# Experimental Investigation of a Device to Restrain the Horizontal Sliding of U-FREIs
## 4 Investigated Constraint System  
In the case of U-FREIs, transfer of the seismic horizontal force from the superstructure to the sub-structure occurs thanks to the friction present at the isolator contact surfaces. Since U-FREIs undergo roll-over deformation, the contact areas decrease with increases in the superstructure displacement (Figure 2b). Moreover, the axial load acting on the isolator varies during earthquake events, depending on the position of the isolator in the plane and on the direction of the seismic action. The friction coefficient, which depends on the materials of the contact surfaces, and the axial load determine the maximum horizontal force that can be transferred by the isolator. When this force is reached, the isolator shear strain does not increase anymore, and the isolator starts to slide with respect to the contact surfaces. Consequently, the transferred horizontal force decreases to smaller values. To prevent the isolator from sliding and to take account of the guidelines of UNI EN 1337-3 [34], a constraint system at least against horizontal forces is required. Considering the peculiar behavior of U-FREIs, the constraint system should allow the upper and lower surfaces of the isolator to detach from the supports and the lateral surfaces to tilt and lean on them in a similar way to what happens in the absence of constraints. This to avoid the occurrence of additional stresses in the isolator and to maintain decreases in the horizontal stiffness with increases in the isolator deformation. Moreover, the constraint system should not preclude the return of the isolator to its initial configuration at the end of the seismic event. Finally, the device used to realize the constraint system should withstand the seismic actions for which the isolator is designed and allow their transfer to the contact surfaces. Some research works already exist on Partially Bonded FREIs (PB-FREIs) [35; 36; 37; 38; 39], and bonded [40] or restrained FREIs [41]. In a PB-FREI, two thick steel mounting plates were bonded to portions of the outer surfaces at the top and bottom of the isolator. The mounting plates were then bolted to the top and bottom contact supports of the isolator. Thus, the vertical lift and horizontal sliding of the PB-FREIs were restrained. In this research work, a different system, restraining only the horizontal sliding of FREIs, is investigated. The constraint to up-lift is not considered herein, because this event is limited generally to isolated tall buildings [42], isolated buildings with vertical setbacks [43], and short piers of isolated bridges with piers of different lengths [44]. In low/medium-rise isolated buildings, this phenomenon usually does not occur or can occur only for a very short time in the accelerogram time history. Moreover, this solution, in comparison with the partial bonding, produces negligible detachment traction between elastomeric layers and fibers. The main criterion used to select the constraint device was to choose a device which is able to maintain these advantages, restraining at the same time the horizontal sliding. In this research work, attention has been concentrated on reclosable hook and loop fastening systems, similar to the Velcro(r) system, as possible devices for the constraint system. Velcro(r) devices (Headquarters, UK) consist of two elements covered with protrusions that interact and fit together with those of the opposite surface, forming a union that can be assembled and disassembled multiple times. Regarding the mechanical characteristics of this system, results from load tests in different material configurations are reported in [45]. It is shown that Velcro's strength does not undergo significant decreases after repeated use. Based on this observation, a device that fulfills the operating principle of hook and loop fastening but with superior strength to Velcro, suitable for applications in the construction sector, was, therefore, sought.