Sill Plate Catilevered Over Foundation Wall

[jimtheengineer10]

A modular home is 27'-8" wide by 52'-0" long. The foundation plan called for the foundation to be 51'-8" long by 27'-4" so 2" rigid insulation could be installed on the outside of the house. The home would bear on 6" of the concrete wall and 2" of rigid insulation.

The contractor, per the home seller's request, installed insulated concrete forms but the outside of the insulation has a dimension of 51'-8" x 27'-4". So basically when the home gets set on the foundation it will extend 2" past the foundation.

If a 2x12 sill plate was installed it would bear on 8" of concrete, 2" of rigid insulation and cantilever out 2". The home comes with a 2x8 sill plate.

Will the 2x12 sill plate be sufficient to carry the load of the home? The roof trusses will also bear on the exterior wall.

A picture is attached to help clarify what I am saying

 

[KootK]

The 2x12 can't carry the load as a cantilever element as that would require it to act in cross grain flexure. Cross grain flexure implies tension perpendicular to grain which is a no-no in wood. That said, the detail might still work. It just needs to be evaluated critically assuming a load path that does not involve using the sill plates in cross grain bending. See the sketch below. I believe that the 2x10 joists need to be the cantilevering members of the system.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

I second KootK's suggestion but with the note that you must then provide a load path for the shear through the 2x10 to the sills. Typically some blocking or a rim board will work if detailed properly.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[mike20793]

I agree with stated above. Keep a 2x8 sill on top of the concrete wall and cantilever the joists and provide a rim joist or a rim board.

 

[jimtheengineer10]

Should the 2x12 plate be included or is it not necessary?

 

[KootK]

Not necessary. A single 2x8 sill would seem to be the way to go.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jimtheengineer10]

Thanks for the great replies and help. A pretty simple solution for what could have been a major issue.

 

[msquared48]

I would add an additional line of 2X10 blocking at the inside edge of the 2X6 stud wall above though.

Mike McCann, PE, SE (WA)

 

[XR250]

I wonder if an LVL works better in "cross grain bending" as it really does not have grain in the same direction as dimensional lumber.

 

[TehMightyEngineer]

I believe I've seen numbers for LVLs in cross-grain load but the capacity was quite low.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[hokie66]

XR250,
Unless you are referring to cross banded LVL, which is not common, all the grain runs in the same direction.

 

[dhengr]

Jimtheengineer10:
I would add a 2x12 rim joist to your detail. I would cut the 2x10 jsts. 1.5" shorter for this, and I would move the 2x8 sill pl. in 1.5", and I would change the 2x12 sill pl. to a 2x10 sill pl. to accomplish all of this. Normally, the rim jst. carries a good share of the wall and roof loads directly down to the sill pl. in direct compression. They also stabilize the jsts. laterally and from rolling and they carry studs which do not line up with the fl. jsts. below down to the found. sill pl., but they are just end nailed to the fl. jsts. But think about it, these rim jsts. are working in a completely different way in this detail than in the normal arrangement. These rim jsts. are hanging off the fl. jsts., so you might consider installing some jst. hangers, upside down, to be sure you get the rim jst. loads into the fl. jsts. while accomplishing the stability duties too. Now the two found. sill pls. primarily provide a good clean edge detail, but otherwise are not loaded appreciably in cross grain flexure, just in compression perpendicular to the grain. Now the jsts. carry all the load down to the sill pls., so check them for C⊥. The rim jsts. should be side nailed into the top sill pl. and things like sill seal and caulking should be detailed properly to provide good air tightness.