Shear walls and stairways?

[MushCreek]

I have a couple questions, and how they are dealt with by code- or if they are.

The first is shear walls. I'm not even sure about the definition of shear walls, much less where they would be prescribed. I know in a basement, a shear wall is a perpendicular wall intended to brace a long stretch of foundation against the force of the dirt outside of it. I assume a shear wall above grade is intended to brace large, flat areas against wind. How does one determine the size a wall can be without bracing? In my latest plan, there is a wall that is 36' long by 18' tall. The wall is braced by the diaphragm of the main floor, and likewise braced by the diaphragm of the second floor, at the 10' level. What about at the eave? is it considered braced by the roof structure? There aren't any perpendicular walls attached to the outside wall on the main floor, but the upper story has a couple. Would these then have to built as shear walls, or does the roof take care of that?

In a semi-related way, my next question is about a stairwell. In one iteration, there is a long stairwell along one of the long walls of the house. Specifically, about 10' of the floor decks are open to allow the stairs that go upstairs, and the set that is stacked under it to go to the basement. Is a stairwell on an outside wall OK? I'm guessing that the inside wall of the stairs becomes structural, with floor joists being attached to a doubled 2X in that area. I'm guessing that the floor joist on either end of the stairs should be doubled as well, although the stairwell will be fully enclosed by structural walls, so the end joists aren't carrying an more load than other joists. Lastly, if the stairs run along an outside wall like this, will the wall be sufficiently strong against wind forces with a 10' gap in each floor diaphragm?

My questions are mainly whether these things are dealt with prescriptively in the code, or am I looking at getting an engineer involved? Here's a floor plan sketch of my current plan. The main area, 19-1/2' X 36, is two stories over a basement. The area on the right, the garage/den/foyer will at least partially be slab-on-grade. The roof over that section is a shed roof, attached to the same wall that the stairs are on.

[MountainDon]

A shear wall is another name for a braced wall, if I understand it correctly.

I believe the IBC and the UBC (codes) call them shear and IRC call them braced.    

The word sheer comes from the nailed in place sheathing being able to, or not be able to, resisting the sheer forces when wind or something tries to "rack" the wall out of plumbness along its length.... trying to sheer off the fateners, sort of, or cause the sheathing panels to rupture.

See IRC Section R602.10.1 Braced wall lines.  PDF page 172 in the Minnesota IRC 2006 (also linked to in the "where can I find.... " section of this board.)

This is a good topic but I'm not sure where this is in our somewhat flexible timetable.... we were thinking of a general planning topicto be next... but we'll see what can be done here and now...

FWIW have a looksee...  (from Washington State University)
http://timber.ce.wsu.edu/Supplements/ShearWall/Segmented%20Design.htm

And, this one has some examples of failures...
http://www.abag.ca.gov/bayarea/eqmaps/fixit/manual/PT07-Ch-3A.PDF


That's all I have to say right now.

[PEG688]

They can be one and the same, but not always.

http://www.coastalcontractor.net/article/104.html

In many cases, the only difference between a shear wall and braced wall is the inclusion of hold-down brackets on the shear wall.

[Don_P]

If I understand it correctly a shear wall is a braced wall designed by an engineer, a braced wall is a prescriptive lateral force resisting wall that is not engineered.
If you can make it work using the wall bracing requirements in the code then an engineer is not required.

[MushCreek]

From what I see here, I'm wrong about what constitutes a shear wall or braced wall. Let me give you an example. A friend of mine is building in rural NC. His Amish builders do nice work, and they are cheap, but they are not engineers. On the gable end of the house, it is completely open, from the first floor, through the second floor, on up to the peak. The other end of the house has a loft. While climbing a ladder placed on this wall, he was alarmed at how much it moved. The entire wall was free to move in and out a lot when a load was applied. There was nothing there to prevent this. His solution is to mount a massive beam about 10' off the first floor as a stiffener for this wall. I could see that without something, that wall could be vulnerable in a high wind. The framing was inspected and passed by the local inspector as-is.

To me, a shear wall would be something like a floor deck, the diaphragm of which would brace such a wall, or an internal wall perpendicular to the wall in question. Apparently, it's just sheathing to prevent the framework from racking in a wind. I would build that way anyhow, so it's not a concern for me. My place will be fully sheathed. Since my design doesn't have anything remotely approaching the condition he has, I think I'll be just fine.

[MountainDon]

Re the flexible wall mentioned... Don_P has mentioned before that he has added a "shelf", basically a beam on its side across a gable wall like that to assist in stiffening the wall against wind, etc. It may not be engineered, but is a help. There is a section in the IRC that gets into some prescriptive methods for things like that.

That flexible wall is most likely just waiting for a big wind to implode it. Or if a tornado passed nearby it would like explode outwards as the twister sucks up the surrounding air.

Without dimensioned drawings illustrating your walls, floors and roof sections it's hard to say what you have to worry about or not, but if you don't have any large wide and tall open walls/windows you are likely safer than your friend.