This paper presents numerical and experimental studies on semi-active seismic response control of structures equipped with variable stiffness and damping structural joints. Such adaptive joints, which are comprised of a shape memory polymer (SMP) core reinforced by an SMP-aramid composite skin, function as load-transfer components as well as semi-active control devices. The SMP core material can transition from a glassy to a rubbery state through thermal actuation resulting in a shift of the structural natural frequencies and a parallel increase of damping ratio, which enables a new semi-active control strategy. Control performance has been evaluated on a three-story frame equipped with 12 adaptive joints and subjected to seismic excitations. Full-transient analysis has shown that when the joints are thermally actuated to the transition temperature (65 degrees C), acceleration and base shear are reduced by up to 62% and 65%, respectively. Shake-table tests have been carried out on a 1/10-scale prototype, confirming that through thermal actuation of the adaptive joints the structural damping ratio increases from 2.6% to 11.3% and the first natural frequency shifts by up to 37%. As the structure becomes more flexible, an increase of displacements and interstory drift might occur. However, depending on the seismic excitation, top-story acceleration and base shear are significantly reduced in the range 43%-50% and 35%-51%, respectively. These results confirm that semi-active control through thermal actuation of variable stiffness and damping structural joints is effective to mitigate the structure response under seismic excitation.