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Which of these will lengthen the natural period of an existing building?

  • A.Adding shear walls in both directions
  • B.Adding diagonal bracing to the frames
  • C.Reducing the height of the storeys
  • D.Adding mass at the upper floors

Answer: D

Correct answer: Adding mass at the upper floors

Period rises with the square root of mass, so adding mass lengthens it - and adding it high in the building is most effective, since the upper floors have the largest modal displacement.

The other three all add stiffness or reduce height, which shortens the period. Note that the change is rarely worth making deliberately: extra mass attracts extra seismic force in proportion, so the lengthened period is bought at a price the spectrum may not repay.

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