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The equation of motion is written about the static equilibrium position so that

  • A.damping may be neglected without loss of accuracy
  • B.the stiffness becomes independent of the displacement
  • C.the self weight and its initial deflection drop out of the equation
  • D.the mass of the structure need not be considered at all

Answer: C

Correct answer: the self weight and its initial deflection drop out of the equation

Measuring displacement from the position the structure already occupies under its own weight makes the weight and the corresponding spring force cancel exactly, since they balance in that position.

What remains is the equation in terms of the dynamic displacement alone, and the response computed is the movement about the static position - which is what matters for stresses caused by the vibration. Damping, stiffness and mass all remain in the equation unchanged.

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