Semi-active devices offer significant promise for their ability to add supplemental damping and reduce seismic structural response in an easily controllable manner, and can be used in some modes to modify or reshape hysteretic structural response. However, many current semi-active devices are highly complex, limiting robustness, while those that can generate larger forces suffer from increased response lag time to do so. Thus, an ideal semi-active device would offer high forces, low complexity, and fast response. The semi-active viscous dampers could offer all these properties and could mitigate not only the displacement response of a structure, but also the base shear. This paper first outlines the structural performance when semi-active viscous dampers, with varying control laws are applied. A spectral analysis over periods of T= 0.2-5.0 sec under 20 design level earthquakes from the medium suite of the SAC project is used to compare three device control laws individually to sculpt the structural hysteretic behaviour. Performance is assessed by evaluating reduction factors (RFs) compared to an uncontrolled structure for maximum displacement (Sd) and total base-shear (Fb), indicative of structural and foundation damage, respectively. These results show that the reduction in terms of both displacement and base-shear demand is only available with the use of the 2-4 control law, which provides damping in the second and fourth quadrants. In the second part, a method to calculate the reduction factor of response for structures using 2-4 devices with different device damping coefficients is presented. Overall, these results indicate the robustness of potentially very simple and robust semi-active viscous dampers to mitigate the risk of seismic damage to both the structure and foundation in a way that is economically suitable for either new designs or retrofit.