Shear Wall Aspect Ratio

[CN-EIT]

Has anyone seen a code provision allowing a blocked wood shear wall to exceed the 3.5:1 aspect ratio? I have a project in which a second story is being added to an existing one story building and I am looking at the load path for lateral forces. The existing structural plans dated 2008 show 3'-0" long shear walls with a 12'-0" wall clear height which has an aspect ratio of 4:1...which is apparently not allowed per the SDPWS Table 4.2.4.

 

[phamENG]

They may have been using the IRC braced wall method and called them shear walls. Can you post the drawing (redacted as necessary to preserve privacy, etc., of course)?

 

[CN-EIT]

Looking a little closer at the braced wall method, I think you are correct that they likely used it. My previous experience did not introduce me to the reduction in wall length allowed per hold downs at the end of wall and as such the plan showing hold downs threw me off. Running the numbers it seems like the WSP method of braced wall would allow for the narrower panel widths that, if looked at as a true "shear wall", exceed the aspect ratios.

 

[phamENG]

Well...I find it odd that there's nothing on the left wall. That would certainly violate braced wall requirements. Another possibility - they ignored the aspect ratio requirements. If you run the numbers on the house as built, what's the D/C ratio on those? If they're very low, then the original engineer may thrown them in expecting the other walls to do most of the work (which they will) and these would be there just in case to keep the house from falling over. For the record, I don't advocate this approach - but I've seen it done.

If you're adding enough lateral load to make the use of those shear walls really important, I wouldn't put much faith in their performance. What's the elevation look like? Are those full height openings, or standard windows? If standard windows, you could look into converting it to an FTAO arrangement.

 

[CN-EIT]

The left wall threw me for a loop as well. Because they nailed all of the walls with 8d @ 6" & 12" w/3/8" wsp that left wall would be one big arse braced wall. My presumption is that because of the length of that left wall they did not need to specify it as a shear wall since the modification factor for a hold down (to reduce required wall length) would not have been necessary thus they did not need to show a hold down. And I agree. Our staff engineers are not big fans of this method either which is why we generally do not use it. However, with the many 3 story apartments and homes built with this method combined with the lack of failures during high winds I can appreciate the perspective of "it still works" even though we do not get to put hard numbers to it.