In this paper, a simple evaluation method of soil stiffness is presented based on the Ra – Rd curve (which reflects the relationship between the rotation angle and the rotation moment of the foundation). Earthquake response records were used to obtain Ra – Rd curves. This study analyzed the Ra – Rd curves of the researched Steel-Reinforced Concrete (SRC) building (with an embedment spread foundation) for seven strong earthquakes that occurred in Japan between 1998 and 2012. And the real measurements of rocking stiffness of the soil were calculated using maximum peak response points of the Ra – Rd curves. The results demonstrated that for the researched SRC building under the condition of strong earthquakes, the calculation method of response and limit strength (published by the Architecture Institute of Japan) underestimated the rocking stiffness of the soil; and the method using vertical ground reaction force coefficient kv estimated in the JARA (Japan Road Association) standard cannot reflect the real measurements of the rocking stiffness of the soil.
