U-Shaped Diaphragm

[cuels]

I am designing a building with a two-story open reception area. The 2nd floor would then create a U-Shaped diaphragm to distribute lateral forces to the shear walls. There aren't any interior shear walls planned. In thinking about the chord forces and the diaphragm, I wasn't sure how those chord forces would transfer across the entire diaphragm, since there is a cut-out in the U-shaped part of the diaphragm. Should the floor be constructed in a certain way to consider this U-Shape, or would I just consider the higher unit diaphragm shear in the skinnier section of the floor diaphragm? Attached is a layout of the building in question.

 

[jdgengineer]

Personally, I don't like big diaphragm discontinuities like this, and I would try and work some type of interior shearwalls at the opening for the "U".

If they are not possible, you'll want to at a minimum design the diaphragm continuity at the diaphragm rentrant corners. You'll also need to amplify your forces per 12.3.3.4 assuming you are in a high seismic area.

When the diaphragm necks down at the U, yes you'll need to check the diaphragm for the higher shear stress (which shouldn't be high since shear is 0 at the middle), but more importantly you'll need to develop chords in the skinnier section. These chords will need to be developed far enough into the remaining portion of the diaphragm to develop the required chord forces.

 

[cuels]

I am assuming that the "development into the remaining diaphragm" would be similar to development length into concrete? If I can develop enough restraint through nailing to an adjoining joist or deck sheathing, that is sufficient to transfer those chord forces into the legs of the u-shape?

 

[msquared48]

If I were you, I would make sure there were a minimum of five diaphragms in that structure. Lots of eccentric rigidities to consider and allow for too. I hate these configurations!

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com