Abstract: In order to meet the demands of simplicity and reliability in active control systems for flexible structures, an inexpensive active truss element and control law has been developed. A decentralized switching control law is used along with a compressible fluid in the truss element in order to dissipate energy during the motion of the structure. However, the energy is not absorbed in the same manner as a conventional viscous damper. The truss element retains its maximum stiffness, but has a resettable nominal unstressed length. Energy is absorbed in the working fluid of the truss element through heat transfer to the environment when the nominal length is reset at the proper switching times. The control law is insensitive to changes in structural parameters such as mass, stiffness, and damping.