Unbonded Fiber-reinforced Elastomeric Isolators Research Articles

Shake table tests on a rigid block isolated with high-damping unbonded fiber-reinforced elastomeric isolators

Fiber-reinforced elastomeric isolators (FREIs) represent a relatively recent type of elastomeric device. In contrast to traditional steel-reinforced elastomeric isolators (SREIs), FREIs are constructed with multiple layers of rubber pads and fiber layers instead of steel laminas. This choice is aimed at cost reduction during the manufacturing and installation process. To enhance cost-efficiency, FREIs can also be used in unbonded applications (UFREIs), where the device is placed between the foundation and the superstructure without bonding or fastening. This approach helps to further minimize installation expenses. Furthermore, the lateral rollover deformation, a characteristic feature of this configuration, improves energy dissipation. This study investigates the seismic isolation capabilities of high-damping unbonded FREIs for low-rise masonry buildings. Specifically, unidirectional cyclic and shake table tests of a rigid block isolated with two UFREIs are discussed. The primary goals of this experimental study include evaluating the effectiveness of UFREIs in reducing seismic demands, analyzing the response of UFREIs under unidirectional ground-motion excitation, confirming the ability of UFREIs to withstand major unidirectional excitation without sustaining damage, and exploring the limitations of this isolation system under a range of seismic inputs and low axial loads.

Unbonded Fiber-Reinforced Elastomeric Isolators for Isolating Masonry Buildings

Unbonded Fiber-Reinforced Elastomeric Isolators for Isolating Masonry Buildings

Bidirectional performance assessment of UFREI-isolated structures with single and multiple tuned mass dampers under earthquakes and winds

An innovative hybrid vibration controller (HVC) composed of unbonded fiber-reinforced elastomeric isolators (UFREIs) and tuned mass dampers (TMDs) is utilized for the vibration control of an existing hospital building under the multi-hazard scenario of earthquakes and (nonconcurrent) winds. A 3-dimensional nonlinear model of the considered hospital building is analyzed. Four configurations of the HVC are investigated, wherein single or multiple TMDs are utilized, having various tuning properties and locations along the height of the building. All the various controlled and uncontrolled structures are subjected to site-specific bidirectional earthquake and wind excitations. Results reveal that the HVC with two TMDs, having favorable tuning conditions for both earthquakes and winds, are the most efficient under the multi-hazard scenario. Interestingly, the increase in the TMD mass ratio always improves the performance of the HVC under wind excitations, however under earthquakes, it may also result in enhanced deterioration of HVC performance in detuned conditions.

ЭФФЕКТИВНОСТЬ КВАДРАТНОГО АРМИРОВАННОГО ВОЛОКНОМ ЭЛАСТОМЕРНОГО ИЗОЛЯТОРА ДЛЯ ЖЕЛЕЗОБЕТОННОГО ЗДАНИЯ ПРИ ЗЕМЛЯТРЕСЕНИЯХ

Un-bonded fiber reinforced elastomeric isolator (U-FREI) is a relatively new type of multi-layer elastomeric isolator in which fiber layers are used as reinforcement to replace steel sheets in conventional steel reinforced elastomeric isolators. It is installed directly between the substructure and superstructure without any connection at the interfaces. Most of the previous studies on the U-FREIs supported to the base-isolated buildings are masonry or stone structures. In this study, the dynamic responses of a reinforced concrete (RC) building supported on square U-FREIs under the action of recorded real time-history ground motions of earthquakes are investigated by finite element analysis using SAP2000 software. A hypothetical 4-storey reinforced concrete building constructed in Vietnam is selected for the study. Comparison of the dynamic responses of the base-isolated building and corresponding fixed-base building is carried out to evaluate the seismic vulnerability of the base-isolated building under earthquakes. Finite element analysis results show that peak values of floor acceleration and inter-storey drifts at different floor levels, and peak value of base shear of the base-isolated building are lower than those of the corresponding fixed-base building. The U-FREIs are found to be very effective in reducing seismic vulnerability of low and mid-rise RC buildings.

