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The number of natural frequencies a discrete system possesses equals

  • A.its number of degrees of freedom
  • B.the number of storeys in the building irrespective of its modelling
  • C.three
  • D.one

Answer: A

Correct answer: its number of degrees of freedom

The eigenvalue problem \( (K - \omega^2 M)\phi = 0 \) is of the order of the model, so an \( n \)-degree system has \( n \) natural frequencies and \( n \) mode shapes.

The count is a property of the model, not of the building: the same structure idealised with one lumped mass per floor and with a full finite element mesh yields quite different numbers. A real structure is continuous and has infinitely many; discretisation is what makes the count finite.

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