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Re: Diaphragm shears

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In a message dated 97-01-02 11:22:53 EST, you write:

<< Subj:	Diaphragm shears
 Date:	97-01-02 11:22:53 EST
 From:	Shake4bake(--nospam--at)aol.com
 Reply-to:	seaoc(--nospam--at)seaoc.org
 To:	seaoc(--nospam--at)seaoc.org
 
 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.
 
 Any thoughts?
 
 Mark D. Baker
 Baker Engineering
  >>

--------------------------

The diaphragm and connection to the vertical element should be designed for
the higher diaphragm shear values.   The commentary in the 1996 SEAOSC Blue
Book (C108.2.9.2) briefly mentions why, higher mode responses.  The base
shear for the building is typically based upon the first mode, and does not
account for possible higher mode responses.   A minimum design force limit is
provided in the code for  diaphragm attachment, as will as a maximum design
force which by code you need not exceed, unless you feel it is necessary.

Another explaination for using the higher design forces is that you have
reduced the expected lateral forces (seismic) by the Rw value and you are
depending upon the ductility of the lateral resisting system to resist the
true lateral forces.  Since the lateral resisting system will possibly be
approaching yield, you want to be sure that you have sufficient shear
strength to transfer the diaphragm inertial forces to the vertical resisting
elements.  You want to prevent premature failure of the diaphragm connection
 which could result in a redistribution of  forces causing other lateral
resisting elements to become overstressed and fail. 

Michael Cochran 
Brian L.Cochran Associates
Los Angeles, CA