Ignoring Skewed Shearwalls and Designing as Cantilevered Diaphragm

[Wandering Wallaby]

Hi everyone, I’ve got a situation where I’m designing a three-story house in seismic category D, wind = 110 mph (ULT). The first level is a daylight basement with full height concrete basement walls at several sides where shown on the attached sketch. The front shearwalls on the lower level are limited by the fact that there is a 16 ft garage door in the middle of an 18’-6” long wall, so I’ve kept those as concrete columns on each side up to the 2nd floor framing, which is a flush glulam across the entire wall. The main issue is that the entire side of the structure is skewed at a little less than 5 degrees from orthogonal (following a property line), which creates a Type 5 horizontal irregularity per ASCE 7-10 Table 12.3-1, which requires it to be analyzed using a 3D representation per ASCE 7-10 12.7.3. My office has WoodWorks (which doesn’t allow skewed shearwalls) and the 3D frame software that comes with the Enercalc SE cloud subscription (which we have never used), so I think the time and money involved in purchasing and/or learning a new software is not yet an option.

To get around this, I was thinking of neglecting the skewed side of the building and treating it as an open-front cantilevered diaphragm. It would satisfy the L’ < 35ft and 0.67:1 L’/W’ aspect ratio requirement for torsionally irregularities per SDPWS 4.2.5.2.2. Would it be acceptable to idealize as a rigid diaphragm in this case even though it’s a light-frame flexible wood diaphragm? Or perhaps envelope rigid and flexible? And would it require slip detailing to keep the skewed side from engaging in a seismic event? Intuition tells me that at such a long length of 60 ft and a minor skew of 5 degrees, that the out-of-plane deflection isn’t going to be an issue and that I would ultimately want it to engage and be a part of the LFRS, but I still need to follow the requirements of 12.7.3.

Seismic Forces (ASD):
4032 lbs at Roof
2559 lbs at Upper
1163 lbs at Main

Any guidance would be greatly appreciated.

 

[molibden]

I'm not from US, so no help with ASCE. Regarding the structure, I would definitely consider are available shearwalls. You have very little walls to really rely on.

This is from Woodworks FAQ:
In the software, a skewed wall could be modelled as two separate walls forming a triangle with the skewed wall. The lengths of these two walls representing the skewed wall would ultimately be up to the designer.

 

[Wandering Wallaby]

Thanks for your response. Yes, I would prefer to use the skewed walls on the left in the design. Afterall, even though they are skewed, they will still have some sort of stiffness in the orthogonal direction and pick up some of the load. The question is, how do I determine their stiffness in a non-orthogonal direction (5 degree skew) so that I can model an equivalent orthogonal shearwall as suggested by WoodWorks? I don’t feel comfortable considering the equivalent length as just the cosine of the length of the skewed wall since there will be other torsionally-induced forces not being considered.

I did model this in WoodWorks however, as a three-sided cantilevered diaphragm and was able to get it to work with an enveloped flexible and rigid analysis. The walls on the main floor became double-sided shearwalls with large 13k holdowns. It’s not pretty but it would work if the entire skewed side of the building didn’t attract any load. The story drift ending up being the governing factor.

 

[molibden]

I would use equivalent length. With 5 degree skew this is the smallest simplification we do when we design. We don't really know the real loads or even real behavior of the timber frame, diaphragm...
You have results without these walls. Now do another analysis with them and make engineering judgement to be on the safe side. I would much rather have these skewed walls as part of my lateral system even if I know there might be some additional forces I didn't account for.