Earthquakes & Dynamic Amplification
An earthquake shakes the ground, but the forces a building experiences depend on its own dynamic properties. A structure acts as a filter: it amplifies ground motion at frequencies near its natural frequency and attenuates others. This phenomenon — dynamic amplification — means a building can experience forces several times greater than the ground motion itself. The response spectrum captures this amplification across all possible natural periods, providing the essential design tool for earthquake engineering.
The Response Spectrum
A design response spectrum has three distinct regions. At very short periods (stiff structures), spectral acceleration equals the PGA. In the constant-acceleration plateau (typically 0.1-0.5 s), amplification reaches 2.5× PGA. At longer periods, spectral acceleration decreases inversely with period as structures become flexible enough to partially isolate themselves from ground shaking. This simulation draws the full spectrum and highlights where your structure sits on the curve.
Damping & Energy Dissipation
All real structures have some inherent damping — energy lost through material hysteresis, friction at connections, and interaction with non-structural elements. Typical values are 2% for steel frames, 5% for reinforced concrete, and 7% for masonry. Higher damping reduces the resonance peak dramatically. Modern seismic design often adds supplemental damping devices: viscous fluid dampers, friction dampers, or tuned mass dampers that can increase effective damping to 15-20%, cutting seismic forces nearly in half.
Soil Effects & Site Classification
The journey of seismic waves from bedrock to the surface fundamentally depends on soil conditions. Soft clay deposits can amplify bedrock motion by factors of 2-3, particularly at periods matching the soil's natural period. This site amplification effect is captured by the soil factor S in design codes. The 1985 Mexico City earthquake and 2011 Christchurch earthquake both demonstrated catastrophic soil amplification. Site classification (rock, stiff soil, soft soil) is therefore one of the first steps in any seismic design.