Fiber-reinforced Elastomeric Isolators Research Articles

Experiment-Based Sensitivity Analysis of Scaled Carbon-Fiber-Reinforced Elastomeric Isolators in Bonded Applications

Fiber-reinforced elastomeric isolators (FREIs) are a new type of elastomeric base isolation systems. Producing FREIs in the form of long laminated pads and cutting them to the required size significantly reduces the time and cost of the manufacturing process. Due to the lack of adequate information on the performance of FREIs in bonded applications, the goal of this study is to assess the performance sensitivity of 1/4-scale carbon-FREIs based on the experimental tests. The scaled carbon-FREIs are manufactured using a fast cold-vulcanization process. The effect of several factors including the vertical pressure, the lateral cyclic rate, the number of rubber layers, and the thickness of carbon fiber-reinforced layers are explored on the cyclic behavior of rubber bearings. Results show that the effect of vertical pressure on the lateral response of base isolators is negligible. However, decreasing the cyclic loading rate increases the lateral flexibility and the damping capacity. Additionally, carbon fiber-reinforced layers can be considered as a minor source of energy dissipation.

Roll-out instability of small size fiber-reinforced elastomeric isolators in unbonded applications

Unbonded fiber-reinforced elastomeric isolators, due to their light weight, low cost and easy installation, are viable devices for seismic mitigation purposes, both for housing and building applications in highly seismic areas, even in the developing world. Important aspects of these isolators are that they do not have thick base plates, they are not bonded to the top and bottom structure, and their reinforcements are flexible. These features allow fiber-reinforced isolators to experience roll-over deformation under lateral loads. Roll-over deformation is stable, if the stabilizing moment due to vertical forces is higher than the overturning moment due to horizontal forces; however, if this condition is not verified, the isolator experiences a “roll-out instability.” This paper proposes a model for prediction of roll-out instability based on the applied forces equilibrium condition and on the assumption that a triangular distribution for the compressive stresses acts on the isolator. The model is developed considering that the isolator contact area varies with the applied displacement, and consequently the compressive stresses vary. An expression for the isolator instantaneous stiffness is proposed. Results are presented for roll-out tests performed on fiber-reinforced isolators that have varied aspect ratios and are subjected to different compression levels. A comparison is made between the measured displacement at roll-out and the results obtained from the aforementioned model. The comparison proves that the model accurately and reliably predicts roll-out displacement of elastomeric isolators subjected to horizontal and vertical forces.

Seismic Isolation of a Shear Wall Structure Using Rectangular Fiber-Reinforced Elastomeric Isolators

AbstractThe seismic response of a 2-story reinforced concrete shear wall structure seismically isolated using unbonded rectangular fiber-reinforced elastomeric isolators (FREIs) is investigated. Using rectangular isolators, rather than square or circular, for buildings with masonry or reinforced concrete structural walls allows a more uniform distribution of the gravity loads and eliminates the requirement of additional wall beam elements. Time history analyses are conducted for both fixed-base (FB) and base-isolated (BI) configurations using 10 earthquake ground motions, selected to match the design response spectrum. The properties of the isolators are taken from lateral cyclic test results carried out on scale model FREIs. A pivot-elastic model, previously introduced by the authors, is used to model the lateral response of the isolators in the time history analyses. The peak response values of the BI structure are significantly reduced compared with the results for the FB structure. It is concluded tha...

A numerical study on the compressive and rotational behavior of fiber reinforced elastomeric isolators (FREI)

Fiber reinforced elastomeric isolators (FREI) possess unique characteristics relative to traditional steel reinforced elastomeric isolators (SREI). FREI can be classified as either bonded or unbonded according to the connection type employed. An understanding of the behavior of FREI under these loading configurations is needed in order to extend the application of FREI to bridges. The objective of this study is to investigate the response of FREI when vertical and rotational deformations are applied separately as well as in combination. Finite element modeling is carried out on five FREI having the same material properties but possessing different geometrical properties. The mean vertical pressure values employed in this studied are 1, 4.5, 7 and 10MPa, which represent the specified upper and lower limits presented in CAN/CSA-S6 and AASHTO-LRFD. The behavior of the isolators under different angles of rotation is investigated up to a maximum value of 0.05 radians. Results obtained from the finite element model study and values obtained from available closed-form analytical solutions are found to be in excellent agreement. In addition, results show that U-FREI experience lower normal stresses and shear strains in the elastomer and lower strains in the reinforcement relative to B-FREI.

