resonance

huangyhg

resonance
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
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.
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