Justifying Shear Wall at upper level of Condo - help please!

[ReverenceEng]

SEE ATTACHED IMAGE FOR CLARIFICATION BELOW.

Got a client with a condo on the 5th floor (top floor) of a low-rise. His unit has high ceilings and a lofted space that creates a "sixth story" whereby this added ceiling space projects above the rest of the main roof, almost like an additional story, but only accessible from within his unit on the 5th floor via stairs and it is very small compared to the rest of the roof. Most units have this feature.

Client wants to put a window in this upper story wall (above the main roof line) to let light in. Gravity is OK, but building dept wants us to check shear due to the reduction in SW length for this upper structure.

Anyone have a quick strategy for tackling this given this is atop a condo building with a very large footprint (upper structure us very small, relative)? I've never justified shear in a large condo where I'm not really looking into tackling the loads of the entire building unless I must.

SKETCH:

 

[atrizzy]

Does the roof of the building consist of one unbroken diaphragm? Seismic zone?
If it is a very big building and you can show that the reduction in the roof shear capacity is very minimal (2% or so) then likely it is fine. Other things to consider:

-Are there hold downs on this wall? (the opening would effectively split it into 2 short walls with much more overturning at each end)
-Is the nailing pattern heavy, are the walls blocked?
-Is this an end wall, or a wall that takes a definitive tributary load, or is it a repetitive wall with many similar types to take up the few percent that this one would fail to support in this case?

 

[HouseBoy]

This doesn't appear to be very distinctive to me. (I could be wrong but...)
Once the shear load gets into the 5th floor roof diaphragm, the rest of the building will not know that there new window up at the penthouse level.
Seems like this can be addressed as a "local" condition i.e. get the load around the new opening and into the 5th floor ceiling diaphragm and that should be all.
Presumably, the original wall construction did that adequately. A perforated shearwall approach looks appropriate now. Might need some tension strap reinforcing around the new opening as I would not expect the original nailing to have been so redundant that it can be relied on for that without some augmentation.

 

[idecharlotte]

The window only effects the shear wall at the upper story, not any of the stories below. You can treat it as a perforated shear wall if you wanted more favorable results. Based on your sketch, assuming the window opening is reasonable (at least 4 ft. of wall on each side), I doubt you will have an issue.

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