Need Concrete Floor Extension Opinions

[ben2go]

I had a 10' wide by 9' deep tin kit building on a 10' wide by 9' deep concrete pad.It was held down at the corners with screw type tie downs.The concrete floor and tie downs were poured into one piece.So the tie downs will be extremely hard to remove.Since tearing down the tin kit building,I have decided I want to build a small work shop for my motorcycles,minibikes,and lawn equipment.I have never poured concrete extensions,only complete floors.I would like to have a 12x12 floor and cover the tie downs up.I would settle on a three foot extension giving me a 10' wide by 12' deep floor.I plan to build a standard stick built building with 8' walls and 6:12 truss roof.

Here is the current pad and description of the lay out.Please excuse my crumby photo chops.


Here are the tie downs.They run at a 45 degree angle into a hole where concrete and rebar for the floor pour.It's all part of the slab.


Here is what I would like to do.12'x12' floor.I wonder about the one foot extensions settling and causing the building walls to shift.Anyone have ideas or insight?This would also encase the tie downs,so no more busted lawn mower blades.


Here's what I'd settle for since I don't have a lot of money.10' wide by 12' deep.


I talked to my county inspector, and since the original building is being replaced, I do not need a permit.I asked about expanding the building.As long as I don't build it more than XX% larger I don't need a permit.I was sweating over this,but luckily I am still in the county.In a few years we will be incorporated into the city limits. 

What's the thoughts and opinions of my plan?

[MountainDon]

I'd definitely do it before you are a part of the city.

I would suggest pouring a slab extension around all four sides with the perimeter all being dug down deeper as a foundation. How deep? Depends on your frost depth. How wide? Depends on the soil but something like 8" would more than likely do. You need a solid foundation if you want that workshop to stay plumb and level over time.

I'd simply cut the offending steel anchor things of with a grinder.

[TexstarJim]

I think it will work fine and if you do a 10x12 building, all you have to do is make sure the one new section of concrete ties into the existing pad.  I'd drill (4) 1/2" holes into the end of the existing pad and use 1/2" rebar into the holes extending out to the edge of the new pour.  Use a couple or (3) pieces of 1/2" rebar running across and tied to the short pieces, it will be all tied together. 

[Redoverfarm]

If you use the combination of the two previous post it should be fine.  In addition by re-pouring all perimeters if the concrete would happend to settle a little most likely the complete building will settle being your sill is on the new pour.  Adversely if the sill is sitting on part of the new and part of the old it could end up being heaved up as the new pour settles. If this occurs and the building could be out of kilter so to speak. Make sense.  

Also I am not sure how far off the "Eye" bolts (tie downs) are from the edge of the old pour but unless they are higher than the concrete floor I would just leave them in.  If they are close then a 20# sledge should bring them below grade of the pour.  You might even want to tie your rebar into those.

[ben2go]

I like the perimeter pour tied into the original slab idea.Pour a 12 inch by 12 inch perimeter footer.I think that will give the building a better chance of settling on an even plane.The tie downs are roughly 8 inches from the slab and would be about 2 inches under the finished surface.If my math is correct and allowing for different wood densities,the finished building would weight roughly 1200 to 1600 pounds.Our frost line is stated to be 12 to 16 inches but I know that to be false.I have never ran into the frost line deeper that 4 to 6 inches.I'm not really sure how frost depths are set but if their is a safety margin in the 12 to 16 inches it's double the actual frost line. 

[MountainDon]

Frost depth figures are usually stated very conservatively, that is, they are given as a worst case possibility. Here in the NM desert I was told that 18" footing depth was required. That's way deep for the normal thing I've seen year over 25 years.

[ben2go]

OK her is the plan.I will pour a 12" x 12" perimeter footing for the building.It will be re-enforced with rebar but not tied into the floor slab.When the building settles it may cause cracks in the original floor.Not good considering I will have a machining center on one side.It's not very big and doesn't weight much but it needs to be level.I need educating.How do I figure out how many sacks of concrete I will need?What mathmatical equation do I use?Anyone have any more insight or advice?Thanks for all the input.It's been a real help.

Here's the plan so far.

[poppy]

I think Don's original suggestion is the best option.

A basic perimeter foundation not tied to the existing slab should work just fine.  After all, we're not talking about a dwelling here that needs flat and level floors for 100 years. 

[MountainDon]

Concrete: whay kind? Mix it yourself from portland cement, sand and gravel? Sacked Quikrete? Or?

Width in ft x depth in ft x length in ft = volume in cu ft

27 cu ft = 1 cu yard


QUIKRETE
80# = .60 cu ft
60# = .45 cu ft
40# = .30 cu ft

[ben2go]

Quikrete is what I had in mind.Thanks.

[ben2go]

New idear.I spoke with a man that built the Bojangles in my town.He is the brick and mortar guy in the building process.His suggestion was to build the building in a pole built fashion using precast concrete piers.Then simply pour the slab extensions after the framing is up and roof is on.His other suggestion was using expanded steel mesh in the extensions instead of rebar.His suggestion may cost a little less but I haven't did the math yet.Now, I am strictly speaking on the concrete perimeter and not the entire building.