Hello,
I have a data of an experimental vibration analysis for a structure, this data are accelerations of the system in a 1/3 Octave band, so i want to estimate forces magnitudes generated by the vibrations of this system, using F= ma this should be easy, however i don't know which method of this 3 should i use:
1-using an average of the accelerations in all the sampled frequencies (1/3 octave band 0-200hz), knowing that i risk an amplification of this force if i include resonance frequency of the system.
2-using an average of the accelerations which coresponds to the 4 or 5 first frequencies to get as far as possible from resonance.
3-summing all the accelerations mesured at all the sampled frequecies (0-200Hz 1/3 octave band ).
thanks to all.
Dear Houcine,
I assumed that you has vibration data, and you want to estimate the modal parameter of the structure but no idea about the source (input). To do this, you may try frequency domain decomposition technique. Please see Smart Mater. Struct. 10 (2001) 441–445 for details. The other way is inversion technique after Huang (JSV (2001) 248(5) pp 789-807). In case you have the data set in different measurement position than you may combine it. For example, please see Journal of Biomechanics Vol 24 Issue 12 (1991) pp 1173-1187.
At least two forces are associated with the vibration - Restoring or spring (proportional to displacement), Inertia (proportional to acceleration). Which force you mean?
dear Iwan Yahya and Chandrashekhar Dharankar
i think that my problem is'nt clear, i have the response of the system in acceleration in a sampled 0 to 200 Hz frequency domain, i want to calculate the forces explicitly from this accelerations , i know the mass of the system, so i thaught i can use F=ma, but my preblem in which frequency i should take the acceleration, or with which method from above should i use.
there's a link of the data table.
best regards
Hello Tarif,
Your solution works good if the structure behaves as a rigid mass, that means that all the parts of your system move in the same way. In that case you can use the mass of your structure to calculate the forces (F=ma). But very few structures behave as a solid rigid mass, especially at higher frequencies, where the mass 'falls apart' in little masses connected with springs. You can see this in your data table, where the x, y and z components of the acceleration don't change evenly with frequency. They should if your structure were a rigid mass.
What you need is the impedance of the structure (F/v). This can be measured with an impedance hammer, which measures the force and the vibration response at the same time.
Best regards,
Ron Houtenbos
The complete equation for the procedure that you are proposing is:
Mx''+Cx'+Kx=F
not only
F=Mx''
anyway, it would be not easy obtain the force on this way. An experimental procedure will be more adequate
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