Seismic response of civil structures utilizing semi-active MR and ER bracing systems

Magnetorheological (MR) and electrorheological (ER) dampers are a promising class of semi-active devices for the control of civil engineering structures for earthquake hazard mitigation. ER and MR dampers exhibit both viscous damping and Friction damping, where the friction damping level is controlled by an applied electric or magnetic field, respectively. These unique characteristics lend them to be very suitable devices for the semi-active control of such seismically loaded structures. Inspection of the equation of motion for such a system yields a non-dimensional variable, beta, which is the ratio of the yield force of the damper to the forcing input (the product of a characteristic mass of the building and the seismic acceleration). It is shown that maintaining beta < 1 prevents a damper lockup condition that will stiffen the structure. This stiffening may cause the structural response spectrum to coalesce with the earthquake spectrum, and worsen the seismic response. beta is also useful because it can be used to scale a damper to a structure for a desired response reduction, and it can be used as a non-dimensional control parameter. It is also shown that for lightly viscous damped structures (like most civil structures), the response reduction is approximately linear with beta. Numerical analyses are performed on a three-story model structure for different damper locations. It is shown that placing dampers near the base of the structure, as opposed to the upper levels, yields better frequency response reduction. Lastly, it is shown that passively controlling structures undergoing seismic accelerations with constant field applied to the damper greatly reduces the seismic response.