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True or false: increasing the damping of a structure shortens its natural period appreciably.

  • A.True, but only for damping ratios below five per cent of critical
  • B.False, because damping has no effect on the response at all
  • C.False, because the damped and undamped frequencies differ negligibly at practical damping levels
  • D.True, because damping removes energy from the system at every cycle

Answer: C

Correct answer: False, because the damped and undamped frequencies differ negligibly at practical damping levels

The relation is \( \omega_d = \omega_n\sqrt{1-\xi^2} \), and at 5 per cent damping the factor is 0.99875 - a difference of about an eighth of one per cent in the period.

So damping does lower the frequency, but by an amount no measurement would resolve, which is why codes use the undamped period throughout. The second option overstates the case in the other direction: damping has a large effect on the amplitude of response, particularly near resonance, and only a negligible one on the period.

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