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From a logarithmic decrement of 0.288, the damping ratio is nearest to

  • A.4.6 per cent
  • B.28.8 per cent
  • C.1.8 per cent
  • D.18.1 per cent

Answer: A

Correct answer: 4.6 per cent

Invert the light-damping approximation:

\[ \xi \approx \frac{\delta}{2\pi} = \frac{0.288}{6.283} = 0.0458 = 4.6\% \]

The distractor 28.8 per cent takes the decrement itself for the ratio, forgetting the factor of \( 2\pi \). A result near 5 per cent is entirely typical of a concrete structure, which is a useful check - a decrement of 0.288 means successive peaks in the ratio \( e^{0.288} = 1.33 \), a visible but gentle decay.

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