Response of structures to earthquakes

Unlike normal loads, seismic loads do not generate "external" forces, as they act on structures through ground movements, and these forces depend on the response of the structure itself. Coupled with the fact that earthquakes stress structures far into their plastic range, the cyclic aspect is very damaging, causing rapid and progressive degradation of the structure's resistance.

After the design phases, structural analysis is used to define the forces that the structure must withstand under seismic loading. The actual behavior of structures is complex, because it is dynamic, cyclic and highly non-linear. Detailed seismic response is beyond the reach of analytical tools, but "simplified" analysis methods are often employed.