High-damping fiber-reinforced elastomeric seismic isolator in different boundary conditions: An experimental insight

Seismic isolation reduces structural vulnerability and is an effective retrofitting solution. The commonly used steel-reinforced elastomeric isolators (SREIs) consist of several rubber pads interspersed with steel laminas for vertical reinforcement. However, SREIs are expensive, making them economically-unjustified for ordinary residential buildings, especially in developing countries. Fiber-reinforced elastomeric isolators (FREIs) offer a lighter alternative with thin fiber layers instead of steel laminas. FREIs can be applied to the structure in bonded (traditional), unbonded, or partially bonded configurations. Unbonded FREIs (UFREIs) are cost-effective as they are not bonded or fastened to thick connection steel plates. Furthermore, they are inherently more flexible in lateral directions and thus require relatively less rubber material to offer the same seismic isolation performance as traditional elastomeric isolators. However, UFREIs cannot resist tensile forces and may slip under certain loads, limiting their use in situations with overturning or high vertical accelerations. To address these limitations, partially bonded applications (PBFREIs) bond only a reduced area of the rubber device to the connection steel plates. PBFREIs retain beneficial characteristics of both unbonded and bonded applications, such as improved lateral flexibility, and resistance to uplift forces and slip. The possibility of UFREI applications remains promising, particularly in developing countries, due to their low cost, ease of fabrication and installation. This paper proposes a FREI made of high-damping rubber and glass fiber fabrics, and examines its performance in bonded, partially bonded, and unbonded applications. As a unique feature, the material and geometrical characteristics of the FREIs are identical. A comparative analysis, including experimental testing, highlights the strengths and weaknesses of each application typology. The paper also provides a detailed explanation of the design process, encompassing rubber compound development, fabrication, and experimental characterization of the device.

Implementation, Verification, and Validation of an Impact Model for Lateral Numerical Modeling of Unbonded Fiber-Reinforced Elastomeric Isolators

Implementation, Verification, and Validation of an Impact Model for Lateral Numerical Modeling of Unbonded Fiber-Reinforced Elastomeric Isolators

Hybrid Vibration Control of Hospital Buildings against Earthquake Excitations Using Unbonded Fiber-Reinforced Elastomeric Isolator and Tuned Mass Damper

Lifeline structures such as hospital buildings need to be specifically designed such that they experience reduced vibrations when subjected to earthquake excitations because it will be difficult to vacate hospital buildings under the event of any earthquake. Therefore, to ensure operational condition under earthquake excitations in an existing hospital building, the present study utilizes flexible unbonded fiber-reinforced elastomeric isolators (UFREIs) for its seismic isolation. The UFREI-based isolation system is designed to restrict the structural acceleration within the tolerable limits for the building inhabitants even during earthquake hazards. However, the use of such flexible isolators results in excessively large bearing displacements, which either may not be practical and/or pose several serviceability issues. Therefore, tuned mass damper (TMD) is attached to the base floor of the UFREI-isolated hospital building to reduce the large isolator displacements. Properties of the hybrid vibration control system are designed according to the site-specific scenario in New Delhi, India. Further, nonlinear time-history analyses of the UFREI-isolated hospital building with the TMD are carried out, and responses are compared with its uncontrolled response. Results show that the peak bearing displacement response of the UFREI-isolated hospital building is reduced by 9% to 27%, due to the addition of the TMD. Importantly, the required design displacement of the UFREI-based isolation system is decreased by 27%, without compromising the effectiveness of base isolation. In fact, the performance of the hybrid vibration control system is superior to the base isolation system alone.

