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Re: Subject: Calculations of Base Shear

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You can find what Mark said in SEAOC Seismic Design
Manual Volumn 1, example 48.
It says,
"Walls parallel to the direction of the seismic forces
are usually not considered in the determination of the
tributary roof weight because these walls do not
obtain support, in the direction of the force, from
the roof diaphragm."

Min
 --- Mark Gilligan <MarkKGilligan(--nospam--at)compuserve.com>
wrote: > Bobby
> 
> There are two approaches:
> 
> In both approaches you must consider all building
> weight above that is
> tributary to the level above the found floor when
> calculating the total
> base shear.
> 
> When distributing the forces to the shear walls you
> can ignore the weight
> of the shear walls at that level when the shear
> walls are parallel to the
> direction of the earthquake motion.  This ignored
> weight times the proper
> coefficient should then be applied directly to the
> shear wall.   This can
> be worthwhile if you have a particularly heavy wall
> or when you want the
> absolute minimum force in the diaphragm.  This is
> more difficult when you
> are using a computer program to analyze the
> structure as opposed to a hand
> analysis.
> 
> The more common approach is to assume all of the
> wall weight tributary to
> the diaphragm is lumped together.  This is quickier
> and less prone to
> mistakes. 
> 
> Mark Gilligan
>  
> 
>
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> Greetings and Salutations:
>  
> I have managed to get myself confused as I attempt
> to determine the weight
> of the structure for base shear calculations.
>  
> When determining the weight of a structure for base
> shear, I understand
> that the weight for the floor or roof system along
> with the tributary
> weight of the walls is "lumped" at each level.
>  
> When making this calculation, is the direction of
> the earthquake force
> considered?  That is, do I include the weight of
> vertical elements parallel
> to the direction of the earthquake force considered?
>  I have seen
> discussion elsewhere that indicated wall weight
> parallel to the EQ force
> was not included in the base shear calculation and I
> have seen examples
> where the total values were determined without
> regard to force direction -
> that is - the earthquake force was the same in each
> direction of the
> structural system.
>  
> What is correct? 
> 
> Thanks in advance!
>  
> Bobby Jenkins, PE
> Jenkins Engineering, Inc.
> PO Box 2101
> 218 S Thomas St  Suite 209
> Tupelo, MS  38803
> (662) 840.1233 vocal
> (662) 840.1103 fax
> E-Mail:  jei(--nospam--at)netbci.com
> 
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