Structural Dynamics

What am I seeing?

• Natural frequency depends only on mass and stiffness: ωn = √(k/m). Damping barely changes it.
• Damping does not change the shape of the motion, only how fast the envelope decays.
• A building with n floors has n natural frequencies and n mode shapes.
• In the first mode every floor moves the same way; higher modes contain sign reversals.
• Press Play and watch the free vibration decay, or switch to a shear building to see the modes.