Hello guys.
I have an question. when we dealing with dynamic analysis, why we set cut off frequency as 33 HZ ?

The assumption is that the ground is unable to transmit enough energy above 33 Hz to cause significant inertial loading to piping systems. I have heard that some wish to extend that up to 40 or 50 Hz.
But all I mention here, so far, is seismic evaluation. The limit of 33 Hz may not be appropriate for other, "faster" transient events such as sudden valve closure.