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[SEAOC] Reroofing

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Without reading a couple of private posts to Michael Cochran, you are correct in your post. My point had to do with structures that had not been designed, but had been built under the provisions of "Conventional Framing" in past codes.
Specifically, I discussed buildings with large aspect ratio's approaching 4:1 - similar to a California Ranch which may be only 25 feet deep, but close to 80 feet in width. The calculated shear may actually exceed the capacity of a stucco shear wall if the short walls are interrupted with windows. This would never have been anticipated in the original drawings since no calculations where run to check the walls capacity. Conversely, if the building were constructed by more current standards and two 4x8 sheets of 3/8" plywood (unblocked with 8d 6:12) the resisting capacity would be no greater than 264plf*8 or 2112#.
Carrying this a bit further (assuming wind governs) an 8' plate height with a 4' rise to the peak of the home might yield approximately 148 plf - with a width of 50 feet the reaction would be 25*148plf or 3700 lbs - exceeding the walls capacity. If the wall follows conventional framing standards it would require bracing at the ends with a 4x8 panel as noted above.
The damage won't necessarily occur if the diaphragm can not transfer greater than 100 plf. But once it is sheathed and the capacity of the diaphragm increases to around 240 plf the roof will be able to transfer all 2700 lbs (25*240=6000 lbs max).
My point was that there would be little damage to even stucco walls (180 plf) if the diaphragm was incapable of transferring the calculated shear to the walls - such as with spaced sheathing (assuming at least 2112/180 or 12 linear feet of stucco shearwall - given a window or two this is reasonable). This would make it seem that other walls - possibly interior walls, exterior entry walls or common walls with cripples for fire stops may be absorbing shear - and this may account for the mass of damage to interior gypsum partitions in older track style homes.
Finally, my comments related that if the diaphragm stiffness where to be increase, then a greater amount of load would be able to be transferred to the end walls. If the walls were not capable of resisting the new demand, additional damage would increase.
One additional observation - if the flexibility of diaphragm due to the use of skipped sheathing where not able to direct the shear to the exterior resisting walls, the interior walls could easily be damaged by the connection of the walls to the ceiling. The deflection of the upper roof diaphragm would force the exterior walls out-of-plane, bringing the rafter ties (ceiling joists) with them. This would drag shear into interior gypsum walls that may have some resistance attributed to the connection at the top and the capacity of the powder driven nails at the sill. Increasing stiffness in the diaphragm might reduce the deflection in the diaphragm yet deliver more reaction to the exterior walls.
Enough for now, let me know if you find fault with this - I enjoy the debate and would be pleased to learn something new that I can apply in practice.
Dennis Wish PE

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