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The Newmark-beta method is unconditionally stable provided

  • A.\( \beta = 0 \) and \( \gamma = 0 \) so that no averaging takes place
  • B.the time step is smaller than one hundredth of the fundamental period
  • C.the structure remains perfectly elastic throughout the analysis
  • D.\( \gamma \ge 1/2 \) and \( \beta \ge \gamma/2 \)

Answer: D

Correct answer: \( \gamma \ge 1/2 \) and \( \beta \ge \gamma/2 \)

These conditions make the scheme unconditionally stable, so the solution does not diverge however large the time step. The average acceleration method, with \( \gamma = 1/2 \) and \( \beta = 1/4 \), satisfies them exactly.

Stability is not accuracy: a stable scheme with too large a step gives a bounded but wrong answer, which is why a step of about a tenth of the shortest period of interest is still recommended. Elastic behaviour is not required, and that is precisely why the method suits inelastic analysis.

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