Hi,

I would like to discuss a budget-saving building method on foundation. I am planning to build a 16' X 24' shop using a 3-bent with queen post design. 2nd story will be a small living quarter (kitchen/bathroom in a 12'x16' section and living room/bed in other 12'x16' section. On the slab, you notice short sill on front.. I am planning to add 4x6 post on the end to support the garage door and 2nd floor beam.

I made a quick drawing using the Sketchup and without putting too much effort into the TF design because my main focus was the foundation itself.. but to give you some rough idea.. the structure will be of 6x6 sill plates, 6x6 posts, 6x8 summer beam to support 2nd floor joist, and a 4x6 rafters.

I want to use Alaskan slab for the shop using 4" thickness. The question is to support the structure using 24"x36" at the edge to support the load. See first pic below on left side. Or the 3'x3'x3' footing seen on 2nd pic below.



The reason I came up with 3'x3'x3' is because when I built a huge outdoor stage which is 40' wide and 20' length.. the architecture had me using 8"x8"x24' treated posts at front corners to support a 40' "board" so it can support the 24' roof rafters (lean-on design). Because it is outdoor in the open and exposed to high wind, a 200MPH wind load requirement design was kept in mind, I had to make a 4'X4'X4' concrete footing for each post. At bottom of the post where the pouring would be, 4-5/8" rebar was drilled through and it was 3' long. So duplicating this idea, I was thinking it might work for the 16'x24' TF structure design? I was using 3'x3'x3' at each post so the anchor can hold down the sill plate and steel strapping to hold down posts. The difference between bottom one and above one is that bottom one needs only 11 yards while above needs almost 19 yards. That is almost $700 in saving.



I am curious on your input!

Hi Ken;

To me an Alaskan slab is a poured concrete slab with thickened edges. This "monolithic pour" is the entire foundation and does not need footings underneath. We use these slabs some here in the frozen north so I am sure they would be even more reliable there the ground does not freeze four feet deep as long as the soil is stable. High quality anchors embedded in the concrete are important to keep the frame tied down. Your local building code enforcement officer may be friendly and would answer detailed questions.

Good luck;
Jim

I'd vote Alaskan slab as well. You won't have animals moving in below building like you would with the other design. Also I doubt that you need 24x36" at the perimeter. The IRC has prescriptive Alaskan (FPSF) foundation requirements now. You can find it online.

Also, are you saying you're putting 2 interior posts under the 6x8? You could get rid of the posts if you beef up the beam. It's only 16', and you've got some nice hardwood in NC.

Jim,

Maybe I was not clear in my first post? I am planning two stories TF structure with 2nd floor containing a floor joint for me to live upstair and will be using Queen Post design as a bent. I do not think that a simple Alaskan Slab will support the structure itself. Please keep in mind that the code in this area require to support 200MPH wind load. All the structures must be bolted down using the anchor. I imagine that the 4" concrete slab will suddenly crack easily under the wind load stress and will collapse? That is why I was seeking professional (or near there) opinion on the 3' X 3' X 3' footing under each post area. The footing would act as an underground concrete pier and joined with the 4" slab.

The local building inspector is nice and friendly, but he doesn't know very much of Timber Frame engineering he said I had one of two options: 1) Get a friend with building license and use his license to build it or 2) Get engineer stamp and he will let me build it as 'Self'. Log home and TF is a very rare structure around here anyway.

Hi Ken;

I think a slab would work for you but I am not an engineer. My advice is to hire a engineer. I think adding 3x3x3 footings under a slab would add enough cost that an engineer's fees would be well worth it. It sounds like you really need an engineers stamp anyway. We are fortunate to have a number of engineers who are very knowledgeable about traditional timber framing in the Guild and they are probably your best bet for foundation and frame design.

Take care;
Jim

Jim,

I have to say that I agree with your comment. I was thinking the same thing about getting a engineer to help out with the project. Based on the research I did last night, a 12" wide with 16" deep at the edge with 4" thick slab would be sufficient because that is anywhere between 3,500 to 5,000 PSI depending on how much cement is mixed into the concrete.

The thing is that I know I do not need a permit to build up to 24'x32' shop but I am not sure about the living quarter above the shop as part of the structure so I plan to check with the inspector today. Another issue would be insurance whether I can obtain a policy for having a 16x24 shop with living quarter if there was no building permit but the electric/sewer permit? I have emailed the insurance agent about that and waiting on reply.

Thanks for helping out!

Why not piles under the bent posts with a thickened slab perimeter say to 18" deep ( or whatever it takes for any frost issues.You can backfill the building lower this way to maintain a concrete perimeter wall keeping the walls/ wood portion of the building 12" above grade this way. Or pour a small grade beam first. What we do is pour a small grade beam instead of the thickened slab. Do the frame and close in the building. Then pour the slab inside. You have weather protection when you pour that nice pad. Also, the slab is poured lower than the top of grade beam so you have your wood off the ground inside and out. Just remember to buck out the grade beam for your doors. Add a few inches to.the side R.O. For trim or overhead door backing.

You can drive a 30,000 lb bulldozer on a 4" slab, so a little wind is not going to hurt it. If you have interior point loads, just put footings under the slab in the interior. The loads in a 16x24 are negligible though. I'd also advise speaking with an engineer. It doesn't have to be expensive. With your wind loads you're going to need some serious wall and roof sheathing too.

This is an Alaskan slab with HTT4 holdowns installed after the pour. You can see the SIP on the other wall.

Good point about the loading. It's a pretty small structure. My advice was for the most part frost and moisture related. Frost obviously is less of an issue in some places. Lots of overkill here, it seems the issue is keeping the building down, rather than up.

Yes that's more of the issue and to prevent from cracking should the loading happen especially at the edge.