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Re: Calculations of Base Shear

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For calculating the base shear you have to use the total weight of the structure (the weight is independent of the direction of the earthquake force). Meaning, you have to include the weight of the parallel and perpendicular structural elements. After that using the stiffness in each direction you will calculate the mass participation ratios (which may require a eigen value analysis) in each earthquake direction. Your total base shear will be a function of the total weight and modal participation ratio. i.e. although the weight that participates in the EQ load is dependent on the EQ direction(do not confuse that statement with the first statement), you have to use the total weight in your equations, and the modal participation factor will show, how much of the total weight participates in that direction.
Have a nice day,
Alper Ucak
----- Original Message -----
To: seaint
Sent: Monday, February 24, 2003 11:55 AM
Subject: Calculations of Base Shear

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