Earthquakes, is it a shock or random vibration event? 1

[Twoballcane]

Earthquakes, is it a shock or random vibration event?

Im looking at a spec and it describes the earthquake in a SRS (Gs vs Hz) profile, but I thought it would be described in a random vibe (G^2/Hz vs Hz) because earthquakes can last longer than typical shocks (pulse, velocity,…).

Any help would be greatly appreciated.



Tobalcane
"If you avoid failure, you also avoid success."

 

[Drej]

Earthquakes are neither of those strictly speaking, as they fall under their own "seismic" type event. The g vs Hz is a response spectrum converted from a time history using single DOF springs on masses. This gives a characteristic shape of spectrum, which shock or impact spectra differ from greatly. The seismic response spectrum will have in general the greatest (most damaging) energy associated with the 5-12 Hz range, whilst the shock/impact damaging frequency will generally be a lot higher (in 100+ Hz range). Industry standard in the nuclear industry for the analysis of systems/structures is to use response spectrum for seismic and time history for shock/impact.


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[Twoballcane]

Thank you Drej for your response (no pun intended).

Tobalcane
"If you avoid failure, you also avoid success."

 

[Twoballcane]

Another question Drej, how would you run that on a shaker table? I mean would you use a G^2/Hz vs Hz profile to simulate the earthquake?

Thanks!

Tobalcane
"If you avoid failure, you also avoid success."

 

[Drej]

No, G^2/Hz vs Hz is a Power Spectral Density input (PSD), which is generally a random vibration signal. You need to obtain either the time history (e.g. acceleration/velocity/displacement vs. time) for the event or the response spectrum (e.g. force/acceleration/velocity/displacement vs. Hz) to represent the seismic input. Whether you use one or the other depends on the results you need as well as computer resources.


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[Twoballcane]

Thanks Drej!

Tobalcane
"If you avoid failure, you also avoid success."