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How to do a modal analysis

  1. Enter the geometry of the structure as you would normally do.
    If your model contains multiple independent structures, make one Diamonds-file for each structure.
    Pay extra attention to the boundary conditions. If you have a lot a of degrees of freedom, you’ll end op with a lot of local modes.
  2. The modal parameters of the structure (eigen modes and -frequencies) depend on the stiffness and mass distribution of the structure.
    • The stiffness is known by Diamonds. It is in the geometry of the structure.
    • The mass distibution is related to the vertical loads (point loads, line loads, surface loads) (0,981kN -> 100kg).
      The correlation coefficient φ multiplied by the combination factor Ψ2 (for Eurocode and Eurocode-like standards) will determine the masses.
      For example: a life load of 5kN/m will lead to 5kN/m²  x  Ψ x φ = 5kN/m  x  0,3  x 1 = 1,5kN/m of mass to be taken into account during the model analysis.
  3. Run the modal analysis using the button .
    • Enter the number of desired eigenmodes. Start with for example 10.
    • Choose an approximation for the damping. Usually Rayleigh damping is used with the default damping factor of 0,05.
  4. When the modal analysis is finished, the eigenmodes and eigenfrequencies can be viewed in the results configuration.
    An overview of the modal parameters belonging to each eigenfrequency can be viewed in this table .

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