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Dear Colleagues,
We are involved in the design of a two-tower office complex, in Mexico City.  It is composed of a 35-storey and 25-storey buildings.  The Mexican Building Code establishes that the seismic design eccentricity be
1.5es + 0.1b, or
     es - 0.1b
where     es is the computed eccentricity, and
              b  the direction perpendicular to the one the seismic force is being considered
For dynamical (modal) analysis the eccentricity should be reduced to 0.1b provided that both horizontal displacements coupling and horizontal displacements and rotations coupling are taken into account.
We are modeling the buildings with ETABS program.   The fact is that if we assign predefined values to the level mass coordinates (in order to introduce such code required eccentricity) having the selfweight command assigned, it changes them once the run is performed.
Does anybody knows how to predefine a fixed value for the mass coordinates having activated the selfweight command?
Opinions and suggestions are all welcomed.
Fabricio Rosales, P.E.
P.S.  We don´t consult CSI because the support period has expired long time ago.