depth.
All you need to do is keep the cold "out" far enough beyond your slab so that the slab edges - actually the ground below it won't freeze and (2) your heated slab is sitting on the insulation directing the heat into the shop instead of the earth.
These guys do a great job explaining this.

 

[jack stand]

Wrong link, sorry.

 

[Leaflessshadetree]

Digging inside a building adds some obstacles. Generally smaller equipment and limited on how you can position it. Also have to be aware of exhaust fumes. How tall is the inside of the building?

 

[billconner]

10" down and up to 53" out with 1 1/2" foam seems to be starting point for my climate for unheated building. Each inch deeper is an inch less horizontal. But much less for heated building. I'm trying to design it for unheated but make it possible to heat in future, and hard to reconcile the differences.

 

[haveissues]

I'm at the same point as you but I am doing things a bit differently. I'm going to dig a trench inside the building and put 2" foam vertically down 16" or so and then 2" under the slab. I'm in upstate ny so probably a similar climate as you and my floor will be heated. I have a backhoe but I'm going to rent a micro excavator to get it done. If I was you I would rent a mini for the weekend and get it done for a fraction of your quote. You will have a little fun along the way also.

 

[platinum_jello]

Thanks for the responses, all.

My idea to use the FPSF design on a post frame isn't to protect the building itself, so much as the slab inside. I plan to install in-floor radiant, but do not feel confident I will be able to run the heat this year. As such, I was interested in this relatively basic configuration to give me some insurance against any movement that might cause the pad to crack. If it provides energy savings in retaining heat, well that's just a bonus. Otherwise, I intend to do all the standard tricks, such as insulating below and around the floor.

Renting a mini-excavator is always fun, but I know my limits. The last time I had one, the the operation and results were far from elegant. Given that I'm looking to dig right up against a very (to me) expensive building, it's the kind of thing I'd like to leave to a pro. I mean some of these guys can scratch a gnat's *** with their bucket...

But I guess then I'm getting into the part of the conversation where I'm answering my own question... lol

 

[billconner]

Did the quote say how long? Around here machine and operator is around $100/hr. If he's done in a day with one machine, seems high.

 

[ConCretin]

You might be confusing the issue by referring to your slab as a FPSF. As you say, you are really just using insulation to protect a slab from frost movement - not installing a foundation at all.

I'm not really clear on your design. Are you turning down the edge of your slab 24"? That wouldn't be necessary in your case because the slab isn't load bearing in a pole structure. Could you scratch in a shorter section of vertical insulation to get some cover and extend the insulation out horizontally an equivalent distance? This would accomplish the same goal.

 

[billconner]

Brings up an interesting point - does a floating slab have a foundation? I'd say yes. Definition from internet: "the lowest load-bearing part of a building, typically below ground level." Seems to me the compacted fill is the foundation for the floating slab, and perfectly legitimate to protect it from frost. Whether its required by code (for over 600 sf) is less clear. My building official says it is.

The IRC says:
"R301.1 Application.
Buildings and structures, and parts thereof, shall be constructed to safely support all loads, including dead loads, live loads, roof loads, flood loads, snow loads, wind loads and seismic loads as prescribed by this code. The construction of buildings and structures in accordance with the provisions of this code shall result in a system that provides a complete load path that meets the requirements for the transfer of loads from their point of origin through the load-resisting elements to the foundation. Buildings and structures constructed as prescribed by this code are deemed to comply with the requirements of this section."

is the floating slab supporting live and dead loads? And thus a complete load path to ground?

LLWillys -not disagreeing with your approach and concepts - just I don't think this is real clear especially in the codes. And keeping the fill and ground under the floating slab frost free in an unheated building (if only unheated for a year) - by combination of drainage and insulation to capture the geothermal energy - is perfectly legitimate.

My building inspector offered an alternative - crushed stone to frost line - no insulation required. About 120 cy for me. Not considering it now.

 

[platinum_jello]

You might be confusing the issue by referring to your slab as a FPSF. As you say, you are really just using insulation to protect a slab from frost movement - not installing a foundation at all.

Fair point, but I did try to clarify this in my posts by saying I wanted to give my pad the FPSF "treatment" and "design".

Bill, he did not give a timeline in the bid, only noted what the end result would be. Using the $100/hr example, I'd agree the figure seems high...

