A new technique to evaluate the real input motion called the soil structure interaction (SSI) appeared; this new tool on structural analysis has clearly shown that a reduction of the motion occurs when a building is on a specific study site compared with free field surface questioning the fixed base design.
Now, the discussion is, if this reduction is so important or it could be neglected in structure design, although the fixed base design have been considered as an overestimation on the seismic design which assure the structure compared with SSI.
The input motion reduction has economic meaning because it means a reduction of base shear force and overturning moment acting on the structures reducing the structural elements size.
This study presents a verification of the input motion reduction on the foundation using strong ground motion records and also presents how the input motion reduction changes the building response compared with that considered in the fixed base design. The effect of the foundation depth was also evaluated by using a numerical method which considers the frequency domain soil response on a massless system. The reduction of the motion was observed and it becomes more important as the foundation depth increases.