Steel-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.

Experimental Study on the Mechanical Properties of Nylon Fabric-Reinforced Elastomeric Isolators (N-FREIs)

In recent decades, carbon fiber, glass fiber, polyester and Kevlar fiber have been utilized to replace the steel shims in conventional steel-reinforced elastomeric isolators (SREIs). This study chose nylon fabrics owing to their extreme low cost, low elastic modulus and good adhesion to rubber, and nine nylon fabric-reinforced elastomeric isolators (N-FREIs) were manufactured with different design parameters. Compression and compression shear tests were, respectively, conducted to investigate the mechanical properties together with their influential factors of the N-FREIs. Results show that the vertical load-carrying capacity of the isolator is high enough to sustain a compressive stress of 10[Formula: see text]MPa without visible damage. The compressive stiffness of N-FREI is much smaller than that of SREI, and the vertical damping ratio under cyclic compression reaches up to 7%. In the compression shear tests, the shear stiffness of the isolator first decreases and then increases as the shear strain increases within 300%, and the equivalent damping ratio varies between 9% and 14% for different sizes of the isolator. Additionally, due to the flexibility and extensibility of the low modulus nylon fabric, both vertical and horizontal stiffness decrease a bit with an increase in the number of fabric layers. Finally, a formula for calculating the horizontal stiffness of N-FREI is proposed, it provides a comprehensive mathematical model to predict the behavior of the N-FREI under horizontal shear conditions.

Investigation of seismic fragility curves of unbonded FREIs: Adaptive characteristics and modeling sensitivity

AbstractFiber‐reinforced elastomeric isolators (FREIs) are composed of layers of elastomer reinforced with either steel or fibers, and can be placed in a bonded or unbonded configuration between the upper and lower supports. The use of fiber reinforcement in FREIs was intended to reduce the production and installation costs compared to common steel‐reinforced elastomeric isolators (SREIs) and to develop an isolator suitable for widespread application, particularly in developing countries. The unique rollover deformation exhibited by unbonded FREIs (UFREIs), due to their flexible fiber reinforcement, enables them to adapt to multiple performance objectives at different hazard levels. In this study, the impact of different numerical models of UFREIs on the seismic response and failure probability of structures is investigated. The research employs incremental dynamic analysis and the development of fragility curves for different limit states, considering three sets of ground motions (including far‐field and pulse‐like). Moreover, the effect of full rollover was investigated by comparing the fragility curves of UFREIs when supported on modified support geometries, which involved three types of support: unmodified, accelerated, and delayed full rollover. The effectiveness of the adaptive characteristics to behave as a displacement restrain is demonstrated. The results emphasize the importance of employing an accurate model to simulate the behavior of UFREIs as an adaptive device for effectively utilizing their potential capacity, particularly at larger displacements where there is more dissipated energy due to full rollover.

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.

Stress distributions of infinite strip steel reinforced elastomeric isolators with a rubber core

Lead-rubber bearings contain a lead core that provides an advantageous increase in initial stiffness and increased energy dissipation during earthquake events. Despite these advantages, the inclusion of the lead core is an environmental and health concern and can be costly. The use of rubber cores in lieu of lead cores has been proposed as an alternative and shown to achieve excellent energy dissipation. The inclusion of the rubber core also decreases the weight of the isolator through the partial removal of the steel reinforcement or the replacement of lead with a lighter material. In this paper, the inclusion of a rubber core in an infinite strip steel-reinforced elastomeric isolator was investigated. An analytical solution was developed based on the assumptions of the pressure solution including and excluding the compressibility of the elastomer for the compression and bending properties. Finite element analysis was subsequently conducted to verify the analytical solution. The analysis considered three different shape factors and a rubber core up to 90% of the width of the isolator. The models were used to determine the elastic moduli as well as the normal and shear stress distributions under pure bending and rotation. The verified analytical solution is an important tool for designers.

Analytical and Numerical Study of the Axial Stiffness of Fiber-Reinforced Elastomeric Isolators (FREIs) under Combined Axial and Shear Loads

Fiber-reinforced elastomeric isolators (FREIs) are rubber-based seismic devices introduced as a low-cost alternative to steel-reinforced elastomeric isolators (SREIs). They are generally used in unbonded applications, i.e., friction is used to transfer the lateral loads from the upper to the lower structure. Under combined axial and shear loads, the lateral edges of the unbonded bearings detach from the top and bottom supports resulting in a rollover deformation. Due to increasing horizontal displacement, the overlap area of the bearing decreases; thus, the vertical properties of the device are a function of the imposed lateral deformation. This paper introduces a closed-form solution to derive the vertical stiffness of the bearings as a function of the horizontal displacement. The variations of the vertical stiffness and of the effective compressive modulus of square-shaped FREIs are given in this work. The analytical results are then validated through a comparison with the outputs of a parametric finite element analysis of FREIs, including different mechanical and geometric parameters.

