This dissertation describes the development of a new passive two-step control damping (PTCD) device for the seismic protection of structures. The new device is a passive system that acts as a semi-active system by delivering two-step variable damping forces. It can be applied directly to structures in series with a linearly elastic brace, or as part of a seismic isolation system by providing an added fail-safe function when the displacement of the structure becomes excessively large. The PTCD device is simple, cheap and reliable, because its operation does not need the external power, sensors, computers or special algorithms that are part of a standard active or semi-active system.

Based on the operating principles of mechanical valves, a prototype PTCD device has been manufactured and tested under both sinusoidal waves and earthquake ground motions in the laboratory. Test results show that the PTCD device consistently provides the analytically predicted two-step variable damping functions. The analytical model was further simplified to allow direct calculation of the two-step force-displacement relationship to encourage use of the PTCD device in engineering practice.

Using a simplified approach, parametric studies were conducted on the seismic responses of a single-degree-of-freedom (SDOF) system with the PTCD device installed and subjected to the MCEER west coast ground motions. When compared to corresponding systems installed with conventional passive dampers, the proposed PTCD device provided superior performance for earthquakes of different intensities. This was illustrated by the significantly reduced displacement responses under earthquakes with near-fault characteristics when two-step damping is provided, and a lower level acceleration response under low and moderate earthquakes.

To illustrate the effectiveness of this device in limiting extra large isolator displacement, a base isolation system was experimentally studied. This isolation system consisted of a roller isolation bearing (RIB), and the proposed PTCD device. Shake table tests were performed on a rigid superstructure equipped with the base isolation system. Experimental results showed good agreement with the numerically predicted results. Both the analytical and experimental results illustrated that the PTCD device effectively reduced bearing displacements and provided better performance for isolated structures.