# Experimental Investigation of a Device to Restrain the Horizontal Sliding of U-FREIs
## 1 Introduction  
In the last century, high-intensity earthquakes have occurred in seismically active regions around the world, evidencing the importance of designing buildings capable of coping with the forces that these events produce on their structures to avoid the loss of human lives. To attain this goal, a relatively recent and undoubtedly effective seismic risk prevention and mitigation strategy is represented by structural control. Among the passive control techniques, seismic isolation [1; 2; 3] and energy dissipation [4; 5; 6; 7; 8; 9] have found greater application in recent years. The present research concentrates on seismic isolation, which involves the interposition of devices, the isolators, between the structure and the foundation, with low horizontal stiffness and high vertical stiffness. The presence of the isolators modifies the building dynamic response to seismic events; it aims to decouple the building from ground motions. In particular, this research is focused on fiber-reinforced elastomeric isolators, which involve the use of fiber fabrics as reinforcements, like those employed in the retrofitting of existing structures [10], instead of the steel plates used for conventional steel-reinforced elastomeric isolators. Moreover, FREIs do not have steel end plates for anchorage to the sub- and superstructures, but they are simply interposed between them. For this reason, they are also called unbonded-FREIs (U-FREIs). There are several studies on the behavior of U-FREIs in the literature, and these isolators are still under development. For evidence, it is sufficient to consider the review of [PERSON] [11], which alone contains mentions of 80 publications. This is because U-FREIs are particularly promising for widespread utilization, while SREI use is limitedto specific categories of buildings of strategic importance or bridges, due to their high cost. The high cost of SREIs is connected to their production method, the thick steel plates used for anchoring to the sub- and superstructures, and their heavy weight, involving transportation and installation technical difficulties. The absence of plates for anchoring and the use of fiber fabrics, much lighter than steel reinforcements, make U-FREIs lighter than SREIs and easier to move and install [12; 13]. Moreover, U-FREIs can be produced in the shape of long strips and then cut to obtain isolators of the desired dimensions [14]. Thus, overall, U-FREIs are cheaper than SREIs and may be employed widely for buildings, as well as in poor developing countries, allowing for the protection of human lives against earthquakes all over the world [15; 16; 17]. The good isolating behavior and effectiveness of U-FREIs in isolated buildings have already been widely demonstrated by experimentation on the devices [18; 19; 20; 21], by tests on small-scale buildings isolated by U-FREIs on a shake table [22; 23], and by numerical simulations [24; 25; 26; 27; 28; 29]. In this research work, the differences in behavior between SREIs and U-FREIs are highlighted. The regulatory constraints for the application of U-FREIs in real buildings are outlined. From this analysis, it results that the main problem to be addressed for the use of U-FREIs in real structures is the necessity to restrain them against horizontal sliding. To this end, a possible constraint system is investigated. The proposed device has the objective to restrain the horizontal sliding of U-FREIs, without producing relevant modifications of their base behavior. An experimental methodology is used to investigate the method to install the device, its influence on the behavior of the U-FREIs and its effectiveness in counteracting the isolator horizontal sliding. Tests under simple compression and combined compression and shear on FREIs in both unbonded and horizontally constrained configurations are carried out. A comparison of the results obtained from these configurations is performed to evaluate the variation in the behavior of FREIs due to the constraint system.