flexible vs rigid buildings - asce 7

huangyhg

flexible vs rigid buildings - asce 7
greetings,
per chapter 6 of asce 7 "slender buildings or other structures are flexible if their natural frequency is less than 1 hz".
in the commentary, it says when buildings have height greater than 4 times least horizontal dimension, then natural frequency should be investigated. it also says that gust energy is smaller at frequencies aobove 1 hz, and hence dynamic response may not be significant if frequency is greater than 1 hz.
suppose i have a building that is 100' tall and 50' x 50' in plan (h < 4 * horiz. dim). assuming the calculated natural frequency is less than 1 hz, does this mean that the building will have to be analyzed as a flexible building with modified gust factors (6.5.8.2) and eccentricities per 6.5.12.3?
when you design buildings, how do you classify them as slender? using h < 4 * horiz. dim. or using the natural frequency.
i must tell you that 6.5.12.3 is a b*tch. hope you can shed some light.
~anantha
the commentary provides general guidance.  the requirement is per the natural frequency.  
a structure with h < 4 * horizontal dimension can still have a natural frequency less than 1 hz depending on the lfrs.
i agree, but i was having an discussion at work and my colleague seemed to disagree with me.
if fundamental frequency is less than 1 hz, i.e, period greater than 1 second, do you calculate your eccentricities for a flexible building and apply them instead of 0.15 as in figure 6-9?
yes, i believe so.  see 6.5.12.3.