Generally, the steel frame structure designed by the equivalent lateral force procedure in earthquake resistant design have concentrated ductility requirement and damage in lower stories. Therefore, the design story force, by which a structure designed must have same ductility requirement over the height of a structure under earthquake load, is needed.
In this study, the cause of ductility requirement and damage concentration in lower stories is investigated an improvement for the vertical distribution of seismic load is proposed.
The cause of ductility requirement and damage concentration in lower stories is that the portion of the gravity load moment in the design yield moment of a beam decrease from top to bottom of a structure. If the beam strength is designed by seismic load alone, that is, if gravity load moment neglected, ductility requirement and damage are greatest at the top.
In this study, in case Taft or El Centro earthquake acceleration record is used as input data for analysis, various steel frame structures which have same ductility requirement over the height are designed. Then, the vertical distribution of seismic load used for design is compared with that according to existing earthquake resistant design (ATC3-06 or UBC-1988), consequently the former is larger than the latter in lower stories, smaller than the latter in upper stories, and similar to the latter in middle stories of a structure.
