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Re: Relative Stiffness of Wood Shearwalls

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Dennis-
The Hardy ES Legacy Report No. PFC-5342 on the www.iccsafe.org website, Reissued June 1, 2003 states, in Section 2.7.1, GENERAL:
"... Where Hardy Frame(R) Panels or Brace Frames are placed in a wall and combined with other shear-resisting elements, the applied loads can be proportioned based on relative stiffness.  The registered design professional is responsible for determining the proper load path to the foundation."
 
They provide capacities based on both R= 4.5 and R = 5.5 in the report.
 
The important factor to look at with any combination of elements is to ensure that sufficient ductility exists. If an element starts to yield, other elements begin to pick up a higher proportional share of the load.  As loading continues, a second element may begin to yield and the still eleastic elements pick up still higher proportional shares of the load. What you want to be sure is that the sum of the capacities of each element at a particular level of deformation, in whatever stress state they exist, do not total less than the required demand.  If a single element fails, there needs to be redundancy among the other elements to share the demand without that element (e.g., this is what happens when a gypsum board panels fails in a brittle manner and the remaining plywood panels must share the load. The caveat is that you want to be sure that the brittle panel does not release its share of the load so suddenly that other panels must share an impact load they are incapable of resisting. It is similar to the reason ACI has a minimum amount of required steel to resist the cracking moment in a beam so the beam will exhibit ductile behavior).
 
Regards,
Bill Cain, S.E.
Berkeley CA