3-Sided Wood Diaphragms for Seismic
3-Sided Wood Diaphragms for Seismic
(OP)
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!
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!






RE: 3-Sided Wood Diaphragms for Seismic
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.
RE: 3-Sided Wood Diaphragms for Seismic
RE: 3-Sided Wood Diaphragms for Seismic
BA
RE: 3-Sided Wood Diaphragms for Seismic
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!
RE: 3-Sided Wood Diaphragms for Seismic
I would also like to hear from someone with more experience in seismic areas.
RE: 3-Sided Wood Diaphragms for Seismic
RE: 3-Sided Wood Diaphragms for Seismic
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.
RE: 3-Sided Wood Diaphragms for Seismic
Thanks for all your input!
RE: 3-Sided Wood Diaphragms for Seismic
RE: 3-Sided Wood Diaphragms for Seismic
Mike McCann
MMC Engineering
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