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Re: Load Paths

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I did look at the JPG you sent - it is the old picture of the hybrid wood shearwall from Simpson. I think I understand what your concern is and I also think that Simpson did not provide a very good picture demonstrating shear transfer for engineers or resistance of out-of-plane forces. There is a large discontinuity at the roof level between the upper roof rafters and the lower chord of the truss that would suggest that it is not finished or still under construction. My comments are related to the condition that would be more commonly shown at the first floor of the JPG file. My comments, while I understand that you did not discuss the king post issues, were related to the last discussion we had with Michael Cochran on the list related to wall buckling on tall walls especially at doors and openings in tall walls with high plate heights. To be more specific, my comments intended to address the "system" rather than the "component". We fail to take into consideration the sheathing on the interior and exterior in tall walls. Instead we isolate the studs in the strong direction and can calculate them as simply supported members where the applied load from wind determines if they are sized sufficiently. My argument is contrary to what you would do and this is fine as your engineering judgment guides you to be conservative in this matter. I just designed a 14-foot wall supported at the upper and lower plates (double top plate and lower sill) where the discontinuity exists at the 12-foot Hardy panels. Theoretically, the failure should occur at the header or pony wall above the Hardy panel where a hinge is introduced. However, this does not take into consideration the exterior wall sheathing (redundant to the capacity of the Hardy panels which control in-plane) nor does it consider the lath and stucco over the plywood or the interior gypsum wallboard. There is not enough information that analyzes the "system" but rather isolates the component and because of this, you are probably correct to use a more conservative means of rationalization in your design. My professional intuition tells me that there really has been little written or identified as a failure for this system and again my intuition tells me that the factor of safety is sufficient for most wind gust areas up to about 90-mph. Above this, I might be more conservative in my design as you are.

Finally, as I noted, Simpson has released a new Metal Strongwall which is now similar to the Hardy Panel. It would be great if we could get AF&PA, for example, to do more testing out-of-plane of walls sheathed with different materials and we could get away from the theoretical numerical aanalysis for something that is empirically evaluated.


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