vertical distribution of seismic forces asce 7-05

huangyhg

vertical distribution of seismic forces asce 7-05
hello;
i have been trying to find out about the "k" factor in the cvx formula (12.8-12). the asce 7-05 states that it is an exponent related to the structure period. but i still do not know where it comes from, is it a factor that relates to the system's ductility?
please let me know if you have some info on this. thank you.
j.l.p.q
i believe that it is a factor that corresponds to the overall period of the structure in that a more flexible (palm tree) structure will behave in a different fashion than a less flexible (oak tree) system.
with the flexible system, under a particular seismic event, the response of the structure could be visualized more as a whipping motion where lateral forces would be more significant in the higher stories.
so, the k factor accentuates the effect of the height of the structure with larger values of k for longer period structures.
this simply places more of the "applied" forces higher in the structure.
as to where it comes from - i would think it simply is the result of analysis studies or tests.  the commentary doesn't even deal with it.