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Re: Plywood shear wall definition

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Mark,
The assumption is that there is a load path from the floor or roof above to the foundation below. Specifically, the load and its resistance is considered to be concentric with the wall framing. Therefore, it is not unreasonable to have the plywood applied to one side at the upper portion of the wall and on the opposite side lower down. With this said, I would strongly suggest that the common edge be continuously blocked with 3x or wider blocking and that this blocking is used to transfer the shear from one side of the wall to the other.

Per your second question - Absolutely. The shear transfer occurs from one panel to the lower panel through the connection of the sill plates. If the shear is much greater above - for example you have a 2x4 wall above a 2x6 wall and it is not continuous then you may need to add connectors (nails, clips etc.) that can make up for the shear. Let's assume that you have a 2x4 pony wall above and you need to sheath the entire length but only have enough 2x6 wall below that you can dedicated for shear. The wall below may be sheathed on both sides while the wall above is sheathed on one side. The trick here is to make sure that the demand from the shear above is properly transfered into the capacity of the wall below. You may need to add mechanical clips like the old Simpson A35F (I forgot the number of their new flat clip) and if there is a discontinuity, you may need to add straps to develop the shear. This is similar to making a connection between a drag truss that is designed for the lateral load you need to accumulate through the roof diaphragm but does not need to be sheathed. The connection of the resisting wall below must be equal to the demand in the drag truss or pony wall above. The wall section(s) below will also need to be designed to resist Overturning and have an adequate foundation.

As for aspect ratio's, this is a gray area since proprietary shear panels do not comply with the code related 2:1 ratio (in seismic zone 4) but are tested for their capacity. I would say that you could do the same with a site built plywood shearwall if you do a deflection calculation on the shearwall and insure that the wall is constructed EXACTLY as you designed it - including the compliance against over nailing. The stiffness calculation of the wall panel that the code relates through the aspect ratio of 2:1 was based upon an emergency measure after the Northridge Earthquake in Los Angeles when it was noted that the older 3.5:1 ratio was never really checked for deflection and tended to be sheared to the max which exceeded deflection or drift criteria.

If you can prove it, I believe that it is appropriate to allow, but then again, you may need to make concessions during plan check.

Dennis

Mark Schroeder wrote:

Does anyone have any input to the following?

Question number 1:  Is it possible to have plywood occur on one face of a wall for the upper portion of a shearwall and the other face for the lower portion of a shearwall if all required blocking, etc. is present?

Question number 2:  Can a setback of 2" occur in the plane of a plywood shearwall?  For example, if there is a transition from 2x6 studs to 2x4 studs that are properly connected together to transfer all shear, etc., would such an assembly be considered one wall or two different walls to determine aspect ratio, holdown requirements, etc.

Any input would be greatly appreciated.  Thanks,

Mark E. Schroeder, S.E.
Ficcadenti & Waggoner, Inc.
16969 Von Karman, Suite 240
Irvine, California 92614
(949) 474-0502 Phone
(949) 474-1801 Fax


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