How is seismic base shear calculated for building design?

Base shear V = Cs * W, where Cs is seismic response coefficient and W is effective seismic weight. Cs depends on spectral acceleration (from seismic hazard maps), site class (soil type), response modification factor R (based on structural system ductility), and importance factor. The base shear is distributed vertically to each floor based on height and weight. Modern codes like ASCE 7 use equivalent lateral force or response spectrum methods.