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[SEAOC] Re: [SEAOC] Re: [SEAOC] Re: [SEAOC] Re: [SEAOC] Re: [SEAOC] Re: [SEAOC] Re: Reroofing (Wa...

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No, your not dense, but I might be for taking this example too far. My point was that there is shear transferred from the roof diaphragm to the existing vertical elements - albeit not exceeding wind or seismic forces that are applied only at the roof projection for wind and the entire roof weight for seismic. As you noted, these forces typically do not exceed the capacity of the diaphragm.
My example was meant to show that the shear transferred through a diaphragm is limited by the capacity of the diaphragm. If, for example only, the shear elements determined from a force equal to the diaphragms capacity then increasing the diaphragm's stiffness (and capacity) could cause damage to the existing resisting elements.
Look at a structure that approaches an aspect ratio of 4:1. If the end walls are only 15 feet long and the roof is spaced sheathing, the capacity of the diaphragm may not exceed 1500 pounds, by the tributary shear may be greater than this. By increasing the diaphragms stiffness you can transfer more shear through the building to the end walls - exceeding the resisting walls capabilities.
I realize that this is not the normal way to view shear transfer, however it has been the *new* way of thinking for those who worked with the ABK method for URM retrofit. The talk among those involved included the possibility of using the ABK in lieu of Chapter 23 typical distribution methods. In multi-story buildings this becomes more critical since the diaphragm is used to a greater degree to transfer shear.
For all of my rhetoric, my point was only that stiffening a roof diaphragm may inadvertently transfer more load to walls that are not capable of resisting it.