A circuit breaker fixed to a spread footing is modeled by a mass-rigid body system. The breaker is represented by a mass and the footing by a rigid body. The mass and the rigid body is connected by a spring and damper. The rigid body is supported by horizontal and rotational springs and their corresponding dampers. By using the recorded input and response data earthquake, parameters for soil-structures interaction are estimated by structural identification techniques based on Gauss-Newton method.