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Two structures have identical mass but one is twice as stiff. The stiffer one has

  • A.no natural period
  • B.the shorter natural period
  • C.the longer natural period and the greater damping ratio
  • D.the same natural period as the other structure

Answer: B

Correct answer: the shorter natural period

Period varies inversely with the square root of stiffness:

\[ T = 2\pi\sqrt{\frac{m}{k}} \qquad \frac{T_2}{T_1} = \frac{1}{\sqrt{2}} = 0.707 \]

so doubling the stiffness shortens the period by about 29 per cent. The square root matters: stiffening a structure is an inefficient way of changing its period, and on a typical design spectrum a shorter period often means a higher spectral acceleration, so the stiffer building may attract more force.

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