Fiber reinforced elastomeric isolators (FREIs) comprise alternating bonded layers of elastomer and fiber reinforcement sheets. In an unbonded application the isolator is located in place without any bonding or mechanical fastening provided at its contact supports. The unbonded application results in an increased seismic isolation efficiency, however, it introduces few design limitations as it relies on friction to transfer shear loads, and no vertical tension can be taken by the isolator. These limitations may restrict the use of isolators where the superstructure overturning is of concern or in regions where large vertical ground accelerations are expected. This paper presents a systematic method for the design of structures supported on unbonded-FREIs. The design calculations conform the equivalent static force (ELF) procedure outlined in current seismic codes and account for the vertical component of earthquake in the analysis. A stability parameter has been introduced to verify both the overturning and sliding stability of the isolated system. The efficacy of the design method is illustrated by extensive response history simulations for several prototype frame structures.
Fiber Reinforced Elastomeric Bearings (FREBs) are a relatively new type of laminated bearings that can be used as seismic/vibration isolators or bridge bearings. In an unbonded (U)-FREB, the bearing is placed between the top and bottom supports with no bonding or fastening provided at its contact surfaces. Under shear loads the top and bottom faces of a U-FREB roll off the contact supports and the bearing exhibits rollover deformation. As a result of rollover deformation, the horizontal response characteristics of U-FREBs are significantly different than conventional elastomeric bearings that are employed in bonded application. Current literature lacks an efficient analytical horizontal stiffness solution for this type of bearings. This paper presents two simplified analytical models for horizontal stiffness evaluation of U-FREBs. Both models assume that the resistance to shear loads is only provided by an effective region of the bearing that sustains significant shear strains. The presented models are different in the way they relate this effective region to the horizontal bearing displacements. In comparison with experimental results and finite element analyses, the analytical models that are presented in this paper are found to be sufficiently accurate to be used in the preliminary design of U-FREBs.
The conventional viscoelastic damper devices employ high damped elastomeric pads that are chemically bonded at their entire contact surfaces to the steel plates of the device. The hysteretic load-deformation characteristics of the device are governed by the inherent response properties of the elastomeric material under cyclic shear deformations. The high damped viscoelastic material of the device is typically made of a particular copolymer of patented formulation. The main objective of this paper is to improve the hysteretic characteristics of a viscoelastic damper by applying physical changes to the configuration of its viscoelastic pads. As such, innovative pre-compressed partially bonded viscoelastic damper devices have been introduced. The proposed dampers employ fiber-reinforced viscoelastic pads that are partially bonded to the inner and outer steel plates of the damper, and are subjected to a desired level of permanent pre-compression. A set of full scale prototype damper devices of different geometrical configurations have been examined experimentally. Results of cyclic shear tests indicate that the pre-compressed stresses increase the effective inherent damping of the viscoelastic material. Moreover, the fiber reinforcing layers provide an additional source of energy dissipation as the partially bonded pads exhibit rollover deformations under cyclic shear loads. Based on the results of this proof-of-concept study, the pre-compressed partially bounded viscoelastic dampers are found to be quite feasible.
One of the most common types of seismic isolators is steel reinforced elastomeric isolators (SREIs) which consist of alternating layers of elastomer and steel reinforcing plates. Unbonded fiber-reinforced elastomeric isolators (UFREI) represent a relatively new type of elastomeric isolators. In this type of isolator, to control lateral strain and provide vertical stiffness, FRP layers are used instead of steel plates. Additionally, to reduce the cost of isolators, the idea of removing the top and bottom connection plates and the unbonded use of isolators has been considered. In UFREIs, due to the rollover deformation and the reduction of the isolator horizontal stiffness, it is expected that the seismic isolation efficiency increases as compared to the bonded isolators. In this research, the performances of UFREIs and conventional SREIs in improving the seismic behavior of liquid storage tanks were evaluated and compared. The isolated water tank was modeled using a mass and spring model of three degrees of freedom with convective mass, impulsive mass, and rigid mass. Time history analyses were performed on the fixed-base storage tank as the benchmark structure and the two base-isolated tanks with steel-reinforced and unbonded fiber-reinforced isolators. The results show that both types of isolators are effective in significantly reducing the demand base shear in the tanks. However, seismic isolation increases the displacement demand in the convective mass. Regarding the comparison of the two types of isolators, it was observed that on average, UFREIs in slender and broad tanks are 33.5% and 23.9%, respectively, more efficient than the SREIs in reducing the maximum base shear forces. Also, there is no significant difference in the maximum displacement of the convective mass in the two isolation systems. The displacement and shear forces developed in the unbonded isolators were found less sensitive to the variations of the Peak Ground Acceleration (PGA) as compared with the conventional bonded isolators.
One of the most common types of seismic isolators is steel reinforced elastomeric isolators (SREIs) which consist of alternating layers of elastomer and steel reinforcing plates. Unbonded fiber-reinforced elastomeric isolators (UFREI) represent a relatively new type of elastomeric isolators. In this type of isolator, to control lateral strain and provide vertical stiffness, FRP layers are used instead of steel plates. Additionally, to reduce the cost of isolators, the idea of removing the top and bottom connection plates and the unbonded use of isolators has been considered. In UFREIs, due to the rollover deformation and the reduction of the isolator horizontal stiffness, it is expected that the seismic isolation efficiency increases as compared to the bonded isolators. In this research, the performances of UFREIs and conventional SREIs in improving the seismic behavior of liquid storage tanks were evaluated and compared. The isolated water tank was modeled using a mass and spring model of three degrees of freedom with convective mass, impulsive mass, and rigid mass. Time history analyses were performed on the fixed-base storage tank as the benchmark structure and the two base-isolated tanks with steel-reinforced and unbonded fiber-reinforced isolators. The results show that both types of isolators are effective in significantly reducing the demand base shear in the tanks. However, seismic isolation increases the displacement demand in the convective mass. Regarding the comparison of the two types of isolators, it was observed that on average, UFREIs in slender and broad tanks are 33.5% and 23.9%, respectively, more efficient than the SREIs in reducing the maximum base shear forces. Also, there is no significant difference in the maximum displacement of the convective mass in the two isolation systems. The displacement and shear forces developed in the unbonded isolators were found less sensitive to the variations of the Peak Ground Acceleration (PGA) as compared with the conventional bonded isolators.
The conventional laminated elastomeric bearing isolators, as a mature technology, have found significant application in earthquake protection of civil structures, which are typically large in size, heavy, and expensive in cost. The technology is not as effective in the seismic isolation of lightweight structures due to the challenges in providing effective period-shift while accommodating large isolator displacement demands. Further horizontal flexibility may yet be required in the elastomeric isolators concerning the Seismic isolation of lightweight structures. A novel hollow circular (HC, also called the annular) type of fiber-reinforced elastomeric isolators (FREIs) is introduced and examined in this study. To demonstrate the validity of the concept, the dynamic response characteristics of two similar full-scale HC-FREIs were evaluated experimentally and compared with those of a corresponding solid circular (SC)-FREI as the control specimen. The response parameters, performance limit states, and failure modes of the two isolator types were evaluated and compared in detail. In addition, the performance of the two isolator types in the base isolation of a lightweight structure was investigated. Given the significantly increased horizontal flexibility and superior damping properties, the unbonded HC-FREIs offer a feasible solution for the effective earthquake mitigation of many lightweight structures. Additionally, the comparatively lower volume of materials and the lighter weight of the isolator provide economic advantages for HC-FREIs over their SC-FREI counterparts.
