Stepped foundation wall to reduce rise from garage to house - joist on concrete wall new construction

[BakingFool]

My new home design calls for 11′ wide by 14’long entryway that sits between the living space and garage, at the same floor level as the living space. Floor support is 2×12 joists, and the exterior walls are 2×6 construction. There is a u-shaped stairway in the entryway to the full basement under the living area.

The house was designed with 8′ foundation walls, and had a rise from the garage floor to the subfloor of 17-1/2″: 4″ for vapors + 1-1/2″ sill+ 11-1/4″ joist + 3/4″ sublfoor. For multiple reasons (steps, topography, fill and grading) I would like to lower the living area and entry to have only one step up from the garage.

I came up with the idea in the attached diagram. The code I researched calls for a sill plate and bearing on concrete of at least the width of the studs. The minimum bearing length required for 2×12 joists is 1-1/2″. For the foundation wall between the entryway and garage I am proposing to use a 9′ high, 10″ thick wall, with a 4″ wide step at 8′. All other foundation walls will be 8′ even with the step at 8', except garage walls which will be 4′ frost walls even with the top of the 9' wall in the diagram.

I am interested feedback on this proposed solution. Any obvious problems with this plan? Thank you in advance,
Don

 

[BakingFool]

Not depicted in the diagram is the bottom plate of the stud wall would be on top of the pressure treated 2x6.

 

[BakingFool]

It just occurred to me that it might make more sense for the step in the foundation to be 11-1/4" high, and extend the subfloor over the 2x6 sill plate - assuming there are not other issues with this proposal.

 

[BakingFool]

I forgot - I can't use a 11-1/4" step, because the other walls would have to be 8'-0 3/4". Back to the proposal depicted in the attachment to the first post.

 

[jar546]

Let's start with the basics. Not only will you have more than 8' of unbalanced backfill on an 8" CMU wall, but there will be a garage slab with cars parked on it. Make sure that is taken into consideration when determining how much steel you want in the foundation wall. Consider a thicker wall on the garage side. Pennsylvania is full of cracked basement foundations where this was not taken into consideration.

 

[BakingFool]

Let's start with the basics. Not only will you have more than 8' of unbalanced backfill on an 8" CMU wall, but there will be a garage slab with cars parked on it. Make sure that is taken into consideration when determining how much steel you want in the foundation wall. Consider a thicker wall on the garage side. Pennsylvania is full of cracked basement foundations where this was not taken into consideration.

Thank you for your reply, as I have no design experience in concrete walls or floors. I do understand the principal of backfill height and loading. A few more details are perhaps in order. This is a one story structure, with full basement mostly 6' to 7' below grade . Poured concrete walls, 2x6 stick construction above the foundation. The initial design was 8' tall 8" thick foundation walls throughout. There is a rectangular entry module 11' wide by 14' long (into and out of the diagram) next to the garage (cross section of basement shown), and a rectangular ~28'x28' living space module is to the left of this (not shown). The wall in question is thus 14' long. Also not shown is the dividing stud wall between the entry and living space, on top of the left foundation wall in the diagram.

The diagram is a little hard to read, but the wall in question is proposed to be 10" thick, poured concrete. All the other walls would remain 8" thick. I have a concrete contractor who everyone (builders, town engineering, etc) says is the best around - very highly regarded. I do not have a personal relationship with an engineer or architect at this point.

As for my proposal, I was surprised that only 1-1/2" is needed for joists bearing on wood, 3" for bearing on concrete. You referred to concrete cracking, and wood distributes the load which is why bearing on wood is less length. Code requires a sill plate to be as wide as studs. Somewhere I found a requirement or suggestion of a 1/2" air gap between untreated wood and concrete. Putting all these together, I came up with the 10" wall thickness. One thing I don't like is the air gap for the joists causing 1/2" of the subfloor to be unsupported. I am wondering if the 2x6 sill plate could have a rabbet in the top left corner of maybe 1" width x 3/4" deep, and run the subfloor over into that. It is a custom house and doing that for 14' of length on one board should be ok. I'll bring donuts for the crew.

I honestly did not consider the surcharge of vehicles on the garage floor. Do you think that a 10" poured wall would suffice?

Don

 

[Paul Sweet]

The rim joist should be treated.

The garage floor needs to be 8" below the top of the subfloor, or else the studs will have to be treated.

How are the floor joists fastened to the rim - toe nail? They can't be nailed through the rim joist as is typical.

I wouldn't worry about the 1/2" of unsupported 3/4" subfloor. If there is a gap it will be covered by the drywall, and it would be good to close it with a rubber joint backer rod.

I would recommend a termite shield.

It would be best to find a structural engineer to size the reinforcing. If you can't find one then use the recommended reinforcing using the right hand column in IRC TABLE R404.1.2(4). This is based on twice the usual lateral load.

 

[BakingFool]

The rim joist should be treated.

The garage floor needs to be 8" below the top of the subfloor, or else the studs will have to be treated.

How are the floor joists fastened to the rim - toe nail? They can't be nailed through the rim joist as is typical.

I wouldn't worry about the 1/2" of unsupported 3/4" subfloor. If there is a gap it will be covered by the drywall, and it would be good to close it with a rubber joint backer rod.

I would recommend a termite shield.

It would be best to find a structural engineer to size the reinforcing. If you can't find one then use the recommended reinforcing using the right hand column in IRC TABLE R404.1.2(4). This is based on twice the usual lateral load.

Thank you Paul. I appreciate these points.

Yes, it occurred to me that the attachment of rim joists to joists can't be standard. Hopefully that can be worked out by the builder. I was wondering about insect vulnerability.

Two questions:
Are the requirements for the rim joist to be treated (it is bearing on a PT plate, and 1/2" from the concrete) related to code?

Also, is the requirement for the garage floor to be 8" lower related to code? I am guessing it is, because that is a specific number.

The house and foundation will be engineered, and stamped plans generated. I am not dealing directly with an engineer, or even with the company. And the company likely outsources the engineering. That is why I asked the question here, where I could see there are knowledgable people. As far as the method and specs of the reinforcing, I will leave that to those doing the engineering. I basically was starting with, "is this concept even feasible" before submitting my request and triggering costs.

Attached is a sketch of the plan. The 8" thick frost walls front and rear have a 2" by one foot step that matches the step in the 10" wall, so that the stud wall won't have a jog, and the exposed top of the frost walls won't protrude 2" from the studs above.

Thank you again!
Don

 

[wwhitney]

The garage floor needs to be 8" below the top of the subfloor, or else the studs will have to be treated.

How do you figure? I don't see anything in IRC R317.1 that would require that. Is there a different section I should be looking at?

Are the requirements for the rim joist to be treated (it is bearing on a PT plate, and 1/2" from the concrete) related to code?

Also, is the requirement for the garage floor to be 8" lower related to code? I am guessing it is, because that is a specific number.

See IRC R317.1. Which doesn't seem to require either of those to be PT.

Cheers, Wayne