The answer in your question should be given in help topic:
but i think that this help topic isn't updated so the informations given in there are wrong.
In one SP of ARSA2012 (i don't remember exactly in which one) the way of calculating the reduced forces has been changed:
- Forces in stories are calculated to the level just above the bottom of each story (and not in the level of center of gravity).
- Total forces in stories are calculated directly from loads applied to nodes and elements of appropriate story. Forces in walls are calculated substracting forces in bars from total forces.