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

Conventional seismic design methods are based on the ductility of the structural elements taking into account their inelastic deformations [1]. Passive friction dampers are well known solutions for improving structural response to earthquakes. The friction force magnitude in these 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 a variable friction force were studied by Ribakov et al. [2]. It was demonstrated that application of such dampers yields significant reductions in structural response to strong earthquakes.

A new passive variable friction damper is proposed in this paper. Its principal distinction from other existing ones is using a wedge-form part, aimed to change the friction force as a function of the displacement transferred to the damper. An additional advantage of the proposed damper is a possibility to design it so, that the centring capacity would be provided.

The proposed damper, shown in Figure 209.1 consists of a square section tube (1), a wedge (2), two elastic strip elements (3) and a bolted connection clip (4). The wedge is located partially inside the tube and can move back and forward along its axis. The strips 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 behavior were studied theoretically and verified experimentally using a small-scale model and a laboratory shaking table. A static scheme was assumed, which enables to obtain the force-displacement relationship of the damper. It was shown that the relationship depends on the flexibility coefficients of the strip elements, which were calculated and also obtained experimentally.

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 behavior 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

I. Iskhakov, "Seismic energy dissipation and ductility of RC elements section", Fifth national conference on earthquake engineering, paper No. AE-025, Istanbul, May 26-30, 2003.

2

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.

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