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[SEAOC] Re: [SEAOC] Re: [SEAOC] Re: [SEAOC] Re: [SEAOC] Re: Reroofing (Was: Basic Roof Rafter Ana...

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In response to RLFOLEY(--nospam--at)
I believe the analogy is still the same. The shear transferred by 1x skipped sheathing is less than the same diaphragm with plywood sheathing. Furthermore, it's unreasonable to think that you can transfer more shear than the capacity of the diaphragm will allow. If the demand of the diaphragm exceeds the capacity of the diaphragm, the diaphragm will fail - i.e., panel buckling, nail popping etc. This is the concept between capacity and demand.
Therefore, if the capacity of the diaphragm were 100 pounds per foot, the resisting wall (whether it be stucco, plywood or masonry) must be able to resist the accumulation from the diaphragm at 100 plf. Any more than this, the diaphragm will fail. If the diaphragm were 20 feet long then it can only impose a maximum of 20*100 or 2000 pounds to all shearwalls in that line. This makes one assumption that I can be faulted on, and that is to assume that the resisting walls are capable of a capacity equal to the connected length of diaphragm times it's capacity (plf). Most of the time, it is much less.
But, following one step further, if you indiscriminately sheath the roof and increase the demand by the diaphragm to 2.5 times it's original capacity, you must increase the shearwall capacity accordingly - which we don't normally do.
So, without seeming too verbose, the increase in the demand of the diaphragm can be detrimental to the existing shearwalls and should be a consideration before recommending a new diaphragm - whether above the existing or in place of it.
Hope this clarifies my point. There is little difference in the URM and wood frame when talking about flexible diaphragm action. The main difference lies in that we automatically design from tributary shear in wood framing and in a modified distribution that considers the diaphragms capacity in URM work.

Thanks for the comments
Dennis Wish PE