Flexible vs Rigid buildings - ASCE 7

[slickdeals]

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

 

[UcfSE]

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.

 

[slickdeals]

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?

 

[UcfSE]

Yes, I believe so. See 6.5.12.3.