 

[PCustoms]

Fair point, but I did try to clarify this in my posts by saying I wanted to give my pad the FPSF "treatment" and "design".

What did you write to the potential contractors?

This may be a miscommunication/terminology issue.

 

[jack stand]

Brings up an interesting point - does a floating slab have a foundation? I'd say yes. Definition from internet: "the lowest load-bearing part of a building, typically below ground level."

is the floating slab supporting live and dead loads? And thus a complete load path to....

Bill the posts are the foundation in "post frame" construction. This is the primary savings for a "pile building".
The OP is solely concerned about potential frost giving him trouble and heat loss from his slab that will only support the slab and anything put ON it, there's no building load transferred to the slab at all.
At least that's what I understand about the OP.

 

[billconner]

I understand that the the building walls and roof are all supported by the footings under the posts (assuming no interior partitions right?) but the code assigns a live load to the floor and says that must be supported by a foundation. Seems to me that makes the slab the foundation, transferring the live load of the building to ground.

But if it suits then perhaps it's a Shallow Frost Protected Non-Foundation or Shallow Frost Protected Slab on Ground (building code used term slab on ground, not slab on grade.)

I know the floating slab without frost protection in unheated buildings is common and I have no problem with it, but just not well resolved in the building codes imho.

I'm studying ASCE 32 now, and sure looks like SFPFs are applicable to these floating slabs that are a part or not a part of the building.

 

[jack stand]

I thought I read some language in your ibc quote that described the load (within their foundation description) basically stating that the structure from the peak down and all loads dead, live and wounded eventually found it's way "there". That would be my argument with the code guy.

 

[billconner]

So you posit the floor is not a part of the building and the live loads it supports are somehow somewhere in the code excluded? But if that same floor slab is supported above grade, they are included?

I suggest it's what happens when a code is revised and added to every few years and some coordinating item is not caught.

Build a store room or office on a slab in a pole barn and it's excluded from the foundation requirements?

I have no problem with the practice but do not believe code languages is aligned with what's being built.

ASCE 32 shows a floating slab - albeit with walls supported on a continuous stem wall rather than posts, and clearly frost protects the slab as part of the SFPF.

 

[Firebrick43]

where to you live platinum Jell-O, it has a lot of bearing in this conversation.

Is the building already up?

What is the drainage situation, ie on a hill, in a swale, in a dale, couldn't help my self sorry, have a perimeter drain?

I believe most locations in the states really don't need a full SFPF. International falls and north shore of alaska would

First, if you have good drainage and washed compacted crushed stone down 8"-12 inches with insulation between I don't think their is any possibility of any heaving in most areas of the US.

As far as building codes, around here they only consider the poles foundation, not the slab as the building should not be attached to the slab in any way.. If poured with foam around the poles and perimeter there is no attachment to the structure and allows the slab to move up and down independently of the building.

SFPF is really more for thickened edged monolithic slabs not pole buildings. This is the only place I used one, My small Piddle shed that I have my lathe, mill, and a bench in.

 

[billconner]

from ASCE 32 for unheated buildings. I'd think right hand is very similar to post frame condition - walks and roof supported separately from floor.

I'd add really more suited to heated buildings.

 

[ConCretin]

Fair point, but I did try to clarify this in my posts by saying I wanted to give my pad the FPSF "treatment" and "design".

Bill, he did not give a timeline in the bid, only noted what the end result would be. Using the $100/hr example, I'd agree the figure seems high...

Not trying to nit pick. I was wondering if your terminology was confusing your potential site contractors and running up the numbers. Maybe I missed it but what is the edge detail for the slab?

A FPSF will have a substantial integral footing to support building loads. Building loads in a pole structure are borne entirely by the imbedded poles. All the speculation to the contrary aside, a slab on grade is NOT a foundation of any sort and as such, foundation codes do not apply. A slab on grade is a non-structural element that depends on the base below it to support loads placed on it.

 

[67CarGuy]

While this doesn't answer the question you asked (because, well, look where you are!), if you were building an Energy Star-rated building, your slab insulation (R10, 2 inch thick rigid foam) would need to extend *either* 24 inches vertically OR 24 inches horizontally from the edge of the slab toward the interior. That's for normal conditioned living space, not a garage.

