Structural Dynamics and Earthquakes
A seed for a note about what happens when loads move: natural frequency, resonance, damping, why bridges and buildings sway, and the basic idea behind designing for earthquakes in Slovenia.
Open in the interactive Digital Garden
Statics assumes that loads stand still. Wind, traffic, machines, people jumping at a concert – and earthquakes – don't. Slovenia is an earthquake country (Ljubljana was badly damaged in 1895), so for me this is not theory but everyday work. This seed is where I'm collecting the parts of the story.
The Basic Idea
Every structure has a natural frequency – how fast it swings by itself when you push it and let go, like a guitar string or a child on a swing. For the simplest case, a mass on a spring, it's
f = 1 / (2π) · √(k / m)
where k is the stiffness and m the mass. A stiffer structure swings faster, a heavier one slower. When the load pushes at the same rhythm, the movement grows – that's resonance, and damping is what keeps it from growing forever.
What I Want to Cover
- Famous cases: the Tacoma Narrows Bridge (1940) – which was actually not simple resonance – and the London Millennium Bridge (2000), which swayed under walking people.
- What an earthquake does to a building, and why a tall building and a short one feel the same earthquake differently.
- The idea of the response spectrum in Eurocode 8, without the whole procedure.
- Why ductility – a structure that bends without breaking – matters more than pure strength.
- Example: the natural frequency of a simple frame by hand, then in OpenSeesPy.
Related: Structural Statics, How the Finite Element Method Works and my Structural Analysis resources.