3-Sided Wood Diaphragms for Seismic

[woody1235]

I hope everyone is doing well.
With respect to a standard stand-alone wood framed garage in California:

Can I design the seismic lateral with a three-sided diaphragm, (not using the short wall segments on either side of the OH door as shear walls)?

CBC 2305.2.5, which deals with criteria for a RIGID diaphragm (which plywood roof sheathing is not) has been interpreted to me by a plan reviewer as stating that ONLY with a rigid diaphragm can a 3-sided be used.

I disagree, interpreting that the code does not PRECLUDE the use of 3-sided, provided L/W ratios are OK, and deflection can be tolerated (keeping in mind this is not a habitable structure)

(This interpretation would affect a lot of wood buildings of all types around the country since all areas have some level of seismic design criteria as well as wind, and there are a lot of 3-sided out there!)

Thanks for your thoughts!

 

[frv]

Plywood diaphragms, like metal roof deck, are actually semirigid. You are allowed to design them as flexible (for the most part), but in short aspect ratios behave more like rigid diaphragms.

If you think about it, the diaphragm MUST be considered rigid to satisfy equilibrium in a cantilevered condition. If it were flexible, it would be unstable.

 

[dcarr82775]

frv hit the nail on the head. A diaphragm as you describe must be rigid (at least semi rigid) or there is no way to resolve the eccentricity. A plywood can easily be rigid.

 

[BAretired]

I agree with frv and dcarr82775. In Alberta, plan reviewers don't address such technical matters, so we don't find ourselves in similar circumstances. What happens if the plan reviewer cannot be convinced?

BA

 

[woody1235]

Thanks for your input!

I know that plywood is actually semi-rigid,(but the code ALLOWS it to be considered as flexible for the purpose of force distribution to walls, which can make it easier for some lateral design.)

In my case(or any using 3-sided), I want to consider its rigidity in distributing lateral force in one direction to the back wall only ("open front"). The issue is basically whether a 3-sided plywood diaphragm can be used at all for seismic per the code, or is it prohibited as my plan reviewer is indicating.

(The code indicates that by default, unless in-depth analysis, wood panel diaphragms are considered "flexible", but the code reference I mentioned (2305.2.5)is only for "rigid".)

My position is that if the plywood has adequate shear capacity to handle the full required force in that direction along the back wall, the building can accomodate the diaphragm deflections, and I am within the code L/W ratios, I am ALLOWED to design as 3-sided ("open front" in the code).

I've taken care of this specific case with this plan reviewer - added short shear walls at the sides of the OH door so now 4-sided. However, my client may have dozens of these all over, and not being allowed to use 3-sided in general will have a big impact. We work in many states, and this is the first time we have had a plan reviewer interpret the code to NOT ALLOW 3-sided to be used for this kind of building (thus my angst!)

Thanks!

 

[frv]

Unfortunately, I am not familiar with the specifics of seismic design, but I'm pretty sure it's at least discouraged depending on seismic design category.

I would also like to hear from someone with more experience in seismic areas.

 

[dcarr82775]

I am not in California so I am not familiar with their code, but I believe they have some severe restrictions on the 3 sided case you are talking about because of a lot of failures in earthquakes.

 

[JoshPlumSE]

Why not run some hand calculations on an "equivalent" rectangular diaphragm? Or, run an FEM analysis on your triangular diaphgram.

You can use the results to justify the assumption of a flexible or rigid diaphragm.

I realize that's probably more work than you really want to do for an unoccupied garage. But, for unusual shaped diaphragms, I think you gotta bite the bullet and do the extra work.

 

[woody1235]

The diaphragm is not triangular. By "3-sided" I meant to indicate that there are shear walls on 3 sides, not the front wall. Sorry that I wasn't clear.

Thanks for all your input!

 

[sandman21]

The code section you reference states "Rigid wood diaphragms are permitted....."(three times) it does not deal with flexible diaphragms as they would be unstable. The only time deflection comes into effect is increasing the ratio. You should just show the plan checker that the diaphragm is rigid per ASCE 7-05 12.3.1.3. We deal with these types of building, they are either shown to be rigid, or designed differently.

 

[msquared48]

What you are talking about is technically a cantilevered diaphragm, and there are limitations on the use of such an animal in the code.

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