resonance

[guyguy]

hello,
long time since i came by this subject. need your help.
let's suppose i have a structure and i calculated the resonance modes of it and found that 1st mode resonance freq. is 70hz.
now, the question is:
are these frequencies: 35hz, 140hz,
can cause the structure to get into resonance (BOTH of them)or is it only frequencies higher/lower than the mode's frequency?

hope i was understood
guyguy

 

[AlohaBob]

Half or double the natural frequency would be a harmonic. It would resonate.

The 35, 70, 140 HZ sound fictitious. Natural building periods should be longer.

 

[GregLocock]

AlphaBob wrote "Half or double the natural frequency would be a harmonic. It would resonate."

Not in a typical linear structure. Only very carefully designed or very simple systems have harmonics that are exact integer multiples of the fundamental.

In a linear structure the system can only respond at the frequencies that are applied to it, that is, if you apply a pure 35 Hz signal, it can only respond at 35 Hz.

In a non-linear structure it is possible to excite resonances away from the forcing frequency. It may be that your non-linearity generates frequencies below the forcing frequency (eg a diode), so in theory you can excite all the resonances in the system with just one input frequency.



Cheers

Greg Locock