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Subject: Calculations of Base Shear

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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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