What is the portal method for approximate frame analysis under lateral loads?

The portal method is an approximate analysis technique for frames under lateral loads assuming: (1) inflection points occur at mid-height of columns and mid-span of beams, (2) interior columns carry twice the shear of exterior columns. The frame is analyzed by cutting through inflection points and applying equilibrium. It's useful for preliminary design and checking computer results, providing reasonable accuracy for low to medium-rise regular frames.