## 3 Experimental Investigation
### Shaking Table Dynamic Tests  
In order to assess the performance of the damper against realistic earthquake-type dynamic loading, six SS-TTDs were installed in a reinforced concrete (RC) structure and tested with the shaking table of the Laboratory of Structural Dynamics of the University of Granada, Spain. The RC structure represents, at 2/5 scale, a portion of a prototype building structure consisting of RC waffle-flat plates supported on isolated RC columns. The prototype structure is shown in Figure 7. The portion of the structure that was selected and built in the laboratory is enclosed with dashed lines. The circles in Figure 7 indicate the sections (located at mid span of the plate and at mid height of the columns) where the bending moment is assumed to be zero under horizontal loads. The prototype structure was designed to sustain gravity loads only, according to the Spanish construction codes [15; 16]. Figure 8 shows the portion of the prototype RC structure with the SS-TTDs mounted on the shaking table. As seen in Figure 8, three SS-TTDs were installed in each storey as diagonal bars. The three identical SS-TTDs shown in Figure 4 were installed in the upper storey, and will be referred to as SS-TTD4, SS-TTD5 and SS-TTD6. Reactive steel blocks were located at the top columns and on the waffle-flat plate to reproduce the mass of the upper part of the structure and to satisfy similitude laws. Figure 9 shows the overall experimental set-up. A detailed description of a similar reinforced structure tested by the authors in previous studies and with a different type of damper can be found in [17]. The seismic tests were performed with a bidirectional MTS \(3\times 3\) m\({}^{2}\) shaking table. The setup of instrumentation included uniaxial accelerometers, displacement transducers (LVDTs) and more than 400 strain gauges set in the whole structure. On the SS-TTDs, a pair of LVDTs for each damper measured the mean values of the relative axial displacements between the tubes. Axial forces were obtained as the average value from six strain gauges fixed to the auxiliary square hollow sections (#80.4 mm) connecting the SS-TTD to the main structure. Figure 8 shows the test specimen equipped with the SS-TTDs and the setup and instrumentation utilized. The RC structure with the SS-TTDs was subjected to eight seismic simulations. In all simulations the two horizontal components, north-south (NS) and east-west (EW), of ground motion acceleration recorded at Bar-Skuptsina Opstine during the Montenegro earthquake (1979) were simultaneously applied, scaled in amplitude by a factor that was successively increased from 15% of the original record to 190%. The NS component was applied in the direction of the axis of symmetry of the test specimen (referred to as direction X), and the EW component in the orthogonal direction (referred to as direction Y). Figure 10 shows the histories of acceleration applied to the shaking table in the X and Y horizontal directions. Figure 11 shows the movements of the reactive steel blocks located at the top of the columns of the upper storey; more precisely, the relative displacements in the X and Y horizontal directions with respect to the RC waffle-flat plate, and the rotations about the vertical axis passing through the centre of mass. It can be seen that the relative displacements in the Y direction (Figure 11b) are about two times larger than in the X direction (Figure 11a). The corresponding maximum inter storey drifts (i.e., the relative displacement divided by the height of the columns) were about 1% in the X direction and 2% in the Y direction. These values are below the limit of 2.5% associated with the performance level of life safety, and far below the 4% associated with collapse in previous studies on RC waffle-flat plate systems [17].