HOW TO CALCULATE NATURAL FREQUENCY IN STAAD FOR STEEL STRUCTURES

Dear all,

I am modelling a lattice tower and need to know natural frequency and mode shapes. I know the command called MODAL CALCULATION REQUESTED to be applied before self weight.

Am i correct? If you can brief me more on this topic, it will be of great help.

Thanks

  • Hi,

    There are 2 methods in STAAD for calculating the frequencies of a structure.

    a)  The Rayleigh Method : This is a one-iteration approximate method from which a single frequency is obtained. It uses the displaced shape of the model to obtain the frequency.  Needless to say, it is extremely important that the displaced shape that the calculation is based on, resemble one of the vibration modes. That means that the loading applied in that load case much be such as to produce a deflected shape which resembles the desired mode shape. Due to the difficulty in estimating the correct mode shape, this method is not recommended.

    b)   The modal calculation method : This method is based on extracting eigen values and eigenvectors using the stiffness and mass matrices of the structure. If the stiffness and mass data are specified correctly, this is a far more reliable method than the Rayleigh method.

    To obtain the natural frequency  using the RAYLEIGH METHOD, you have to specify the command CALCULATE RAYLEIGH FREQUENCY in the load case in which the load data which produce a mode-shape type deflected shape are specified.

    To use the eigen value method, specify the command MODAL CALCULATION REQUESTED in the load case in which the load data for the mass matrix are specified. For an example which illustrates this method, please go to Help-Contents-Application Examples-American Examples-Example 28.

    Also, the Rayleigh cases should not have the MODAL CALC or dynamics in the same case.  Remember to leave these Rayleigh cases out of the Load List for design.

    Please also take a look at this Forum thread:

    communities.bentley.com/.../18180.aspx



    Answer Verified By: Gautam paul 

  • Answer is available at

    communities.bentley.com/.../3003.aspx

    See question number 1 as shown in the screenshot.



    Answer Verified By: Gautam paul 

  • I have one query regarding the modal analysis frequency. Can we use it to calculate static base shear value? If yes then corresponding to which mode? For the better explanation I will mention the steps 

    1. Get the natural frequency from STAAD 

    2. Calculate corresponding time period.

    3. Calculate design spectral acceleration for the time period using the formula from code or the acceleration plot

    4. Calculate design horizontal seismic coefficient Ah

    5. Finally Vb = Ah*W

    Kindly correct me if anything wrong.

  • So when we use model calculation requested , do we need to give the self- weight command to check the self weight of whole structure?