Seismic response of semi-active friction-damped reinforced concrete structures with self-variable stiffness

The influence of structural self-variable stiffness and semi-active friction dampers on the behavior of reinforced concrete (RC) buildings during strong earthquakes is discussed. A fully braced six-story beamless RC frame is analyzed. The effect of concrete braces (with only constructive reinforcement) as a self-variable mechanism is studied. It is shown that up to a certain limit the frame itself controls its behavior by adapting its dynamic characteristics in the real time of the earthquake. This self-adaptation is achieved by autonomous disengagement of the braces under tension and their further nonlinear action under compression. The system has several levels of seismic adaptation, and it selects one of them for enhanced response to the given earthquake. However, when the limit is reached, further self-adaptation of the frame becomes impossible. The occurrence of an earthquake of higher magnitude can then lead to disengagement of the concrete braces under compression, intensifying structural damage and even causing collapse. The use of semi-active controlled friction dampers is proposed as a means of preventing the collapse of braces under compression, thereby enabling structures to withstand earthquakes. The forces in the friction dampers are regulated according to an optimal control algorithm. Modulation of the friction level in real time during the earthquake yields additional improvement of structural seismic behavior and obviates the need for retrofitting.