Time history analysis on a 2D frame problems

[Anonymous]

Hi I´m trying to do a time history analysis on a 2D frame. I´ve created the a random excitation in MatLab using the rand() function and saved it on a thf. file with time on the first column and force on the second (see attachment).

When i try to open it on robot it crashes and i have to close the program. What did i do wrong?

The second problem I’m having is with the frame that I’m studying (attachment figure 1). For some reason when running a modal analysis I’m not getting the natural frequencies that I should be getting. Any idea why?

Thanks a lot. 

[Rafal.Gaweda]

Replace commas by dots in thf file

Last value should be set at 600 

[Anonymous]

Hi and can you please check the model I´ve made 🙂 the frequencies should be 2.7 Hz for the first mode and 7.0Hz for the second. But robot is giving me different numbers.

Thanks a lot

Edit. i forgot to mention I calculated the natural frequencies with:

det(K-w^2*M)=0

being:

K- stiffness matrix 

w- natural frequencies

M- mass matrix 

[Pawel.Pulak]

First two vibration frequencies calculated for your original model by Robot (disregard density active, considering only horizontal masses resulting from conversion of UDL load) are f1= 1.17 Hz and f2= 3.10 Hz.

In you last post you have mentioned that you have calculated the frequencies yourself (f1= 2.7 Hz and f2= 7.0 Hz) from:

  det(K-w^2*M)=0

  being:

  K- stiffness matrix 

  w- natural frequencies

  M- mass matrix

But is the above formula w is not a natural frequency (in Hz) but pulsation (in radian/s)

Even considering this mistake and converting from pulsation to frequency your results (f1= 0.43 Hz and f2= 7.0 Hz)  are not coherent with Robot.

But what was the model calculated by you - have you really "manually" calculated the model with 4 dynamic degrees of freedom as the model in Robot?

I have calculated manually the simplified model with 2 dynamic degrees of freedom  (vertical cantilever with 2 masses - in the middle and on the end) and I have received the following frequencies:

f1=1.18 Hz and f2= 3.09

So they are very close to results of Robot.

Please double-check your manual calculations - especially calculation of stiffness and considered masses.

Regards,

[Anonymous]

Hi thanks for the reply. On the manual calculations I´ve only considered 2 dynamic degrees of freedom, how do I tell robot that only want the horizontal direction of each floor of the frame as degrees of freedom?

Edit: and the values 2.7 Hz and 7.0 Hz are already after converting pulsation to frequencies 🙂

Plus the beam has infinite stiffness so there are no rotations and deformations of the materials should not be considered. How do i implement that on Robot?

Thanks a lot for all the help 🙂

[Pawel.Pulak]

It seems that you have made some mistake in your manual calculations:) Attached are mine for your reference.

As concerns the infinite stiffness of beams it can be modeled defining rigid links overlapping these beams. Defining them results in almost no change of the frequency of vibrations - as could be predicted looking at shapes of vibration modes in your original model.

Regards,

[Anonymous]

ok thanks a lot for all the help. You were great. I´ll review my manual calculations then 🙂