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RE: Vertical Force Distribution - UBC97

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Look carefully at the distribution section of the code.  Somewhere there is a section which gives you an out if the lower stories are 10x more stiff than the upper story.  You would do you vertical distribution based on two buildings.  First do a vertical distribution with the "soft" structure & it's appropriate R value.  Then do a vertical distribution for the "rigid" structure and scale the forces at the interface by Rsoft/Rrigid.  This is all in chapter 16 somewhere, but I don't have my code with me.
 
Hope you can find it,
 
Jake Watson, P.E.
Salt Lake City, UT
-----Original Message-----
From: YI [mailto:YI(--nospam--at)summit-sr.com]
Sent: Monday, May 17, 2004 5:23 PM
To: seaint(--nospam--at)seaint.org
Subject: Vertical Force Distribution - UBC97

UBC97 distribute seismic force vertically, basically using the ratio of floor weight times the height.   Consider a three story building, if the roof is a plywood diaphragm, and the floors below are all concrete deck, e.g. rigid diaphragm.  Would the vertical force distribution still occur as described in UBC97, 1630.5?  That is, does the lateral force in the roof diaphragm need to be calculated according to section 1630.5 of UBC97, which mostly like means a higher force than the base shear calculated at the roof level?
 
SEAONC bluebook says that when the structure deformation differs significantly from the assumed liner mode behavior, in this case the roof diaphragm will deform a lot more than the concrete diaphragm below, consideration should be given to alternate methods of distributing the forces sucha s dynamic analysis. 
 
Is there any other method you use in this situation, to avoid doing a dynamic analysis?  Do you just ignor the vertically distributed force in UBC97, and use just the base shear at the roof level as the design force?
 
Appreciate any comments.
 
 
Y i   Y a n g,   P. E.             
STRUCTURAL DIVISION
SUMMIT ENGINEERING INC.
707.527.0775.x15
Santa Rosa, California