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Bridge and own frequency
Hello,
I introduce myself, I'm a student and I currently conducting a study on the stability and robustness of the bridges. In this study I have to determine the behavior of a bridge subjected to a sinusoidal input. For this I wanted to use the software on which robot, I made a plank supported by two bearings and I would now determine its natural frequencies. but how?
Thanking you for your help,
Sincerely,
Re: Bridge and own frequency
Natural frequencies can be found defining dynamic modal analysis - see the attached screen capture. The table with appropriate results will be available in for instance Results>Advanced>Modal Analysis.
By default only the selfweight of the structure will be taken into account. Additional masses can be defined in the way analogous to loads or converted from loads ("Load to Mass Conversion" tab in the attached screen capture).
The steady-state harmonic response to sinusoidal (harmonic) input can be found using harmonic analysis (for single frequency of excitation) or FRF harmonic analysis in the frequency domain (for the range of frequencies). These types of analysis are available after defining modal analysis. Forces for these types of analysis are defined analogously to static cases - defined values correspond to amplitudes. Results are also analogous to static ones - they are interpreted as amplitudes too. In case of FRF analysis additional ways of output are available in Results>Advanced - like diagrams in frequency domain.
dynamic.png
I hope these explanations will be helpful.
Re: Bridge and own frequency
Hello,
Thank's you very much for your answer.
Sincerely,
Pierre
Re: Bridge and own frequency
Hello,
Excuse me, but I have an other question, indeed i can now calculate own frequency, nevertheless, the software give me only two mods. That's why i have to increase number of nodes on the bridge to have other node. Nevertheless, i don't understand why , end what increase node change ?.
Can you help me ?
Thank you
Re: Bridge and own frequency
The maximum number of modes, which can be calculated, is equal to the number of dynamic degrees of freedom of the model. By default (if "Disregard density" is not active) the number of dynamic degrees of freedom is equal to the number of static degrees of freedom.Originally Posted by pelboat354433
Your description suggests that your model consists of one simply supported bar and the available degrees of freedom are only the end rotations.
It is recommended to divide this bar in smaller parts - especially that opposite to static linear analysis the precision of dynamic and static nonlinear analysis depends on the number of divisions of bars.
Regards,
Re: Bridge and own frequency
Hello,
I don't present my experience, so i'm giving you a photo of this experience ( there is juste a bar support by two pliers and animate by a vibrator ( only movement up and down ) on wich i can adjust the frequeny. I will try to cut my bar in many other little bar, but in how many ? to have satisfactory result to your mind ? ( my bar measure one meter)
dispositif expérimental bis.jpg
Thank you very much
Re: Bridge and own frequency
Division in 10 should give satisfactory precision. Of course you can try diferent numbers of division and observe the convergence of frequencies for analogous vibration modes.
Re: Bridge and own frequency
Hello,
I try to cut my bar in 10 part, i have results and want your point of view( I'm giving you the result with a picture ). But i have a question , indeed, the more i'm impose nodes et the more frequency looks to be important, is normal ?
Sans titre.png
Thank you very much
Re: Bridge and own frequency
Yes, it is normal.
Increase of the number of divisions results in:
1/ convergence of frequencies of appropriate vibration modes (for instance the 1st one) to the precise analytical values
2/ increase of the number of available vibration modes - their shapes can be displayed using Diagrams/Deformation and selecting appropriate mode
Last edited by pawel.pulak; 2013-03-07 at 07:13 AM.
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