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.

Experimental testing of full-scale fiber reinforced elastomeric isolators (FREIs) in unbounded configuration

Fiber reinforced elastomeric isolators (FREIs) have shown to be a promising option as an alternative to classical steel reinforced elastomeric isolators (SREIs). Previous investigations were limited to scaled-geometries without any attempt to test such devices under code-compliant protocols as per international standards. In the present study the authors investigated two circular full-scale (diameter 620 mm) FREIs manufactured with a non-standard process adopting a soft rubber compound and polyester fibers. Experimental tests were performed in unbounded configuration through the large anti-seismic device test facility at the EUROLAB of the University of Messina, Italy. A significant number of protocols were imposed to the prototypes in order to demonstrate the effect of different loading conditions, i.e., strain level, frequency of the excitation, axial load and repeated loading. The tests confirmed the significant dependency of mechanical behavior on axial load which tends to increase damping (i.e., higher friction mechanisms) while reducing stiffness (i.e., lower stability limits). Due to internal slippage at the fiber-rubber layers interface, damping capacity of FREIs achieved 20% of critical, i.e., significantly higher than that commonly achieved in SREIs with the same rubber compound. Even if adequate capacity was reached in compression, the run-in effect would limit the axial stiffness of FREIs. A significant roll-over phenomenon was detected under lateral loading up to a maximum shear strain of 100% without damage and permanent deformation after unloading. A numerical study finally demonstrated the effectiveness of FREIs when compared to SREIs in a base isolation system designed for a 3-storey reinforced concrete frame located in a high seismicity region in Italy. Lower axial stiffness of FREIs did not affect the seismic performance of the building due to limited rocking motion component and beneficial higher damping mechanism. This paper provides a significant contribution to the standardization of FREIs to be adopted in base isolation of conventional buildings.

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.

Parametric studies and application of fibre reinforced elastomeric isolators to low-rise buildings

Base isolation systems are efficient in decoupling the building from the ground during earthquakes, leading to a reduction in seismic demand to be experienced by the isolated building. Although Steel Reinforced Elastomeric Isolators (SREIs) have been proven to perform well under earthquakes, they are mostly applied to special buildings and are not a cost-effective solution for low-rise buildings in developing countries like India. Although there are existing studies on Unbonded Fibre Reinforced Elastomeric Isolator (UFREIs), effectiveness of alternative low-cost fibres was not studied. Hence, this paper focuses on parametric studies of UFREI systems with various locally available fibres. Initial dimensions of UFREI required for seismic isolation of a two storeyed framed building were approximated. For these specific dimensions, performance of UFREI numerical models with various low cost locally available fibres were studied and compared with high strength fibres by conducting parametric studies using ABAQUS. Interesting results obtained by varying the strength, thickness and number of fibre layers are discussed in detail. A new parameter, peak stress ratio (Sn) was proposed which helps in deciding the type of fibre to be used in UFREI systems. Based on the observations from these studies, a suitable configuration of UFREI was proposed. Significant improvement in the seismic performance of the base isolated building was observed from the linear static and dynamic analyses.

On the response of fiber reinforced elastomeric isolators (FREIs) under bidirectional shear loads

Fiber Reinforced Elastomeric Isolators (FREIs) have been proposed as an alternative to Steel Reinforced Elastomeric Isolators (SREIs) and several studies have highlighted the advantages of this technology over conventional devices. When FREIs are unbonded to the top and bottom substructure, the bearings exhibit softening under imposed lateral deformations with or without instability depending on the aspect ratio of the device. Research studies available in literature have focused on the response of rectangular bearings when loaded in shear along their base. Very little is known on the lateral response of these devices under bidirectional shear loading. This study aims to assess the response of rectangular FREIs under combined axial and bidirectional shear loads and to show the effects of (i) the secondary shape factor of the bearing, of (ii) the axial pressure, of (iii) the horizontal loading direction on the stability of these devices. As result of a large set of analyses, stability domains have been derived which take into account all of the variables listed above. Based on the results of this work, simple to use design considerations can be derived. Results of this study contribute towards the real implementation of this technology particularly in developing regions of the world.