Full-scale experimental tests on unbonded fiber reinforced elastomeric isolators under bidirectional excitation

Unbonded fiber reinforced elastomeric isolators (UFREIs) represent a lower-cost alternative to conventional steel reinforced elastomeric isolators (SREIs), in which the internal steel shims are replaced with fiber reinforcement and the installation is accomplished without anchorage bolts or dowels, but only relying on frictional mechanisms and interfacial roughness. In the literature, experimental tests on UFREIs were generally presented under unidirectional seismic excitation and considering small-scale devices. In this contribution, the hysteretic behavior of UFREIs is investigated experimentally by testing full-scale devices (diameter 620 mm) not only under unidirectional loading, but also including bidirectional orbits, thus quantifying the degree of biaxial coupling. A simple phenomenological model is then set up to numerically simulate the experimental outcomes. This model consists of a set of nonlinear springs arranged in a circular configuration. The spring parameters are calibrated by either curve-fitting 1D test results or considering 2D tests, thus selectively incorporating the lateral coupling. This model makes it possible to investigate the influence of bidirectional interaction of UFREIs on the seismic performance of base-isolated structures. Nonlinear time-history analysis on a regular three-dimensional reinforced concrete building with UFREIs modeled through the proposed approach demonstrates that the lateral coupling of the UFREIs leads to lower seismic response, of around 10-20%, in comparison to a simpler, uncoupled model that ignores the bidirectional interaction of the isolation devices.

Performance of prototype seismic isolators reinforced with fiber and a recycled rubber tire matrix

Seismic isolators are passive control devices that reduce the energy transferred from an earthquake to the superstructure, this avoiding damage to building elements. Seismic isolators are classified according to their connection to the structure; for example, unbonded isolators lack mechanical connections with the building. Unbonded fiber-reinforced elastomeric isolators (U-FREIs) are formed of intercalated layers of bidirectional polyester mesh used as reinforcement and agglomerated ground rubber from end-of-life tires (ELTs) as an elastomeric matrix. This paper presents the mechanical behavior of U-FREIs built at a reduced scale to provide an affordable alternative for the mitigation of seismic forces that affect low-rise buildings. The isolators were designed for a low-rise residential frame structure located in an area of high seismic hazard and were subjected to shear and compression tests to characterize their mechanical behavior. The experimental results show good behavior of the prototypes with respect to standards and design requirements.

Impact of the Implementation of Unbonded Seismic Isolators on the Design and Retrofitting of Bridges

The existence of road infrastructure such as bridges that do not meet current design criteria, the deterioration of these due to use, environmental factors and extraordinary or unforeseen conditions, and the increase in seismic and gravitational design loads due to regulatory updates, generate the need for search of new design and retrofitting alternatives for this type of structure. Thus, the conditions of these structures need to be assessed and corresponding interventions need to be identified. It is important to implement alternatives that are both economically and functionally efficient. For example, base isolation can reduce direct construction costs by using devices designed and built in the country of origin, but the unfamiliarity and importation costs of these devices are the main obstacles to mass use. In this article, a comparative analysis is performed of a conventional design and an integrated design for base isolation using low-cost unbonded fiber-reinforced elastomeric isolators (U-FREIs) made in Colombia. A methodology is proposed for the analysis of new and existing isolated structures, in which the isolators are placed between the substructure and superstructure. This methodology is used to design and strengthen a typical prestressed concrete girder bridge, and the results are compared against those obtained using a conventional scheme. Finally, an existing bridge is analyzed using on-site measurements to determine the dynamic properties and modify the numerical model used for analysis, design, and retrofitting. The effectiveness of the proposed methodology as a tool for the design of new bridges and the retrofitting of existing structures was verified for all cases. Economic and structural benefits were evidenced by reductions in direct costs of up to 15% and the seismic force of up to 20%, respectively.

