Continuous or Discontinuous diaphragm help

[TycoSpot]

I'm trying to determine if I can make a "continuous diaphragm" out of a second floor that has 1/3 of the square footage is concrete on metal deck and 2/3 is wood sheathing on wood joists. A little more background: it is a three story building that is framed out of all wood members except for approximately 1/3 of the second floor required the concrete deck framing system (due to architectural reasons). I'm concerned that a separation joint would cause undue complications. Is there a way that I can satisfy ASCE 7 section 12.10.1 by connecting the two different diaphragm materials? Thank you for any help you can provide to me.

 

[msquared48]

I hear what you want to do, but have never run into the need. Sounds challenging...

The inherent problem that strikes me is two structurally different diaphragms - one flexible and one rigid. I would have to think how to do this, and would also have to consider the shearwall/frame locations as to whether this would be feasible.

Can you post a sketch with the shear walls shown and the diaphragms dilineated. Itr would help immensely.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[TycoSpot]

Here's a simplified sketch of the situation. I have plenty of shear wall possibilities as the building is divided into many units. I'm using wood shear walls at the wood diaphragm locations and CMU shear walls under the conc deck diaphragm location.

I was hoping to make the wood diaphragm on the second floor blocked and therfore at least semi rigid to work better with the rigid concrete diaphragm....?

Yes, this is very challenging! Again I appreciate your help!

 

[a2mfk]

Not sure I see any big problem. Its a shame that your interior CMU wall does not align with the exterior wall on the left side of the building. If you need an expansion joint though this would be the place to put it.

You'll need drag struts extending out from your return walls, and then you will essentially have two diaphragms divided into two sections. Rigid diaph on the left side and flexible on the right side, though you may model the wood diaph as rigid. The interior CMU wall will take a small amount of shear from the left conc diaph. and then 1/2 of the right side wood diaph. shear (between the CMU wall and the drag strut to the right).

Dig in a little more and post your prelim analysis if you want...

 

[TycoSpot]

Part 2 of my question: Can this building qualify as a "Simple Diaphragm" and therefore allow me to use Method 1 wind design?

 

[a2mfk]

Go through the "checklist" of things it must meet, if they don't all apply to your building your stuck with Method 2. Its not that bad actually, just had to do it for the first time. Expansion joints, drag struts, etc. I think would kick you out of that, don't have ASCE in front of me.