Recycled versus Natural-Rubber Fiber-Reinforced Bearings for Base Isolation: Review of the Experimental Findings

ABSTRACT Over the last decade, significant efforts have been dedicated to develop novel low-cost seismic protection systems that could be used on a large scale in developing regions of the world. In the attempt of reducing the cost of conventional base isolation devices, the feasibility of using flexible reinforcements to replace the steel sheets of traditional steel-reinforced elastomeric isolators (SREIs) has been validated. It has also been verified that these bearings can be installed in unbonded configurations. Previous research has verified the performance of fiber-reinforced elastomeric isolators in unbounded configuration (U-FREIs) using recycled elastomers. The use of recycled rubber to manufacture U-FREIs is very appealing because of its low-cost and its limited environmental impact. In this paper a thorough comparison of the performance of natural-rubber unbounded fiber-reinforced elastomeric isolators (NU-FREIs) and that of recycled rubber bearings (RU-FREIs) is proposed. To this end, the paper discusses the manufacturing and the main features of the two different technologies, as well as the results of experimental tests on the devices and small-scale structures tested on a shaking-table. Overall, the performance of NU-FREIs was found to be better than that of RU-FREIs. Nevertheless, recycled rubber devices are very promising when it comes to protecting non-engineered constructions from moderate to high seismic inputs at a fraction of the cost of conventional technologies.

Investigation of partially bonded fiber-reinforced elastomeric isolators (PB-FREIs) with nominal vertical tensile loads

Unbonded fiber-reinforced elastomeric isolators (FREIs) were initially proposed as a potential low-cost alternative to conventional steel-reinforced elastomeric isolators (SREIs). FREIs are similar to SREIs but comparatively lightweight as the steel components from SREIs have been replaced with polymer fibers in FREIs. Subsequent experimental investigations identified that unbonded FREIs have desirable characteristics for seismic isolation due to the unbonded application and fiber reinforcement. The unbonded application removes mechanical fasteners, relying on friction to transfer horizontal loads, further reducing the cost. However, the unbonded application also introduces limitations, being susceptible to slip in certain loading conditions and being incapable of resisting tensile forces. In this paper, the concept of partially bonded FREIs (PB-FREIs), a proposed solution to these limitations, is further investigated experimentally with nominal vertical tensile loads. It is shown that PB-FREIs can achieve similar properties to an unbonded FREI with a vertical compressive load.

Method to design fiber-reinforced elastomeric isolators (U-FREIs) and application

This paper proposes a method to design unbounded fiber-reinforced elastomeric isolators (U-FREIs) which, on the basis of the building mass, loads and target isolation period, allows the designer to determine the isolator geometrical dimensions and stiffness characteristics. Many studies in the literature demonstrate that U-FREIs are a viable alternative to Steel Reinforced Elastomeric Isolators (SREIs), making it feasible to employ seismic isolation in residential buildings, even in developing countries. However, currently there are only few references in the literature which give precise directions on how to design U-FREIs to take account of their peculiarities and differences with respect to SREIs. The method proposed herein imposes the fulfillment of three conditions, which take account of U-FREIs peculiar behavior. A step by step procedure developed for square shaped isolators is provided as guide to the application of the proposed design method. A case study is presented, both utilizing the proposed U-FREIs and equivalent SREIs. The differences in the performance of the building, obtained with the two different devices, are discussed.

Effect of shear modulus on the performance of prototype un-bonded fiber reinforced elastomeric isolators

Un-bonded fiber reinforced elastomeric isolator (U-FREI) is light weight and facilitates easier installation in comparison to conventional steel reinforced elastomeric isolators (SREI), in which fiber layers are used as reinforcement to replace steel shims as are normally used in conventional isolators. Shear modulus of elastomer has significant influence on the force-displacement relationship of U-FREI. However, a few studies investigated the effect of shear modulus on the horizontal behavior of prototype U-FREI in literature. In this study, effect of shear modulus on performance of prototype U-FREIs is investigated by both experiment and finite element (FE) analysis. It is observed that reduction in horizontal stiffness of U-FREI with increasing horizontal displacement is due to both rollover deformation (or reduction in contact area of isolator with supports) and shear modulus of elastomer. Reasonable agreement is observed between the findings from experiment and FE analysis.
 Keywords: base isolator; prototype un-bonded fiber reinforced elastomeric isolator; rollover deformation; shear modulus; cyclic test.

A new bi-linear constitutive shear relationship for Unbonded Fiber-Reinforced Elastomeric Isolators (U-FREIs)

Unbonded Fiber-Reinforced Elastomeric Isolators (U-FREIs) are valuable devices that can be used for seismic isolation of buildings. U-FREIs are cheaper than Steel Reinforced Elastomeric Isolators (SREIs), due to the U-FREI’s light weight, ease of installation, and elimination of the need for steel end plates for anchoring to the structure. Moreover, good isolating properties of U-FREIs are demonstrated by a substantial number of publications available in the literature. A model to define a bi-linear constitutive shear relationship for U-FREIs on the basis of given experimental loading cycles is proposed in the paper. The relationship serves to simulate the isolator behavior for use in structural analysis software programs for the design of isolated buildings. The procedure to derive the relationship, provided by the model, is simple to implement. A comparison between results of shear tests and the relationship obtained from the proposed model and from previously existing models is performed. The comparison shows that, among the models considered, the proposed new model provides the most adequate relationship for the description of U-FREIs shear behavior.