Experimental and finite element study on the lateral response of modified rectangular fiber-reinforced elastomeric isolators (MR-FREIs)

Fiber-reinforced elastomeric isolators (FREIs) have been shown to be viable and potentially inexpensive devices for seismic mitigation of low-rise buildings. FREIs utilize fiber material for the reinforcing layers resulting in lower weight and manufacturing costs compared to conventional steel-reinforced elastomeric isolators (SREIs). Experimental test results have shown that modifying FREIs, by cutting holes in the center portion of the isolators or removing sections from the sides, can enhance their lateral response characteristics. This study investigates the lateral response of modified rectangular FREIs (MR-FREIs) through experimental tests and 3D finite element analysis. Experimental test results of five FREIs with and without modifications are used to evaluate the 3D finite element models. These models are subsequently used to investigate the effect of modifications on the lateral behavior of the isolators, in addition to the stress and strain demands in the elastomer and fiber reinforcement layers.

Model of the Shear Behavior of Unbonded Fiber-Reinforced Elastomeric Isolators

AbstractIn this paper, an analytical model is developed and used to predict the horizontal behavior of unbonded fiber-reinforced elastomeric isolators (FREIs). An advantageous feature of unbonded FREIs is the ability to undergo rollover due to the lack of flexural rigidity of the reinforcement and the unbonded support conditions of the isolator. The rollover causes a characteristic softening, followed by stiffening, as the initially vertical faces of the isolator come into contact with the upper and lower supports—defined as full rollover. The force-displacement relationship is modeled by dividing the isolator into three sections: a central section, which experiences pure shear, and two rollover sections, which experiences combined shear and bending. The displacements of the rollover section are used to establish the curved deformed profile and predict full rollover. The model is evaluated with experimental data from four unbonded FREI designs. A parametric study is conducted to determine the influence of...

Seismic Response of a Historic Masonry Construction Isolated by Stable Unbonded Fiber-Reinforced Elastomeric Isolators (SU-FREI)

We study the implementation of Stable Unbonded Fiber-Reinforced Elastomeric Isolators (SU-FREI) for the seismic protection of a typical historical masonry construction from Puglia. The effectiveness of this innovative isolation technique is analyzed by means of a non-linear dynamic analysis, which shows substantial improvements of the seismic response with respect to the corresponding fixed base construction.

Partially bonded fiber-reinforced elastomeric isolators (PB-FREIs)

Summary Although stable unbonded fiber-reinforced elastomeric isolators (SU-FREIs) have desirable characteristics for seismic isolation, the unbonded application also introduces limitations in comparison with bonded elastomeric isolators. SU-FREIs are not capable of resisting tensile forces, making SU-FREIs unsuited for situations where overturning is of concern or where large vertical accelerations are anticipated. Furthermore, as SU-FREIs rely on friction to transfer horizontal forces, the isolator could potentially slip under certain loading conditions, resulting in permanent displacements. This paper proposes that concerns over the transfer of tensile forces and potential slip can be addressed by partially bonding the SU-FREI to the upper and lower supports. In this way, partially bonded FREIs (PB-FREIs) not only retain the beneficial characteristics of an unbonded FREI but also inherit characteristics of a bonded isolator; notably tensile and horizontal forces can be transferred through the partial bond. Experimental results from isolators tested unbonded and partially bonded under vertical compression are used to evaluate a finite element model. The experimental data and finite element analysis demonstrate that portions of a FREI can be bonded without substantially altering the rollover characteristics of the isolator within the range of average vertical compressive and tensile stresses considered. Despite an unconventional deformed shape under tensile vertical stress, the horizontal force–displacement relationship exhibits negligible deviation from a conventional unbonded SU-FREI under a compressive vertical stress. It is postulated that with further development, partially bonded FREIs can retain the beneficial characteristics of unbonded FREIs while addressing concerns over tensile forces and slip. Copyright © 2014 John Wiley & Sons, Ltd.

Experimental and finite element study on the compression properties of Modified Rectangular Fiber-Reinforced Elastomeric Isolators (MR-FREIs)

This study investigates the compressive behavior of Modified Rectangular Fiber-Reinforced Elastomeric Isolators (MR-FREIs). The geometric modifications are introduced to reduce the horizontal stiffness and increase the energy dissipation of the isolation system, allowing long rectangular isolators that provide uniform support along walls to be utilized. It is of critical importance that MR-FREIs maintain adequate vertical stiffness to satisfy the requirements for an isolation system. Experimental data from vertical tests of four rectangular FREIs with and without geometric modifications is used to evaluate a three-dimensional (3D) finite element (FE) model. The 3D FE model is then used to conduct a parametric study on two MR-FREI configurations with varying geometry. The parametric study investigates the effect of the geometric modifications on the vertical stiffness and compression modulus in addition to stress and strain distributions in the elastomer and fiber reinforcement. The study identifies that, similar to annular isolators, introducing a minor geometric modification to the interior of the isolator results in a significant decrease in vertical stiffness and compression modulus. This influence is considerably less for geometric modifications positioned on the exterior of the isolator.

