Variable Friction Dampers (VFD) for a modulated mitigation of the seismic response of framed structures: Characteristics and design criteria

In this paper a new approach for the energy dissipation is discussed based on the use of Variable Friction Dampers (VFDs). The VFD device is borrowed from braking systems mechanical engineering, able to modulate its capability of dissipation, providing a constant pure friction force coupled with an additional (variable) damping force with the increasing of the displacement. Resorting to a previous study, the characteristics and the efficiency of the “braking” system in terms of reduction of the displacements and the restoring forces is described for SDOF systems under seismic excitations and compared with the performances achievable by classic constant friction dampers (CFDs). Then, a design criterium based on structure stiffness, viscous damping and seismic demand is proposed evaluating its effectiveness through a statistical assessment of the dynamic response of SDOF models subjected to natural accelerograms. The results obtained by the proposed design approach have proven that is a robust approach and indicate that the device can be really considered suitable for the improvement of the energy dissipation and the reduction of the drift in framed structures in an extended range of the seismic demands (from low to high) differently from the classic friction devices whose characteristics make them effective in a smaller range of the seismic intensities.