to each floor and check the "Calculate Diaphragm Center of Rigidity" option in the analysis settings.

If ETABS reports total mass = 100,000 N·s²/m, then $W = 100,000 \times 9.81 = 981,000 N = 981 kN$. This must match the sum of dead load plus applicable live load per ASCE 7.

When you open the table, you will see several columns. Understanding these is vital for a precise seismic analysis: