There are two ways to determine seismic loads:
to be fixed at the base
to consider the flexibility of the foundation (Soil - Structure interaction)
It is determined in accordance with Section 12.13.3 or Chapter 19.
Structural systems are formed from three subsystems: framing, foundation and supporting soil. The ground motion depends on the complex interaction among these subsystems. The dynamic response of the structure depends on the foundation system and on the characteristics of the soil that support the system. Soil–structure interaction (SSI) is meant the dependence of the response on the structure–foundation–soil system.
If the foundation is considered to be rigid, the computed base shears are usually conservative, which is why rigid foundation analysis is permitted. The user should choose whether neglect soil– structure interaction in accordance with Section 12.13.3 or implicitly in accordance with Chapter 19.