distribution of base shear
The manner in which base shear is distributed over the height of a structure according to the displacements that would occur during an earthquake. For a building of regular rectangular shape with equal floor weights and heights and no irregularities in stiffness or mass, base shear is distributed to each horizontal diaphragm above the base in proportion to the floor weight at each level and its distance from the base. This results in a triangular load configuration varying from zero at the base to a maximum lovalue at the top. For structures having a natural period of vibration greater than 0.7 seconds, a portion of the total base shear is assumed to be concentrated at the top of the structure to account for the whiplash effect of seismic forces. For structures with irregular shapes or framing systems, the distribution of lateral forces should be determined according to the relative stiffnesses of adjacent floor levels and the dynamic characteristics of the structure. (Ching, 1995)