Performance of fiber-reinforced elastomeric base isolators under cyclic excitation

Summary This paper presents three-dimensional finite element models for the analysis of square fiber-reinforced elastomeric isolators (FREI) subjected to cyclic horizontal displacement and constant vertical load. Direction of applied displacement has been varied to evaluate their influence on the isolator performance. Both bonded and un-bonded boundary conditions at the top and bottom contact surfaces of isolator have been simulated in the analysis. It is observed that the un-bonded FREI are more effective in seismic isolation as compared with bonded FREI, and stresses developed under lateral displacement are also significantly lower in un-bonded FREI. Experimental validation has been carried out for un-bonded isolators and very good agreement is observed in terms of mechanical properties and deformed configuration. In the absence of any close form solution of un-bonded FREI due to its complex deformation pattern, finite element solution is observed to be sufficiently accurate for arriving at the design parameters of an isolator. Copyright © 2014 John Wiley & Sons, Ltd.

Finite element analysis of unbonded square fiber-reinforced elastomeric isolators (FREIs) under lateral loading in different directions

Compared to conventional elastomeric bearings, fiber-reinforced elastomeric isolators (FREIs) are expected to cost much less and be easier to install, particularly when unbonded isolators are utilized. Due to the complex lateral response of unbonded FREIs, only a limited number of analytical studies are available in the literature. In addition, most of these analytical studies are based on a number of simplifying assumptions, applicable only to a few basic geometries. In this paper, three-dimensional finite element analysis (FEA) is carried out to investigate the lateral response of square FREIs, having aspect ratios of 1.9, 2.6 and 2.9, when loaded in different directions. Since square isolators are not axisymmetric, variations in lateral response can be expected to occur when subjected to different loading directions. The finite element (FE) models are validated using experimental test results for bearings loaded at 0° and are subsequently employed to analyze the lateral response characteristics of the bearings when loaded at 15°, 30° and 45°. The results show that in general, the effective lateral stiffness of the bearings increases as the loading direction changes from 0° to 45°. In addition, as the aspect ratio decreases, the sensitivity of the lateral response to the loading direction increases.

Performance of carbon fiber-reinforced elastomeric isolators manufactured in a simplified process: experimental investigations

SUMMARY Fiber-reinforced elastomeric isolators (FREIs) are relatively new elastomeric bearings in which fiber-reinforced polymer composite plates are used as reinforcements rather than steel shims. Here in this research, a number of carbon FREIs are produced in scaled size using a simple and fast manufacturing process, which has potentials to be applied in developing countries. The aim is to show how efficient the carbon FREIs in bonded applications can operate under different loading conditions. Experimental results show that under cyclic displacements, although a partial delamination occurs between rubber layer and steel supporting plate because of the rollover deformation at shear strains greater than 50%, the rubber bearings perform properly up to 100% shear strain. The vertical stiffness increases with increasing the fiber-reinforced layers' thickness and with decreasing the elastomers' thickness. The flexibility in the horizontal direction increases by increasing the total thickness of rubber layers, while the energy dissipation capacity enhances with increasing the thickness of both fiber-reinforced and elastomeric layers. Copyright © 2014 John Wiley & Sons, Ltd.

Sensitivity analysis of carbon fiber-reinforced elastomeric isolators based on experimental tests and finite element simulations

Conventional steel-based rubber bearings are being replaced by fiber reinforced elastomeric isolators (FREI) due to their high weight and manufacturing cost. Compared to existing rubber bearings, FREIs have superior performance and as a result can control the seismic response of structures more efficiently. This study aims to simulate the performance of rectangular carbon FREIs (C-FREIs) produced through a simple and cost-effective manufacturing process. Additionally, the effect of different factors including the number and the thickness of rubber layers, as well as the thickness of carbon fiber reinforced sheets are investigated on the performance of C-FREIs through sensitivity analyses based on the results obtained from finite element simulations. The results show that by increasing the number and thickness of rubber layers, the efficiency of C-FREIs degrades in terms of vertical strength and damping capacity, however, the performance improves in terms of lateral flexibility. Another important observation is that the increasing thickness of fiber-reinforced layers can increase the vertical rigidity of the base isolator. The vertical stiffness has the most sensitivity to the thickness of elastomeric layers and the thickness of CFR sheets while, when the number of rubber layers increases, the effective lateral stiffness is mostly affected.

