This paper includes analytical studies for an investigation of the redistribution of the gravity load moment for strong earthquakes and an evaluation of the effect of gravity load on the seismic response of building structures. In the earlier stage of strong earthquakes, gravity load results in different plastic rotations and bending moments at both ends of beams in multistorey building structures. After some significant inelastic excursions, the gravity load moments in beams are redistributed, so that the gravity load moments at both ends of the beams are significantly reduced. Reduction of gravity load moments at the beam ends changes the distribution of bending moment demands over the height of multistorey building structures. Therefore, beams in lower floors of the building structure continue to experience inelastic deformations while beams in the upper floors behave within elastic limits after the component of the gravity load moment disappears. As a result, excessive structural damage may occur in the lower floors of multistorey building structures subjected to a strong earthquake. The implications of the effect of gravity load moment for earthquake resistant design are that it is desirable to reduce the contributions of gravity load and to increase seismic load in determining the design moment.