I may have missed it, but are you planning to heat your slab at all (radiant)? I think that you and a mini excavator could dig out an extra 2 inches down, 24 inches in from the stem wall / footing, for the entire perimeter of your slab in one weekend. Might be a bit of a learning curve, but most fun things have one (or several).

 

[jmdirk]

So you posit the floor is not a part of the building and the live loads it supports are somehow somewhere in the code excluded? But if that same floor slab is supported above grade, they are included?

I suggest it's what happens when a code is revised and added to every few years and some coordinating item is not caught.

Build a store room or office on a slab in a pole barn and it's excluded from the foundation requirements?

I have no problem with the practice but do not believe code languages is aligned with what's being built.

ASCE 32 shows a floating slab - albeit with walls supported on a continuous stem wall rather than posts, and clearly frost protects the slab as part of the SFPF.

In this kind of case, no, the live loads experienced by the floor, do not transfer to the foundation. Even in the thickened slab design, most of the floor load gets dispersed into the material beneath the floor - usually compacted aggregate.

Now if you've got a second floor, mezzanine or something like that, it's attached to the walls of the building, and the live loads of that floor must be taken into account, but not the 'ground' floor. In that thickened slab design you posted later, the fact you've got a monolithic pour between the foundation and the floor is irrelevant to the transfer of load from the floor to the foundation. That connection is not structural. Some designs will even call for the control joints running around the perimeter so that if/when it cracks the floor floats free from the foundation.

To have the floor to be structurally connected to the foundation, you would need a fair bit of rebar throughout to tie the floor into the foundation.

The frost protection part of that thickened slab design is more the rigid insulation that needs to be installed vertically on the wall of the foundation and then extending out horizontally for some distance. In my area that's about 4' horizontally.

 

[billconner]

So at what thickness or change is a raft or mat slab not a foundation? Or do you disagree that may slabs are ever a foundation?

I'll accept the code is screwed up in requiring all the loads in a building including garage floor live and dead loads to go to a foundation.

 

[ConCretin]

It's not a matter of the slab's thickness. It's whether the slab is supporting loads imposed by the structure. A thick 4" monolithic slab with an integral footing i.e thickened edge could be considered a foundation. An 8" thick slab placed within walls that support the structure is not.

I'm not sure what you are seeing in the code but it is physically impossible for a typical slab on grade in a garage to transfer loads to the building foundation. It's not a structural member. All loads are transferred directly to the soil below.

I'm curious if the OP is thinking he needs the thickened edge of a FPSF. This would add considerably to the cost and is entirely unnecessary since no building loads are imparted to the slab. A slightly thickened edge is not a bad idea but you don't need a 24" turndown since it's not load bearing.

 

[jmdirk]

So at what thickness or change is a raft or mat slab not a foundation? Or do you disagree that may slabs are ever a foundation?

I'll accept the code is screwed up in requiring all the loads in a building including garage floor live and dead loads to go to a foundation.

To me it depends on what function the slab is performing. If the slab is supporting the building loads (live and/or dead), then yes indeed it is a foundation.

In the OPs case, we're talking about a pole barn structure. The foundation is the piers supporting the posts. The floor and the loads bearing on it are irrelevant. You can have a dirt floor, or as is often done, you have a floating slab. Which is decoupled from the building itself. Yes, you do have to account for the loads on the floor itself when deciding on the thickness and composure of the floor slab, but it is not a 'foundation'. There's a big difference in the requirements between driving around a passenger car on a slab vs a 10,000 lb forklift. Yes, you can install a partition walls to create an office or store room in a pole structure and not have it included in the foundation requirements. But depending on the loads imposed by those walls (supporting a mezzanine for example) You could be required to put footings under those walls, but that still is not considered part of the foundation.

Again, even for a thickened monolithic slab, the loads bearing on the floor are not transferred to the perimeter footings, Therefore the loads imposed on the floor are not included in the loads for the foundation

 

[matt_i]

Imo the OP has the wrong idea. Any excavation next to posts compromises their moment-bearing ability (the ability of the building not to fold over like a house of cards) due to the sideways forces of winds, etc.

If you want thermal performance insulate the entire plane under the slab. If you want even more thermal performance than that with a post frame, continue widening the insulation horizontally with holes cut where the posts protrude.

The posts must sit below the frost line anyway and thus the need for FPSF adds zero help to the quality of construction.