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Queensferry

Module 18

Supplemental damping devices

Manufactured viscous dampers: velocity-dependent force, the velocity exponent and what it is for, brace geometry and the cos²θ penalty, damper placement, sizing and the analysis they require.

What this module covers

  • State the damper force law and explain why the force peaks where the displacement is zero
  • Explain what the velocity exponent does and why α < 1 is chosen
  • Apply the cos²θ brace-geometry penalty to damper effectiveness
  • Compute the damping ratio a linear damper adds to a mode
  • Place dampers using the modal drift across each storey
  • Say what analysis a damped structure needs and why a spectrum will not do

Lessons

  1. 18.1

    How a viscous damper works

    Force from velocity, the phase shift that makes it almost free, and the exponent that keeps it from becoming expensive.

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  2. 18.2

    Sizing and placing dampers

    Where a damper earns its keep, how much damping it actually adds, and what analysis a damped structure needs.

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Module checkpoint

Check what you have taken in

4 questions

  1. Question 1

    A damper with C = 1 200 kN·(s/m)0.5 and α = 0.5 is driven at 0.50 m/s. What force does it deliver, in kN?

  2. Question 2

    What fraction of a horizontal damper's effectiveness is retained by one braced at 45°?

  3. Question 3

    A linear damper with C = 1.5 MN·s/m braced at 45° spans a storey with first-mode relative displacement 0.28. The mode has ω = 3.5 rad/s and modal mass 750 tonnes. What damping ratio does it add?

  4. Question 4

    Why is a response-spectrum analysis not valid for a structure with nonlinear viscous dampers?