always the vectors that troubled me.

[hnQ_9999]

I have a hard time understand this.
Can someone please help?

[smac]

I think....the center of rigidity is closer to wall 2 because it is more rigid (longer attached on 2 ends) than wall 4. That forms a couple where the center of seismic load and center of rigidity equal out (each wall taking part of the seismic load). Because they are not at the same point, this causes torsion. Each of the walls have to counteract that torsion with an additional load creating a couple opposite of the one causing the torsion. Wall 2 has forces acting in opposite directions so that they will cancel each other or make the larger force smaller (a-b) where as on wall 4 they are in the same direction so the force is more (a+b).
Hope I didn't confuse you more...