Is walkout basement a "story" in residential

[shaneelliss]

Ok, I am reviewing some calcs for a two story stick framed home with full basement. The concrete foundation wall is stepped down in the rear for a walk-out basement and conventional stick framing is used for the walk out portion.

Should this house be considered a three story home? Typically with only two stories above ground, I would say no, but because the whole rear of the home is stick framed, I would have to say yes. Am I wrong?

Should the rear basement walls that are stick framed be designed to resist shear? I think they should, though no calcs are provided for shear resistance on the lower walls.

Also, the house has in-plane and out-of plane irregularities. If I remember correctly, ASCE7-05 says that the engineer cannot use the simplified seismic method on an irregular structure that is 3 stories. Is this correct?

And one more question. The house has a raised roof portion over a staircase that is separate from the rest of the main roof structure (engineer calls it a turret). The front exterior wall that supports this roof is rounded (maybe 8'-10' diameter) and the back of this raised roof is supported by a wood beam and posts on the interior of the home. I think because the roof is a separate diaphragm that this part of the structure will act somewhat independently from the rest of the home. The engineer shows shear walls on each side of the raised roof and across the rounded front, but nothing on the rear. Two things jump out at me: one is a rounded shear wall (which seems too small of a round to bend OSB around without breaking it), and the other is no shear resistance in the rear. Any ideas on how the shear resistance should be handled? I have told the engineer that I do not like what I see and that I cannot accept a round shear wall but maybe I am being too hard on the guy or otherwise unreasonable. He seems at a loss of what to do. Any thoughts?

Long post, with lots of questions. Any help will be appreciated.

 

[ishvaaag]

Since, you are addressing a structural question, I think the normal interpretation is that it has 3 stories. Other thing is that nor codes nor books are particularly clear on a proper way of addressing the building-soil interaction at basements.

Over the ground stories are mostly counted (here, Spain) to address architectural permitted volumes and stories; where I live there's also an unwritten understanding that no more than 5 underground levels are to be permitted wthout special consent ... most of what underground over 2 levels is quite discretional for the council.

 

[MiketheEngineer]

I believe IBC/IRC 2006 would consider it as 3 story - at least in this portion of the building.

 

[RockEngineer]

Should the rear basement walls that are stick framed be designed to resist shear? I think they should, though no calcs are provided for shear resistance on the lower walls.

Lateral loads from the upper floors must be taken all the way to ground so yes they need to design the framed daylight basement wall for shear.

Also, the house has in-plane and out-of plane irregularities. If I remember correctly, ASCE7-05 says that the engineer cannot use the simplified seismic method on an irregular structure that is 3 stories. Is this correct?

ASCE 7-05 12.14.1.1 (5) states "the structure shall not exceed 3 stories in height". There are many other restrictions that would need to comply. Acquire and read section 12.14.

And one more question. The house has a raised roof portion over a staircase that is separate from the rest of the main roof structure (engineer calls it a turret). The front exterior wall that supports this roof is rounded (maybe 8'-10' diameter) and the back of this raised roof is supported by a wood beam and posts on the interior of the home. I think because the roof is a separate diaphragm that this part of the structure will act somewhat independently from the rest of the home. The engineer shows shear walls on each side of the raised roof and across the rounded front, but nothing on the rear. Two things jump out at me: one is a rounded shear wall (which seems too small of a round to bend OSB around without breaking it), and the other is no shear resistance in the rear. Any ideas on how the shear resistance should be handled? I have told the engineer that I do not like what I see and that I cannot accept a round shear wall but maybe I am being too hard on the guy or otherwise unreasonable. He seems at a loss of what to do. Any thoughts?

The lateral and vertical loads need to go to ground. There are various ways to do that including transferring the loads through collectors or drag struts. Whether they have been properly handled depends on the whole structure not just what we can assume from this post.

 

[shaneelliss]

Thanks for the responses. So from what I understand, the consensus, from an IBC/IRC perspective, is that this is a three story structure.

Per ASCE7-05 (sorry I didn't have it in front of me last night when I wrote this post) the simplified seismic procedure can be used for up to a three story building that meets all the other limitations defined in that section. Limitation 11 (12.14.1.1(11))is that the structure cannot have irregularities with an exception that allows them in two-story buildings of light frame construction. What I still don't see is how a story is defined in ASCE7-05. To me it seems fairly straight forward that this is a three story structure, but I just wanted something to back up my thoughts when I am speaking to the engineer who designed the structure.

Anyone have any other thoughts on whether or not a rounded wall would work as a shear wall?

 

[OldPaperMaker]

In ASCE7-05 look at the Commentary, Figure C11-1 (page 354) for the defination of "Story Above Grade". For the defination of "Grade Plane" look at page 111 of ASCE7-05.

That might clear up the situation.

I don't know what to do with the circular shearwalls. I have always managed to talk the architect into changing them into octagons and just using the segment of wall length parallel to the rest of the walls in the direction being designed.

You might talk to the engineer that did the calculations to find out his thinking on the issue.

 

[structuresguy]

For circular shear walls, you could analyze them like a vertical tube cantilevering up from the foundation. That is really how any shaft behaves, whether circular or rectangular, or even C-shaped. Not traditional, and probably difficult for most typical software to solve, but it could be done by hand fairly easily.

 

[boo1]

A few years ago we framed the circular walls with structural grade Thermo-Ply. Its tough and bends easy to the radius.

http://www.covalencecoatedproducts.com/en-US/products/Thermo-Ply/content.aspx