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Re: Steel straps on wood shear walls

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I may have misunderstood the original question--my earlier answer was directed at more or less continuous, horizontal straps.
 
For strap-type tiedowns, optimal installation is directly to the studs/post.  Simpson catalogs state this, with advice that deflection is increased if the strap was placed over the plywood, and refer to their form "T-PLYWOOD"  ( http://www.strongtie.com/ftp/bulletins/T-PLYWOOD08.pdf ) which tabulates the deflections for several products and conditions, and states that it's the designer's responsibility to determine whether the added deflection is acceptable.
 
The additional deflection occurs because the plywood under the tiedown acts like a shim between the post and the tiedown, and the nails from tiedown to post deform within the thickness of the plywood.  All of the uplift cannot be taken out by gripping the bottom corner of a shear panel--the post above the tiedown collects the majority of the uplift, and this force needs to get to the tiedown somehow.  Delivering the force through a plywood or OSB spacer is questionable.  The increased deformation of the tiedown nails in the plywood 'shim' will undoubtedly reduce overall performance, especially in cyclic loading.
 
My standard detail shows strap tiedowns attached directly to a studs/post and a 3-foot block attached to the studs/post for the shear panels to connect to, with the panels being cut out around the tiedown.  I almost never use this, because I tend to specify the HDU or HTT tie-downs.
 
Thor Matteson, SE