Mass Source in Etabs

[maniafridi]

When I calculate the weight of the building for a combination D.L +0.5 L.L similar to that Mass source defined then why the vertical reaction Fz is Greater than the Seismic Weight used for Base Shear Calculation
See the Image Below

 

[Agent666]

It's because the bottom half of the lowest level is not considered in the seismic mass. But it is considered in the same constructed load case.

 

[maniafridi]

Thank u for your answer and I confirmed it manually you are 100% right.