What is the primary effect of Soil-Structure Interaction (SSI) on the natural frequency of a structure?

The primary effect of Soil-Structure Interaction (SSI) on the natural frequency of a structure is to *decreasethe natural frequency compared to the fixed-base condition (where the structure is assumed to be rigidly supported by the ground). SSI refers to the interaction between the structure, its foundation, and the surrounding soil during an earthquake. When SSI is considered, the soil's flexibility is accounted for, which effectively increases the overall flexibility of the system. This increased flexibility results in a lengthening of the structure's fundamental period (the inverse of natural frequency), thereby *reducingthe natural frequency. The extent of the reduction in natural frequency depends on the relative stiffness of the soil and the structure. Softer soils lead to a greater reduction in natural frequency, while stiffer soils have a smaller effect. This change in natural frequency can significantly affect the structure's response to earthquake ground motions, as it can shift the structure's response closer to or further away from the dominant frequencies of the earthquake.