Keywords: variable friction damper, stepped force characteristic, seismic response, base isolation system.

Using passive friction dampers is a well known solution for getting enhanced structural response to earthquakes. The friction force magnitude in commercially available dampers is usually constant. It is known that buildings with supplemental friction dampers do not always return to the initial position after the earthquake, and remain in a deformed stage. Semi-active friction dampers with variable friction force were studied by Ribakov et al. [1]. It was demonstrated that application of such dampers yields significant reductions in structural response to strong earthquakes.

Recently developed passive variable friction dampers [2] can be alternatively applied in order to get structural behaviour similar to that with an active selective controlled base isolation system. Using dampers with stepped force characteristic allows more effective displacements limitation and dissipating of larger amounts of seismic energy yielding further improvement in structural response to earthquakes. This paper presents a preliminary study focused on a variable friction damper with stepped force characteristic.

The proposed damper investigated in the previous study [2], consists of a square section tube, a wedge, two elastic strip elements and a bolted connection clip, and has additional strips yielding stepped force characteristic. The wedge is located partially inside the tube and can move ahead and back along its axis. Both strips pares have a cantilever static scheme and are fixed on the tube by the connection clip, forming an elastic strip system. The stiffness of this system may be regulated by changing the location of the connection clip along the tube. The free ends of the cantilever strips have a contact with an inclined surface of the wedge.

The damper's mechanical properties and its hysteretic behaviour were studied theoretically and verified experimentally using a small-scale model and a laboratory shaking table.

The displacements and the damping forces were measured using analogue sensors with an output to a PC through a data logger. The displacements and the forces records were received by cyclic moving of the wedge, using the shaking table. Using these records, the hysteretic behaviour of the damper was further obtained.

The results of the dynamic test are in good correlation with the proposed theoretical model. It was demonstrated that the loading and unloading lines of the damper's hysteretic loops are linear and have similar slopes as the calculated ones. Hence, the theoretical model, proposed in this study, can be successfully used for design of full-scale variable friction dampers for real structures.

1

Y. Ribakov and J. Gluck, "Active Controlled Friction Damped MDOF Structure with Variable Stiffness", Eighth Canadian Conference on Earthquake Engineering, Vancouver, Canada, 409-414, 1999.

2

Y. Ribakov, B. Blostotsky and I. Iskhakov, "Theoretical Model and Laboratory Tests of a Variable Friction Damper", European Earthquake Engineering, XX, 43-47, 2006.

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