# Experimental Investigation of a Device to Restrain the Horizontal Sliding of U-FREIs
## 5 Test Specimens
### Constraint System Configuration  
The constraint system should guarantee the transfer of at least horizontal forces, but it should also allow the roll-over deformation of the fiber-reinforced isolator, i.e., the detachment of the isolator ends. The last requirement can be fulfilled through a constraint that does not involve the entire isolator contact surface. The realization of a partial constraint was, therefore, sought.  
By considering U-FREIs under roll-over deformation, the length of the isolator portion detaching from the contact surface depends on the applied displacement [39]. By neglecting the isolator vertical deformation due to the compression force, it can be assumed that the detaching length is equal to the applied displacement. Therefore, to allow the isolator to roll over in all directions of the seismic motion, the constraint device should be placed in the central portion of the isolator and should extend for a length equal to  
\[\mathrm{l}_{\mathrm{c}}=1-2d_{\mathrm{max}} \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Name** & **Elastomer** & **Fabric** & \(l\) & \(t\) & \(n\) & \(t_{e}\) & \(H\) & \(S\) & \(A\) \\ \hline  & & & **[mm]** & **[mm]** & & **[mm]** & & **[mm]** & & **[mm\({}^{2}\)]** \\ \hline B1, B2 & \(ldn\) & \(bd\) & 210 & 2.5 & 20 & 50 & 52.02 & 21 & 44,100 \\ Q1, Q2 & \(hdnr\) & \(qd\) & 240 & 2.5 & 20 & 50 & 54.03 & 24 & 57,600 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of the specimens.  
Figure 4: Photo of the isolators. Note that the names of the specimens in this paper were shortened from QA1, QA2, BA1, and BA2 to Q1, Q2, B1, and B2, respectively, for brevity.  
where \(\rm l_{c}\) is the length of the constraint device, \(\rm l\) is the isolator dimension in the considered direction, and \(\rm d_{max}\) is the isolator maximum design displacement. Depending on the isolator base shape, rectangular or circular, the constraint device will also have a rectangular or circular shape, respectively. Thus, \(\rm l_{c}\) represents the rectangle side length or the circle diameter, respectively.  
Since the specimens during the test campaign were subjected to a displacement equal to the overall elastomer height, according to Equation (1), the side length of the constraint system had to be lower than (\(\rm 210\,-\,2\,\times\,50\)) = \(\rm 110\,\,mm\) for specimens B and (\(\rm 240\,-\,2\,\times\,50\)) = \(\rm 140\,\,mm\) for specimens Q.  
Taking into account the constraint device used (Figure 3), the configuration chosen for the constraint system is shown in Figure 5a, with a side length of \(\rm 90\,\,mm\) and an empty square space in the center. This length satisfies the previous limits for both specimen types. Considering that the hooks have greater stiffness and flexural strength along the strip longitudinal direction, the arrangement of the metal strips was defined with the aim of ensuring the same stiffness and strength along the two main directions of the isolator, as it should be for the isolator in real applications. For this reason, two strips were oriented in one direction, and the other two in the orthogonal direction. The red arrows in Figure 5a indicate the direction of the hooks' greater strength. It is expected that the acting shear force will be distributed according to the hooks' horizontal stiffness; hence, the hooks having maximum strength along the loading direction will bear the greater part of it.  
A two-component thixotropic epoxy resin with a pasty consistency was chosen to bond the constraint system to the concrete blocks (Figure 5b). In the application of the resin, attention was paid to avoid the formation of cavities between the metal strips and the concrete surface and to prevent excess resin from emerging through the holes and enveloping the individual metal hooks.