Supporting exterior wall at parallel rim joist

[John Smith 00000]

Hi guys,
I have a question about the general floor system design. If you look at the below picture, two exterior wall (let say 6'' wall) of the house are vertical to the floor joists (16'' O.C) and they sit on top of the rim joist and floor joists (good enough), but the other two walls are parallel to floor joists and they sit on top of 2'' rim joist only. I think this is a widely used design, but personally I have two questions
1. 6'' wall is sitting on top of 2'' rim joist only, does the rim joist provide enough support? (Let's consider 2 story and 3 story house)
2. With the load of above exterior wall, rim joist might want to roll towards outside, I know most people install blocks every 4 feet, do you think it's enough?

 

[phamENG]

Well...this is how 99.8% of the light frame wood houses in the US are built. I don't see many falling over as a result, so I think it's okay.

The rim joist can't roll out - it's attached to the floor sheathing, blocking, etc. As long as all the code mandated pieces are there and attached per the fastener schedule, you're good. Unless it's a freakishly large house requiring special detailing by a structural engineer. Then you might be okay, or the engineer may need to put something a little more robust in place. (That's quite rare.)

Why do you ask? What's your background?

 

[John Smith 00000]

Thank you phamENG for your reply. I'm new to structural and was surprised by this design when I looked at it and obviously it's just my ignorance.
But 6'' wall on 2'' rim joist, with two stories' weight above, it doesn't make sense if you think about it. I understand it'll be more difficult for pipes and wires to pass through if 6x6 rim joist is used, but at least it should be designed to match the strength of the other two wall by installing blocking every 16'', why every 4 feet?

 

[jayrod12]

Personally I specify blocking at 24" o/c for exactly the reason you indicate. But depending on the roof framing and geometry, there may be next to no load on that wall that's parallel to the floor joists, and therefore might not require more than 2" (1 1/2" I assume, a 2x6 is 1 1/2" x 5 1/2") of bearing to be adequate.

 

[phamENG]

It's not about matching the strength everywhere - it's about matching the strength that matters. That 6" wall is 6" not necessarily because you need 6" for vertical load, but because you need it for out of plane bending from wind loads (or combined bending and axial). Or, you need it to meet a certain R value and the loads could be handled by toothpicks. You also have to consider that comparing the strength of a rim joist loaded on its side and continuously supported along a foundation wall and that of a 2x6 stud wall is sort of like comparing apples to space ships.

 

[John Smith 00000]

That makes sense. If that's the design adopted by vast majority of houses, obviously it's proved already and should not be questioned.

 

[phamENG]

I didn't say that. Question everything! It's the only way to learn. It's also a good way to make your boss hate you. But who needs a boss, anyway?

 

[Eric C.]

You should question any and everything that doesn't pass your smell test. Especially, as you say, you are new to structural, or any other discipline for that matter. Run some numbers and prove to yourself whether or not that the single rim joist works.

 

[John Smith 00000]

Thanks all of you for your reply and support!

 

[KootK]

But 6'' wall on 2'' rim joist, with two stories' weight above, it doesn't make sense if you think about it.

I agree in some respects, as shown below. If I were to design a wood building without ever having seen one before, I probably would be inclined ot use at least a 3" rim member. The one that really bothers a lot of us is the platform to basement wall connection: Link

 

[molibden]

I always demand blocking in this case. We also need it in order to install shear brackets between wall and floor.

 

[kissymoose]

The one that really bothers a lot of us is the platform to basement wall connection

Stupid IRC still just calls for anchor bolts at 6ft oc, and the rim joist to sill plate with toenails 6in oc, joists to sill plate with 3 toenails.
Tops of basement walls detached from the floor diaphragm (leaning in) is not rare.