Wood shearwall not extending to roof with windows above

[jdgengineer]

I'm working on a wood framed project which has several exterior walls where the solid wall does not extend all the way to the roof. Above the wall there are clerestory windows with structural posts between each window. See below for a photo I found on the internet. Not the exact situation but similar.

We are in an area of high seismic. It's a single-story structure and the architect has been resistant to steel moment frames. He has done quite a few of these in the past and the engineer he was working with has found ways to make them work, often times with all wood framing. I don't have their calcuations so I'm not sure what that engineer's assumptions were.

How have you all designed these in the past? It seems one option would be to run full height columns between the windows and design those as cantilevered columns which are braced by a wood shearwall below the windows. If we use an R=1.5 for a wood cantilever column or R = 1.25 for ordinary steel cantilever column the drift and forces become fairly large. I've heard an argument to treat those columns instead as collectors to the shearwall and therefore design with an R=6.5, but I'm not sure how I feel about that interpretation.

How have you all handled similar situations?

 

[msquared48]

Cantilevered columns to a shear wall.

The tops of the columns all must deflect the same, and the wall top plate/beam will serve as a drag strut. Distribute the force to the columns based on the cube of the projected length if all the columns are the same size. Otherwise factor in relative S's too. The shortest column will take the most load. Use R=1.5.

Mike McCann, PE, SE (WA)

 

[jdgengineer]

Thanks Mike. That's what I was thinking too.

I can handle the columns, but the design of the wood shear wall becomes tough with an R=1.5. The cantilevered columns also magnify the shear force to the wall due to the cantilever effect which makes the problem worse. Seems like we should design the whole the shear wall also for 1.5, but what have you done? On one hand I think that is what the code intends, On the other hand it seems pretty conservative considering a WSW strongwall is essentially a wood cantilevered column with an R = 6.5. For a single family residence we would be allowed to design the lower lateral system for a higher R than the upper system if this was multi-story which seems to give some credibility to using a higher R for the shear wall.