INVESTIGATION THE BASE SHEAR – HORIZONTAL DISPLACEMENT CURVES OF FIBER REINFORCED ELASTOMERIC ISOLATORS SUPPORTED A BASE-ISOLATED BUILDING UNDER EARTHQUAKES

Fiber reinforced elastomeric isolator (FREI) is a relatively new laminated elastomeric isolator and is expected to reduce the damage for mid and low-rise buildings under earthquakes. It has been studied and developed with two types: bonded fiber reinforced elastomeric isolator (B-FREI) and unbonded fiber reinforced elastomeric isolator (U-FREI). In this study, the response of a reinforced concrete frame building supported on U-FREIs under the action of various ground motions of recorded real earthquakes in the past is investigated by finite element analysis using SAP2000 software. Result is to determine the hysteresis loops shown the base shear - horizontal displacement curves of the U-FREI supported the base-isolated building under the action of various earthquakes.

Investigation the base shear – horizontal displacement curves of fiber reinforced elastomeric isolators supported a base-isolated building under earthquakes

Fiber reinforced elastomeric isolator (FREI) is a relatively new laminated elastomeric isolator and is expected to reduce the damage for mid and low-rise buildings under earthquakes. It has been studied and developed with two types: bonded fiber reinforced elastomeric isolator (B-FREI) and unbonded fiber reinforced elastomeric isolator (U-FREI). In this study, the response of a reinforced concrete frame building supported on U-FREIs under the action of various ground motions of recorded real earthquakes in the past is investigated by finite element analysis using SAP2000 software. Result is to determine the hysteresis loops shown the base shear - horizontal displacement curves of the U-FREI supported the base-isolated building under the action of various earthquakes.

Vertical response of unbonded fiber reinforced elastomeric isolators (U-FREIs) under bidirectional shear loading

Fiber Reinforced Elastomeric Isolators (FREIs) were generally studied in unbonded configuration. Due to combined axial and shear loads, the contact area between the bearing and horizontal supports reduces with the horizontal displacement. As a result, both the vertical and the horizontal stiffnesses decrease with the horizontal deformation while also the vertical deformation increases. This paper presents the results of a large set of full-scale 3D Finite Element Analyses on unbonded fiber reinforced bearings with different geometries, subjected to combined axial and multi-directional shear loads. The main vertical response parameters were studied, namely the vertical displacement, the vertical stiffness, and the effective compressive modulus, thus highlighting the influence of both geometry and horizontal loading direction on the vertical response of the FREIs. Conclusion of this study demonstrate to what extent the combined influence of geometric properties and loading conditions affects the vertical response of elastomeric bearings with flexible reinforcements.

Experimental tests and numerical modeling of full-scale unbonded fiber reinforced elastomeric isolators (UFREIs) under bidirectional excitation

Unbonded fiber reinforced elastomeric isolators (UFREIs) can be used for the seismic protection of structures as a lower-cost alternative to conventional laminated rubber bearings, by replacing internal steel shims with fiber reinforcement and exploiting the frictional mechanism at the rubber-concrete interface to avoid anchorage bolts. In this paper, the hysteretic behavior of full-scale (diameter 620 mm) UFREIs is investigated in both an experimental and a numerical framework when subjected to triaxial loading (i.e., simultaneous imposed displacement along two horizontal directions with concurrently applied vertical load), to account for the inherent multi-directional nature of a real earthquake scenario. Experimental results of UFREIs tested under two bidirectional orbits involving different velocities and amplitudes up to 100% shear strain are presented, and peculiar effects ascribed to the lateral coupling of the devices are identified by comparison with monodirectional test results. Then, an efficient nonlinear phenomenological model is proposed to simulate the isotropically coupled biaxial hysteretic behavior of UFREIs detected from the experiments. This model consists of a set of nonlinear springs arranged in a circular configuration and governed by just three parameters with clear mechanical significance. Novel analytical (closed-form) expressions for the model calibration are ad-hoc developed in this paper. This modeling approach is used to quantify the impact of biaxial coupling of UFREIs on the structural performance of base-isolated structures under bidirectional seismic excitation. To this aim, the seismic response of a three-dimensional reinforced concrete building isolated with UFREIs is numerically simulated. Numerical results show that isolator displacements and superstructure accelerations tend to be higher by using an uncoupled model, calibrated upon monodirectional tests and that neglects bidirectional interaction, as usually performed in practice.

