looking for option similiar to staad rayleigh/modal frequency

[wrxsti]

Hi im looking for option in Etabs similar to STAAD rayleigh frequency function that gives the approximate natural frequency of frame
or modal frequency

i see an option Structure Output ->Modal information ->table modal periods and frequencies

Where i see the 12 modes from the default modal cases

i tried some comparative models but etabs periods are not coming back to staad periods

 

[Agent666]

Did you define the mass source correctly to suit your situation? You can change the number of modes under where you define the modal cases.

 

[wrxsti]

i have the mass source as default and put applied loads dead and live in modal cases

 

[Trenno]

Perhaps you need to include the vertical mass too? By default, ETABS will only consider the lateral mass of the structure.

 

[wrxsti]

yes and took off lumped mass at story levels

 

[wrxsti]

i am getting closer to the value now it was just a beam alone
so i set DOF To XZ plane

thanks for the help agent666 and trenno

 

[wrxsti]

ok i tried 3d model with both
and the frequency is converging i have to unclick include lateral mass in etabs
to get corresponding frequency in staad
how do i turn this on in staad?

 

[Agent666]

You most likely only have a vertical mass applied in staad so are probably only calculating vertical modes of vibration.

I'm not familiar with staad, but however you apply the mass there should be an option to apply the mass too all 3 translational degrees of freedom. Either by entering it 3 times as the same value for x/y/z degrees of freedom or via a option to apply one value in 3 directions.

 

[wrxsti]

i will check my analysis in staad i didnt even use the mass option but just put UDL on the beam in the vertical direction

so makes sense i will check the mass option and see
i am totally new to staad

 

[wrxsti]

i read somewhere that i have to create nodes along the beam between the supports as not to "lump mass at supports"
for modal analysis

so i created a note at center of spans

i also had to do this in etabs to get similar values for modal analysis

splitting the beams lowers the length of the individual pieces which would cause issues
for lateral torsional checks?

i guess thats another thread for modelling with joints splitting beams
but how can i use this methodology and still have torsional buckling in mind