This paper presents experimental testing of a new type of semi-active variable stiffness and damping control device in the form of an adaptive joint for truss and frame structures. The adaptive joint is made of a shape memory polymer (SMP) core that is reinforced by a SMP-aramid skin. Actuation through resistive heating transitions the SMP core material from a glassy to a rubbery state, which causes a significant stiffness reduction and a parallel increase of damping due to viscoelastic effects. Experimental testing on a 1325 mm x 650 mm x 650 mm three-floor frame is carried out to investigate the capabilities of this new semi-active control device to mitigate the structure dynamic response. The prototype frame is made of 30 aluminum tube elements connected through 12 adaptive joints which have been 3D printed. Stiffness and damping characteristics are controlled independently through PID control using a temperature sensor and a resistive heating wire which are embedded in the core of each joint. A free vibration test is carried out to measure the structure natural frequency and damping change from ambient (25°C) to the transition temperature (65°C). The fundamental frequency shift (reduction) is 27.4% while the structure damping ratio increases from 2.6% to 8.0%. Such a frequency shift and increase of damping allows for a significant reduction of the dynamic response under base excitation. Through thermal actuation of the joints to the transition temperature, acceleration and displacement responses under base excitation reduce by 88% and 78%, respectively.@en