Seismic response of isolated structures with an improved model of the UFREI

An improved formulation of the horizontal shear behavior of the unbonded fiber-reinforced elastomeric isolators (UFREI), which predicts both the pre-rollover and the post-rollover behavior considering the effect of the vertical load, is proposed. A simplified and computationally inexpensive formulation of the horizontal shear behavior of the UFREI has also been proposed afterward. Further, these formulations are verified using experimental results and compared with the results obtained using the other earlier methods in the recent literature. The horizontal shear behavior predicted by both of the proposed formulations is observed to fit the experimental results very well. The shear behavior predicted by these formulations is observed to be more accurate than the corresponding predicted by the other formulations from the recent literature. A parametric study characterizing the effect of the vertical pressure and the initial elastomeric shear modulus on the horizontal shear behavior of isolators is also carried out. The proposed formulations are further used to model the behavior of the UFREI employed to isolate the benchmark structure considered in the study.Supplementary damping devices are further installed alongside the nonlinear UFREI isolation system to mitigate the excessive displacement demand on isolators under near-fault ground motions. The isolated benchmark structure is thus coupled with a Magneto-rheological Damper (MRD) with an improved control algorithm for nonlinear isolation systems in three configurations, i.e., semi-active, passive-on, and passive-off. A comparative study of these configurations shows that MRD in passive-on mode provides the minimum isolators displacement response, but an overall improved structural response is achieved with MRD in the semi-active mode of operation.

Tuning the lateral response of unbonded fiber reinforced elastomeric isolators (U-FREIs) through horizontal holes: Experimental and numerical findings

Fiber-Reinforced Elastomeric Isolators (FREIs) are elastomeric devices introduced as a low-cost alternative to the well-known Steel-Reinforced Elastomeric Isolators (SREIs). As common in rubber-based devices, the application of FREIs to lightweight structures is challenging as slender bearings tend to be unstable under large horizontal displacements. This paper discusses the effects of a simple modification, consisting of drilled horizontal holes through the elastomeric layers of the bearings, on the lateral response of FREIs. A desirable hardening response and a stability range up to 100% of shear strain has been obtained on modified bearings. This result has been confirmed by experimental tests and finite element analyses. Findings of this work show how the proposed modification allows (i) to reduce the horizontal stiffness of FREIs, (ii) to increase the stable range of lateral deformations of the devices and (iii) to obtain a desired hardening behavior under large horizontal displacements without instability.

Seismic performance of Unbonded Fiber-Reinforced Elastomeric Isolators (UFREI) made by recycled rubber. Influence of suboptimal crosslinking

Seismic base isolation is considered effective to reduce the vulnerability of structures and it represents an optimal retrofitting solution in terms of reliability and effectiveness. Nowadays, one of the most promising devices is the Unbonded Fiber Reinforced Elastomeric Isolator (UFREI), which is considered a low-cost device, thanks to its lightweight, easy installation, and the total absence of steel. The rubber used to assemble the devices must be vulcanized correctly to create the polymer crosslinking to be ready for structural application. All rubber mechanical properties are strongly affected by curing temperature and curing time. In this paper, a series of experimental tests and numerical analyses have been performed to investigate the influence of crosslinking on the seismic performance of UFREIs made of regenerated Ethylene Propylene Diene Monomer (EPDM) combined with glass fiber reinforcement. Two prototypes have been considered, one vulcanized correctly at 150 °C for 40 min and the other at 130 °C for 40 min. Results obtained from Finite Element (FE) cyclic shear tests analysis and nonlinear time history analyses of a preliminary structural application have shown that the devices vulcanized at 130 °C for 40 min, although having suboptimal crosslinking density, are suitable to isolate low-rise masonry buildings properly.

