Abstract: A semi-active variable stiffness control (SAIVS) device, which can change its stiffness continuously and smoothly between a maximum and minimum stiffness. It comprises four spring and telescoping tube elements arranged in a rhombus configuration with pivot joints at the vertices. A control rod powered by a DC servomotor reconfigures the aspect ratio of the rhombus under computer control. This aspect ratio determines the stiffness of the device, and is continuously variable using little power. The SAIVS is installed as a connection between structural elements, and varies the connection stiffness smoothly by changing its configuration in response to a control algorithm in the computer. The algorithm obtains accelerometer and displacement input as feedback to optimize the result. Thus it maintains the structure in a non-resonant vibration state under dynamic disturbances such as earthquakes and windstorms.