Experimental assessment of utilizing fiber reinforced elastomeric isolators as bearings for bridge applications

Abstract Fiber reinforced elastomeric isolators (FREI) can be implemented in a variety of structures, including buildings and bridges. A FREI consists of alternating layers of elastomer and fiber fabric (often carbon fiber). The main role for the fiber reinforcement is to limit the bulging of the elastomer layers, and is the dominant parameter affecting the vertical stiffness. Compared to tradition steel-reinforced elastomeric isolators (SREI), the use of fabric reinforcement could potentially lead to a substantial reduction in both weight and manufacturing costs. To date, FREI have been tested under either vertical loading or combined vertical and lateral loading. Although, these loading scenarios are typically sufficient for building applications, extending the use of FREI to bridges requires an understanding of their behaviour under rotational deformation. This paper presents results from an experimental study conducted to 1) investigate the behaviour of FREI under different types of loads/deformations expected in bridge applications and; 2) determine the viability of FREI as bridge bearings. The test program covered a range of axial loads, lateral displacements as well as angles of rotations. In general, under a different of loading scenarios, U-FREI has shown considerable success in resisting loads and accommodating deformations with no damage/delamination of the isolators observed.

Fiber reinforced elastomeric isolators for the seismic isolation of bridges

Unbonded fiber reinforced elastomeric isolators (U-FREI) have been proposed as a viable alternative to traditional steel reinforced elastomeric isolators (SREI) for use in low-rise building base isolation systems. The viability of U-FREI for this particular application was confirmed through an extensive evaluation of their lateral response under the condition of no rotational deformations of the loading supports. In order to extend the use of U-FREI to bridge applications the consideration of rotational deformation in the analysis is necessary as it is an important component in bridge isolator design. Currently, no data exists in the literature for investigating the influence of rotational deformation on the lateral response of U-FREI, to the best of authors’ knowledge. Accordingly, an experimental and numerical study was completed on U-FREI to investigate their lateral behaviour under a range of vertical loads and rotational deformations in order to determine their suitability as a seismic isolator for bridges. The lateral stiffness and damping were computed experimentally for different levels of vertical pressure, angles of rotations and lateral deformations. Additionally, the resulting stress and strain state within the isolators under peak deformations was also evaluated numerically via 3D modelling and presented in this study.

Seismic vulnerability assessment of a steel-girder highway bridge equipped with different SMA wire-based smart elastomeric isolators

Shape memory alloy wire-based rubber bearings (SMA-RBs) possess enhanced energy dissipation capacity and self-centering property compared to conventional RBs. The performance of different types of SMA-RBs with different wire configurations has been studied in detail. However, their reliability in isolating structures has not been thoroughly investigated. The objective of this study is to analytically explore the effect of SMA-RBs on the seismic fragility of a highway bridge. Steel-reinforced elastomeric isolators are equipped with SMA wires and used to isolate the bridge. Results revealed that SMA wires with a superelastic behavior and re-centering capability can increase the reliability of the bearing and the bridge structure. It was observed that at the collapse level of damage, the bridge isolated by SMA-HDRB has the lowest fragility. Findings also showed that equipping NRB with SMA wires decreases the possibility of damage in the bridge while, replacing HDRB with SMA-HDRB; or LRB with SMA-LRB increases the failure probability of the system at slight, moderate, and extensive limit states.

Effect of different steel-reinforced elastomeric isolators on the seismic fragility of a highway bridge

Seismic fragility assessment of highway bridges is a technique to predict the probability of the structure reaching a certain level of damage under a given seismic excitation. Vulnerable structural components have significant contributions to the failure probability of a bridge system. The effect of elastomeric isolators on the seismic fragility of highway bridges has been discussed in the literature; however, the impact of different types of rubber bearings, including natural rubber bearing, high-damping rubber bearing, and lead rubber bearing has not been investigated yet. The objective of this study is to address this problem in detail. Seismic fragility of isolated bridges is analytically estimated by considering pier and isolation system as two major vulnerable components. Results showed that the isolation system is more fragile than the bridge pier. This finding represents those scenarios where the structure (and especially its seismic isolation system) is designed only according to the usually adopted probabilistic seismic hazard assessment, with a frequently insufficiently too short useful life. The bridge isolated by natural rubber bearing, which has the lowest lateral stiffness and energy dissipation capacity among considered bearings, is the most vulnerable system, and the bridge quipped with HDRB has the minimum risk to undergo damage. Copyright © 2016 John Wiley & Sons, Ltd.