In an Earthquake,
- Seismic Waves arise from sudden movements in a rupture zone in the earth’s crust.
- Waves of different types and velocities travel different paths before reaching a building’s site.
What happens to the buildings?
During earthquake, if the ground moves rapidly back and forth, then the foundations of the building are forced to follow these movements. While upper part of the building prefer to remain where it is because of its mass of inertia.This causes strong vibrations of the structure with resonance phenomena between the structure and the ground, and thus large internal forces. This frequently results in plastic deformation of the structure and substantial damage with local failures and, in extreme cases, collapse.
The effects of an earthquake on a building are primarily determined by the time histories of the three ground motion parameters; ground acceleration (ag), velocity (vg), and displacement (dg), with their specific frequency contents. They depend on numerous factors, such as the distance, direction, depth, and mechanism of the fault zone in the earth’s crust (epicentre), as well as, in particular, the local soil characteristics (layer thickness, shear wave velocity). In comparison with rock, softer soils are particularly prone to substantial local amplification of the seismic waves. As for the response of a building to the ground motion, it depends on important structural characteristics (eigenfrequency, type of structure, ductility, etc). Buildings must therefore be designed to cover considerable uncertainties and variations.
Apart from structural hazards due to ground shaking, extensive loss can be caused by the so-called induced hazards such as landslides, liquefaction, fire, retaining structure failures, critical lifeline failures, tsunamis and seiches.