What is Significance of 33Hz in Seismic?

[m_ridzon]

***Repost from my earlier thread in Mechanical Engineering. My work actually relates to mechanical engineering, but it didn't occur to me that "structural" would be the more appropriate subforum for this question. My apologies.***

Folks, in many seismic discussions I have had over the years, I have frequently heard reference to 33Hz. In terms of seismic, what is the significance of that frequency? Is there some geological reason that seismic vibrations around/above/below that frequency may not matter? Is that a magical seismic frequency of some sorts? Why do I often hear reference to that frequency?

By the way, I'm asking in terms of the continental USA.

 

[canwesteng]

That is a much higher frequency than we consider or earthquakes and a much higher frequency than the important natural frequencies for structures.

 

[IRstuff]

Please refrain from double posting

 

[driftLimiter]

Take a look into IEEE 693. structures with 33hz or greater natural frequencies are effectively rigid.

 

[JoshPlumSE]

Yeah, if you look at the Seismic codes, you'll see that there are different frequency ranges that determine how the building responds. If the natural frequency is above 33Hz (or a period below 0.03 second) , it is generally considered to have a rigid response.

It's sometimes a little disguised in the code, because the code equations often start with a design force / accelerations that is a little higher... Associated with the acceleration controlled portion of the minimum / code spectra rather than a true rigid response. In the US, we call this the transition range.