Base shear load vs. base shear capacity?

[drile007]

I'm wondering about base shear load (induced by earthquake) and base shear capacity? I'm working with Etabs so I got base shear (tables) and shear forces in individual wall (pier).OK, so far. But here are the things which one I wonder about:
If I sum shear forces (just walls in particular direction) in individual walls-piers I've got approx. 20% higher force than base shear (from tables). I know that this difference came from combining the mode shapes. OK, but If I look on the structure as a whole on which base force should I dimension the walls. My philosophy in dimensioning is that I don't pay attention on the individual wall I'm just take care that sum of individual walls shear capacity exceed the base shear load. In this philosophy I'm counting on redistribution of seismic forces which my code (Eurocode 8) allows me. Again, on which base force should I dimension the walls (base shear from tables or sum of of individual wall shear forces)? I'm aware that the base shear (from Etabs tables) is the "clearest" force and have the minimum "errors" induced by combining the mode shapes and the most it's 20% smaller.

Thank you for any comment. Hope I was clear.

Drile007

 

[eduardotano]

Are you considering in your analysis rotational ("torsional") effects? They may cause a given wall get a "total shear" greater than its "direct shear" (base share distribuited between walls according their stiffnesses).
Perhaps it may be the cause of the difference you found out, specially on the walls located in the perimeter of the building plant.
I hope this may help!