# Experimental Investigation of a Device to Restrain the Horizontal Sliding of U-FREIs
## 9 Conclusions  
In this research work, a possible FREI constraint system made out of metal strips with duck head hooks was investigated. The strips are bonded to the sub- and superstructures and the hooks penetrate the external elastomer layers of the isolator in such a way that it is constrained against the horizontal sliding. Experimental tests were performed, and the following conclusions can be drawn. For both the specimens with quadri-directional fabrics (Q) and bi-directional ones (B), the presence of one constraint had a negligible influence on the vertical stiffness; indeed, the \(K_{V}\) percentage variations were lower than 3%. Regarding the ability of the proposed constraint system to prevent the isolator from sliding horizontally, the presence of one constraint improved the bond behavior of the Q isolator's moving bond failure to lower compression stresses (from 0.375 MPa to 0.25 MPa). For isolator B, the compression stress at which bond failure occurs remained the same as that without constraint. However, an improvement was observed in the maximum horizontal force exerted by the isolator before bond failure, which increased by 13%, and in the corresponding displacement, which increased by15%. The improvement given by two constraints was a little lower, due to damages that occurred during the test with one constraint. In the bond test under low compression stress, the hooks with less flexural strength in the direction parallel to the load application bent and the isolator slid. The hooks with greater flexural strength orthogonal to the loading direction did not bend; hence, the isolator that slid was superficially cut by these hooks. After this damage, for compression stresses greater than that producing bond failure, the B isolators' behavior with two constraints under shear was similar to that obtained with one constraint. For both specimens Q and B, the introduction of one constraint did not substantially affect the secant stiffness measured under the quasi-static cyclic tests. Indeed, the secant stiffness variations were lower than 5% for the B specimens and 9% for the Q specimens. For the B specimens, the introduction of two constraints produced variations in the stiffness lower than 6%. In the case of the dynamic cyclic tests, for both specimens Q and B, the introduction of the constraint system did not substantially affect the horizontal stiffness, whose variations were lower than 5%. Regarding the damping ratio measured under the dynamic cyclic tests, the variation in the presence of one constraint was negligible for the Q specimens, while it was lower than 10% in the presence of both one and two constraints for the B specimens. In conclusion, the advantage of the proposed constraint system is the improvement in the bond behavior under low compression stresses, even if this improvement is limited by the shape of the hooks of the constraint device. Moreover, for undamaged isolators, the constraint system does not substantially modify the isolator's behavior, neither under simple compression nor under combined compression and shear. It is underlined that the use of a commercial product (Metaklett(r)), created with completely different purposes from those pursued herein, obviously shows limitations. The present study essentially investigated the feasibility of using a constraint device of the described type and, from this point of view, represents a valid basis for a specific and more advanced design of a device based on the same principle. The current shape of the hooks themselves, equipped with protrusions, has proven to be favorable in achieving the retention objective for which the constraint itself was created. However, it is suggested that the stiffness and flexural strength of the hooks should be the same in the two principal directions, not different, as in the presented case. Moreover, the use of hooks of different lengths is suggested.