Disjointed Diaphragm Aspect Ratio

[EngineeringEric]

I have a building with a plan area of 140ft x 58ft. This gives an overall aspect ration of 2.4, not a problem. However, when looking at the section of this building the roof is divided (see attached). There is a clear story in the center. If i look at the aspect ratio for each half I get a ratio of 4.8, problem.

I do not know if i can count on this complete 58ft roof to act as a single acceptable diaphragm, or do i need to count for it as two separate ones? The short vertical clear story will be sheathed in wood it does have many openings but i can utilize decent panels between windows for shear transfer as required.

I am wondering what your opinions are on the acceptable vs run-away nature of this building and what are some serious concerns you may have with the lateral loading in the 140ft length.

As per AWC 2008 Table 4.2.4
Max Diaphragm Aspect Ratios for Slope Diaphragms
Blocked Wood Structural panels = 4:1
Unblocked Wood Structural panels = 3:1

Roof System
I-joist rafters, no ceiling joists
Low eave=9ft, Top of 5' clear story=22ft, highest eave=30ft
The center wall will be full height 2x6 studs and i probably will be sheathing it in wood for the in-plane loading.

I will be checking deflection... just haven't yet

 

[azcats]

I think you have two distinct diaphragms unless there's some means to transfer out of plane forces through the clear story.

 

[mike20793]

This is a nasty condition for wood. Are there any interior shear walls? You've definitely got two separate diaphragms there. You can either bring the force from each one to your shear walls below or you can drag the force from one into the other using a transfer diaphragm. I'd try to put a shear wall directly below the discontinuity and also some on the exterior. It'd probably be pretty easy to drag the force in that way, depending on the configuration.

If your worried about aspect ratios, then just add an interior shear wall for the other direction. Since your probably using a flexible diaphragm, the area between each shear wall is treated as a separate diaphragm. Either way, there is some detailing required to make sure the forces are going to the right places.

 

[EngineeringEric]

No interior walls of any sort. Only movable partitions. Looks like a case for moment frames!

Thanks for the verification. time to go hurt the architect's feelings

 

[KootK]

You could sell me on the single diaphragm concept so long as you're careful about the vertical force transfer at both the clerestory and the chord elements at the end walls. My gut feel is that this system will attempt to behave as a single diaphragm no matter what you do.

I'd feel better about it all if the clerestory could be a steel truss with vierendeel panels. That would get expensive of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[EngineeringEric]

I agree it would take some rather detailed analysis on the chords. I found a transverse wall in the higher roof section which i may be able to dump load into through a collector. The clear story has a cased opening in the middle of a long hall that i will have to convince the Arch to relocate 3ft. This in theory should reduce the aspect ratio in half. Just some serious detailing and the use of LVL instead of the I-joist in that location. Or go with steel.

Thanks!