Module 13
Earthquake ground motion and base excitation
No force is applied to the structure at all — the ground moves and the structure's own inertia does the rest. Which is why mass is a liability in an earthquake and an asset under wind.
What this module covers
- Derive the base-excitation equation and explain why üg appears as an effective force
- Distinguish relative from absolute response and say which quantity matters for what
- Explain why the equation is written in relative displacement
- Describe how amplitude, duration and frequency content each affect the response
- Explain why records are baseline-corrected and filtered, and what that costs
- Recognise the limits of a uniform base-excitation idealisation
Lessons
Where −m üg comes from, why the equation is written in relative displacement, and the distinction between two answers that are routinely interchanged.
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Module checkpoint
Check what you have taken in
4 questions
Question 1
A 12-storey building has a total seismic mass of 9 600 tonnes. At an instant the ground acceleration is 2.5 m/s². What is the total effective earthquake force, in MN?
Question 2
A structure's relative displacement peaks at 95 mm, and the record's peak ground displacement is 110 mm. What is the peak absolute displacement?
Question 3
Two records have the same peak ground acceleration. One lasts 8 s and the other 40 s. For which type of structure does the difference matter most?
Question 4
Ground motion is harmonic at 25 mm and 1 Hz. A structure has a natural frequency of 0.5 Hz and 5% damping. What is its relative displacement amplitude, in mm?