# Spring beam - MDOF vibration simulation

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Hello,

I am developing a Vibration set that simulates a free 4 degree of freedom system using beams as the spring element. I am an inexperienced user and am trying to make a dynamic simulation of the system. This simulation should give me the position of each mass by the time the set vibrates. It would be nice to have some other data as natural frequencies and mode shapes as well.

I was trying at first to use flexbodies to simulate the spring beams, but I couldn't figure out how to have initial displacement as the input of the system. With the calculated force to move it to the initial displacement position, I couldn't use the force only for one second an than having a vibration response.

What do you suggest as a method for me to simulate these? Should I try it with NLFE?

I appreciate any help given!

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HI,

We can use flex bodies or beam bodies, springs and we can define initial conditions too in the model and run the static + Linear analysis to get the natural frequencies and static simulation will give us displacements of the bodies due to initial conditions.

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Thank you very much! I made a lot of progress with the tips you gave me!

I made the set with flexbodies and ran the static+linear analysis. With that I was able to find the modes and natural frequencies of the system. However, it didn't match the ones I found in the algebric analysis in Matlab. Most of modes in the Static+linear analysis presented some sort of torsion or buckling so the modes and natural frequencies presented in the analysis were a lot different from what I expected. No matter wich joints I used, trying to prevent the torsion and buckling from happening, the frequencies would only assume higher values.

Later on, I tried to build the set using NLFE beams, but with this method, the static+linear analysis would deliver an error presenting only the transient mode and not the other vibration modes.

Is there a way to get the modes and natural frequencies of a simulation with NLFE?

Is there something I could do to make the flexbodies simulation to work?

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Could you please share results from analysis and theory?

Please use the file transfer link in my signature to share the model file

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HI,

1. Should put a force on one end and run a static plus linear. This will make the model Fixed+Forced condition and the linear analysis should give him the frequencies with model in this condition.
2. Cannot use NLFE for this as as of now Linear analysis doesn’t not support NLFE elements and hence the error which you see.

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