sister 2x6 in a 2x4 bearing wall?

[Ben29]

I have a job where they want to add a second story to an existing single story house. The existing exterior bearing walls are 2x4. For strength, they really need 2x6 studs. Per IRC R602.3.4, studs shall have full bearing on a 2x plate, where the width of the plate is at least the width of the stud.

Is the only option to demo the existing 2x4 walls and build back new 2x6 walls? Is there anything wrong with scabbing on a 2"x 1 1/2" ripped board to the existing sill plate per image below?

 

[jayrod12]

In the Canadian code you can overhang a stud up to d/3 provided the actual bearing area provided works. Is there some similar argument that could be made in your local code?

 

[Enable]

Have you actually checked the strength of the 2x4 members and found them to be inadequate?

The change from 2x4 to 2x6 in most codes was not a strength issue rather it was to increase cavity depth to accommodate additional insulation.

 

[KootK]

I feel as though you could do this as you've proposed so long as you're willing to venture into engineered design rather than prescriptive space. Other than for finish attachments, I don't know that the reinforcing pieces even need to make it down/up to plates. The axial load component can probably just stay in the 2x4's. I'd be inclined to throw in some 1/3 point blocking.

Any chance double 2x4 would work? That sounds simpler in many ways and you're already messing with standard insulation widths etc.

 

[Ben29]

Enable, I did check it. They want to raise the height of the wall, so the existing 2x4 cannot be used for strength anymore.

I got (2)2x4 to work. So that means I will have (3)2x4 at 24"o/c.

I guess we will just have to do it that way.

 

[KootK]

They want to raise the height of the wall, so the existing 2x4 cannot be used for strength anymore.

How tall are the new and original walls? Even if they don't make it full height, the existing studs can still:

1) Resist wind loads and;

2) Increase the axial buckling load of the assembly.

 

[Ben29]

KootK - we are raising the wall height 12".

 

[KootK]

12"?? Pish-posh... use those studs.

 

[Ben29]

KootK - do I need to add blocking above them? or take some kind of safety factor?

 

[KootK]

KootK - do I need to add blocking above them? or take some kind of safety factor?

I don't believe that you need to do either. What you do need to do is engineer the system carefully in what will be a non-standard way:

1) Assume the axial stress stays in the short studs and make sure they don't squash.

2) Let the new and old studs share the wind load and fasten them accordingly.

3) Is the new studs to add EI to the existing studs and, in effect, brace them for strong axis buckling.

4) Probably some other stuff that I haven't thought of...