Three-dimensional finite element analysis of circular fiber-reinforced elastomeric bearings under compression

Fiber reinforced elastomeric isolators (FREIs) are a relatively new type of elastomeric bearing. FREIs utilize fiber material for the reinforcing layers instead of steel which is used in conventional steel reinforced elastomeric isolators (SREIs). Unlike SREIs, where the steel reinforcing material is usually assumed to be rigid in extension, the reinforcing fiber layers are considered to be flexible in extension. As a result, the vertical response characteristics of FREIs differ from those of SREIs. In this study, a 3D parametric numerical finite element analysis (FEA) is carried out on sixteen circular FREI bearings. All bearings investigated have the same overall dimensions, while the thickness of elastomer layers and the axial stiffness of reinforcement are the parameters that are varied. FEA results, in terms of deformed shape, stiffness, stress and/or strain in reinforcing and elastomer layers are compared with predicted values obtained using the pressure solution and the pressure approach methods. Good overall agreement was observed between the FEA results and the results obtained from analytical solutions. It was concluded that the end boundary effect, which is often neglected, becomes a contributing factor on the vertical response for bearings reinforced with flexible reinforcements, or bearings with low shape factor values.

Study on dynamic and mechanical characteristics of carbon fiber- and polyamide fiber-reinforced seismic isolators

Seismic isolators are used to decrease the energy and forces of earthquakes. The weight of conventional steel-reinforced elastomeric isolators (SREIs) is high, mostly due to the use of multiple steel shim plates. On the contrary, the damping ratio of SREIs is relatively low. Accordingly, this research utilizes a new approach in which the steel shim plates are replaced by carbon and polyamide fibers. This study attempts to obtain the dynamic and mechanical properties of such carbon fiber- and polyamide fiber-reinforced elastomeric isolators, in comparison with SREIs. In this work, a number of specimens were initially designed and manufactured. Afterwards, compression and cyclic shear tests were performed on them. In the shear tests, due to the limitations of the testing machine, a constant vertical load was not applied. All three types of isolator specimen were cylindrical, with identical diameter and height. The steel-, carbon fiber-, and polyamide fiber-reinforced elastomeric isolator specimens had 16, 23, and 23 reinforcement layers, respectively. To decrease the effect of manufacturing errors on the dynamic and mechanical characteristics of the specimens, 6 samples of each isolator type were manufactured, i.e., a total of 18 samples. The outcome of the experiments revealed that the use of flexible reinforcement resulted in a damping increase of up to 20 and 30 % for the carbon fiber- and polyamide fiber-reinforced elastomeric isolators, respectively. Furthermore, the carbon fiber design provides more reasonable performance for the isolators compared with the use of polyamide fiber.

A study on experimental shear behavior of fiber-reinforced elastomeric isolators with various fiber layouts, elastomers and aging conditions

Fiber-reinforced elastomeric isolators are innovative devices for seismic isolation that employ fibers, rather than steel, as reinforcement material. Their light weight and low cost could help to widen the application of seismic isolation, to housing for example, and to buildings in highly seismic areas of the developing world. Experimental studies available in the literature have already demonstrated the suitability of fiber-reinforcing isolators to protect buildings from earthquake. Theoretical analyses have been carried out to describe the isolator behavior under compression and bending. However, no treatment of its behavior under compression and shear has been performed yet. This because of objective difficulties due to the non-linearity both of the geometry and the properties of these materials. The present paper proposes a simplified geometric model to describe the isolator’s deformed configuration under compression and shear, based on the observation of its experimental behavior. The results of compression and shear tests carried out on 17 pairs of fiber-reinforced isolators are reported. The specimens have different characteristics as regards rubber typology (neoprene and low and high damping neoprene), reinforcement (bi-directional or quadri-directional carbon fiber fabrics), shape factor, and aging (unaged and aged specimens). On the basis of the proposed model, an expression for the equivalent linear horizontal stiffness of fiber-reinforced isolators is proposed. This expression predicts well the experimental results. The predictions are more uniform and accurate than those obtained from the stiffness expression of traditional isolators, which is commonly used in the literature also for fiber-reinforced isolators.