Bi-directional loading of unbonded rectangular fiber-reinforced elastomeric isolators

Elastomers are ideal for application as base isolators due to their ability to accommodate large recoverable strains. The formation of a composite with reinforcement, either steel or fibers, takes advantage of the near incompressibility of the elastomer to enhance the vertical and bending properties of the bearing. Although the conceptual design of a fiber-reinforced and steel-reinforced elastomeric bearing is similar, the lack of flexural rigidity of the fiber reinforcement results in unique performance characteristics, especially in the lateral direction. In this paper, an experimental program is conducted to evaluate the effect of lateral coupling in rectangular unbonded fiber-reinforced elastomeric isolators (UFREIs). An apparatus with six degrees of freedom was used to apply a vertical load with simultaneous displacement in both primary lateral directions, whereas previous experimental programs are mostly based on a two degree of freedom analysis (vertical and lateral). The UFREI specimen was subjected to cyclic lateral displacements including tests with initial lateral offsets in the secondary lateral direction, in directions angled off the principal lateral axis, and in a circular motion. This experimental program is the first to investigate lateral coupling in rectangular UFREIs. A comparison was conducted based on the effective lateral stiffness, equivalent viscous damping, and energy dissipation characteristics. The considered rectangular UFREIs were found to exhibit lateral coupling but the degree of coupling and the impact on the structural performance under bidirectional loading require further investigation. Residual displacement was observed in the experimental program and is discussed in depth. Also, a theoretical equation is derived to calculate the lateral stiffness based on the angle of loading and lateral displacement. Theoretical values are compared with experimental results to verify the equation.

Finite element modeling of horizontal load-displacement hysteresis loops in unbonded elastomeric isolators

Finite element simulation of the dynamic response of elastomeric isolators is challenging due to the rate and amplitude dependency of elastomer material within the isolator. Current design codes mandate the use of experimental cyclic tests on prototype isolators to evaluate their dynamic response characteristics for the final design. This paper presents a step-by-step systematic procedure to construct a novel FEM for simulating the cyclic load-displacement hysteresis loops of unbonded fiber-reinforced elastomeric isolators (FREIs). The FEM employs a visco-hyperelastic model together with a piecewise calibration technique and the Nelder-Mead simplex optimization method to simulate both the nonlinear stress-strain relationship and the strain-history dependency of the rubber material within the isolator. Results of the uniaxial tensile test on the rubber specimen and preliminary cyclic shear tests on a prototype isolator are employed to calibrate the model. The proposed FEM leads to an effective response simulation of the unbonded FREIs. Thus, it may serve as a valuable tool to finalize the preliminary design and curb the expensive costs of test repetitions required to achieve the optimized isolator design.

On the wind‐induced motion of unbonded fiber‐reinforced elastomeric isolators: Designing for habitability

A large number of studies have verified the satisfactory seismic performance of unbonded fiber-reinforced elastomeric isolators (UFREIs). Typically, the total height of a UFREI will be shorter than its conventional elastomeric isolator counterpart. The shorter height results in a relatively larger isolator initial horizontal stiffness. The relatively large initial stiffness and damping values of UFREIs might be potentially sufficient in many cases to meet the wind serviceability requirements. However, in real practice, this must be examined for any UFREI system. The purpose of this study is to examine the dynamic wind response of a real UFREI system that its satisfactory seismic response was previously verified through shake table tests. The stochastic wind excitations were simulated in the time domain based on the code-specified basic wind velocity, gust factor, and exposure category using the Cholesky decomposition technique. The root-mean-square (RMS) acceleration response of the UFREI system was calculated and then compared with the threshold of human comfort. The cases where the UFREI system could satisfy the wind serviceability requirements were determined. For the cases where the performance was found unsatisfactory, the wind response of the UFREI system was mitigated with the aid of a set of brittle fuses that serve as supplemental springs to increase the initial stiffness of the base isolation system. A simplified procedure is developed for the design of brittle fuses.