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Re: Diaphragm shears

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> Mark Baker wrote:
> 
> In a multisory building (light commercial), when the floor diaphragm shear at
> a vertical resisting element is higher than calculated building shear at the
> vertical resisting element, which load should the vertical element be
> designed for?
> 
> It doesn't make sense to me to design the floor diaphragm and its connection
> to the vertical element for the higher floor diaphragm shear and then design
> the vertical element for the lesser building shear. I do not find anything in
> UBC or Blue Book Commentary which addresses this issue.
> 
Mark,
	When you distribute siesmic forces vertically per UBC 1628.4, and the
floor diaphragm forces per UBC distribution 1631.2.9 the lower level
wall/bracing forces will be less than the floor diaphragm forces unless
the above wall/bracing forces are included.  My understanding has always
been that the forces at each level can not distrubute up in the
structure unless they at least get to the vertical system.  This works
in reverse at the roof level where the vertically distributed forces
will be less than the roof diaphragm force.  I do on occasion design the
roof diaphragm for the larger vertically distributed shear force in
order to be conservative but this is the exception.  The new SEAOC Blue
Book page 165 on diaphragm forces tries to explain this in the third
sentence.
	Hope this helps.

			Jim Dane, P.E.