Multi-criteria optimization and seismic performance assessment of carbon FRP-based elastomeric isolator

Elastomeric base isolators minimize the structural damage in moderate seismic events and prevent structural collapse in extreme conditions like earthquakes. They considerably decrease the earthquake force transmitted to the structure by providing a flexible damping mechanism between the substructure and the superstructure, thanks to their very low horizontal-to-vertical stiffness ratio. This paper presents a multi-criteria optimization process for fiber-reinforced elastomeric isolators (FREIs) used in bridges. FREI is made of high damping rubber and carbon fiber reinforced polymer (FRP) composite plates. Here, a numerical material model is proposed for high damping rubber using finite element (FE) simulation to capture its highly nonlinear behavior. After validating this material model with experimental results, the effect of different parameters on the efficiency of the elastomeric bearing will be investigated through a sensitivity analysis performed by proposing regression models. The performance of FREI is optimized by assigning different weights to its operational specifications which are the effective horizontal and vertical stiffnesses and the equivalent viscous damping. Results show that the effective horizontal stiffness and viscous damping are highly dependent on the shear modulus of the elastomer layers. Also, the number of rubber layers and thickness of FRP composite plates have large effects on the vertical stiffness. Finally, an isolated three-span continuous steel girder reinforced concrete pier supported bridge is fitted with the optimized FREI in order to analytically study the performance of the rubber bearing on the seismic behavior of the bridge structure through dynamic time history analyses.

Sliding instability of fiber-reinforced elastomeric isolators in unbonded applications

Fiber-reinforced isolators not anchored to the structure represent a new development in seismic isolation. Their light weight, low cost and great ease of installation, in comparison with conventional elastomeric isolators, could allow to extend the use of these valuable devices to seismic protection of housing and commercial buildings, particularly in developing countries. As demonstrated in studies available in the literature, fiber-reinforced isolators match the behavior of steel-reinforced ones. However the friction behavior between such new devices and the structure has not been investigated yet. This paper describes an experimental study on elastomeric isolators reinforced by bi-directional carbon fiber fabrics performed to investigate the static friction at contact surfaces. The isolators were placed in contact with concrete surfaces and subjected contemporaneously to compressive stress and shear strains. Friction behavior was analyzed in terms of isolator sliding with respect to the concrete sub- and super-structure. Influence on friction behavior of different parameters, such as level of compressive stress, rubber typology, concrete roughness, aging and loading rate, was investigated. The tests showed that, in relation to the value of the applied compressive stress, uncontrolled sliding of the isolator can occur by increasing the shear force. This unstable condition is called sliding instability herein. The experimental results are explained and the limit values of the compressive stress needed to avoid sliding instability is provided both for new and aged isolators, under quasi-static loading conditions. It is demonstrated that quasi-static tests are reasonably conservative with regard to safety. Patterns of stresses and relative displacements based on the experimental results are also proposed.

Stability of fiber-reinforced elastomeric bearings in an unbonded application

Stable unbonded fiber-reinforced elastomeric isolators (SU-FREIs) are a viable device for seismic mitigation purposes in low-rise structures. SU-FREI bearings consist of alternating layers of elastomer and fiber fabric with shape factor and aspect ratio such that stable rollover occurs during lateral displacement. This study investigates the stability of SU-FREI bearings through experimental testing. Two experimental test procedures are employed. The first investigates stability during incrementally increasing axial loads under lateral cyclic excitation, and the second investigates the ultimate shear properties of SU-FREI bearings through monotonic lateral displacement under design axial load. Dynamic stability testing reveals that the tested SU-FREIs remain stable at axial loads significantly greater than the design load under a wide range of cyclic lateral displacement amplitudes. In addition, an increase in axial load results in lower effective stiffness and greater effective damping in the SU-FREI bearings. Although rollout instability was not encountered during ultimate shear property testing, shear tests provide insight into the stiffening effect encountered as the originally vertical faces of the SU-FREI bearings make contact with the top and bottom contact supports.

Bonded versus unbonded strip fiber reinforced elastomeric isolators: Finite element analysis

This paper presents a finite element (FE) model for the analysis of strip fiber reinforced elastomeric isolators (FREIs) that are subjected to any given combination of static vertical and lateral loads. The model is able to simulate both bonded and unbonded boundary conditions at the top and bottom contact surfaces of the isolator. Compared to bonded (B)-FREIs, the FE-analysis of stable unbonded (SU)-FREIs presents additional analysis challenges. SU-FREI refers to unbonded FREIs that exhibit stable rollover deformation under lateral loads. Additional analysis challenges are attributed to changes in the boundary conditions of SU-FREI as a result of rollover type deformation. To address these challenges, the utilized FE-mesh is updated during analysis consistent with the deformed geometry of the isolator. Using the proposed FE-model, the lateral responses of a B-FREI and a SU-FREI were evaluated. Both isolators had the same material and geometrical properties and were subjected to identical constant vertical loading. Comparing the lateral responses, it was found that the SU-FREI was considerably more efficient than the B-FREI as a seismic isolator. In addition, the in-service stress demands on the SU-FREI were found to be significantly lower than the B-FREI.