Unbonded Fiber-reinforced Elastomeric Isolators Research Articles

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.

Numerical study on rubber compounds made of reactivated ethylene propylene diene monomer for fiber reinforced elastomeric isolators

AbstractA huge amount of rubber waste coming from tire industry or other sources (eg, weather strips producers) has attracted interest from researchers to investigate the possibility of recycling used rubber for various products, particularly in construction sector. This article presents a series of experimental and numerical investigations to evaluate the feasibility of regenerated synthetic rubber ethylene propylene diene monomer (EPDM) for production of a building seismic isolation system. Four rubber blends using two different sources of regenerated EPDM are considered in this study. The mechanical properties of the recycled rubber under study are evaluated through uniaxial tensile and relaxation tests, considering the accelerated aging effect. Based on several hyperelasticity and viscosity models of rubber available in the literature, several numerical test simulations on rubber compounds and on a prototype of fiber reinforced elastomeric isolators (FREIs) are performed. The test results reveal that the proposed regenerated EPDM compounds can be particularly suitable for the production of unbonded FREIs, especially because they are able to exhibit low/moderate tensile stresses at high‐shear strain. The results also show that not all regenerated rubber blends fulfill the durability requirements of rubber for seismic isolation after accelerated aging tests. The durability of the specimens seems to be mainly affected by the choice of the regenerated rubber source.

A simple strategy to tune the lateral response of unbonded Fiber Reinforced Elastomeric Isolators (FREIs)

The study of Fiber Reinforced Elastomeric Isolators (FREIs) has gained momentum over the last decade for their potential to be adopted directly under the foundation of masonry buildings with no need for costly rigid diaphragms and transfer structures, which are required for the application of conventional base isolation (BI) devices. While the possibility of base isolating masonry structures with no diaphragms at the isolation level is fascinating, this requires FREIs with a tunable response in all 3 directions (i.e., devices that can be engineered to perform in a specific way in each direction of loading). It is known that unbonded FREIs exhibit softening and instability under large displacements. While softening of the bearings is beneficial because it reduces the force demand on structural elements, instability is not. This is particularly true as the unstable branch of FREIs is followed by a sharp hardening response. With the aim of contributing (i) towards the development of tunable FREIs and (ii) towards the development of FREIs with tunable hardening under large lateral deformations, this manuscript describes the findings of a wide set of Finite Element Analyses (FEAs) performed to assess the effectiveness of modifying the stiffness and the lateral response of FREIs through lateral holes. Results of the analyses show that, a simple to implement strategy can be used to tune the lateral response of FREIs to a desired hardening and lateral displacement capacity. The findings of this work constitute a step towards the development of scalable technologies for the isolation of non-engineered buildings in developing regions of the world.

Predicting Stability of a Prototype Unbonded Fiber-Reinforced Elastomeric Isolator by Finite Element Analysis

Unbonded fiber-reinforced elastomeric isolator (U-FREI) is an improved device for seismic mitigation of low-rise buildings. The horizontal force — displacement behavior of U-FREI is nonlinear due to rollover deformation and the horizontal stiffness is a function of both vertical load and horizontal displacement. In this paper, stability of a prototype U-FREI is studied based on the dynamic response utilizing finite element (FE) analysis. Furthermore, the horizontal response of the isolator is also evaluated under the design vertical load and increasing horizontal displacement up to [Formula: see text] [Formula: see text] is the total thickness of rubber layers) to observe the rollout instability.

Effect of Temperature on the Response of Unbonded Fiber-Reinforced Elastomeric Isolators

AbstractOne of the most commonly used types of isolator is the steel-reinforced elastomeric isolator (SREI), but recent research has been completed on the use of fibers as a replacement to the stee...

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.

Hybrid seismic base isolation of a historical masonry church using unbonded fiber reinforced elastomeric isolators and shape memory alloy wires

One of the most promising devices for seismic base isolation of structures is the Unbonded Fiber Reinforced Elastomeric Isolator (UFREI) due to its low manufacturing cost and horizontal stiffness. This paper investigates the possibility of combining UFREIs and shape memory alloy (SMA) wires to increase the energy dissipation capacity of the isolation system for the seismic protection of a historical masonry church. Detailed 3D finite element (FE) analyses are performed to characterize the response of UFREIs under cyclic displacements. The behavior of SMA is simulated through a thermomechanical constitutive model implemented in a user-defined material (UMAT) subroutine available in the software package Abaqus. To reduce the computational effort in non-linear dynamic analyses of large isolated structures, an Abaqus user element (UEL) is developed to represent the 3D behavior of the isolation system proposed in this study. Non-linear dynamic time history analyses are then carried out to evaluate the seismic response of a historical masonry church in different configurations (fixed-base model and model equipped with different base isolation systems) for moderate and severe seismic intensity levels. Numerical results show that the damage observed in the masonry church can be considerably reduced through the insertion of UFREIs. The utilization of SMA wires with a specific pre-strain significantly increases the energy dissipation capacity of the base isolation system and decreases the horizontal displacements of the masonry church.

Experimental assessment and analytical modeling of novel fiber-reinforced isolators in unbounded configuration

Base isolation system is one of the most commonly used technologies implemented around the world for seismic protection of infrastructure. Its objectives are the protection of human life and the reduction of damage to buildings as a result of earthquakes. However, the system is rarely used in developing countries such as Colombia, due to its relatively high costs, including the cost of importing the devices. The development of an isolation system using local technology therefore eliminates the latter expense. This paper focuses on the experimental assessment and analytical modeling of low-cost seismic isolators for low-rise buildings, which represent most construction projects worldwide. Two types of unbonded isolator with a high damping rubber matrix and different reinforcement fibers were employed: carbon and polyester. Scaled prototypes were manufactured and tested under compression and shear loads. Despite the lower mechanical properties of polyester, the results revealed an adequate comparison between the vertical and horizontal properties of the two isolators, with both satisfying minimum required design values. Nevertheless, when taking into account the fact that the price of polyester fiber is one order of magnitude less than of carbon, this seems to be the option with greater potential to be implemented as a low-cost seismic isolation system. Based on the experimental results, an analytical model was proposed to estimate the horizontal stiffness of unbounded isolators, taking into account the reinforcement characteristics, the effective area and the shear modulus of the rubber. In comparison with other formulations, the proposed model was found to be sufficiently accurate to be used in the preliminary design of unbonded fiber-reinforced elastomeric isolators.

Implementation of a simple novel Abaqus user element to predict the behavior of unbonded fiber reinforced elastomeric isolators in macro-scale computations

The need for seismic protection of structures located in earthquake prone areas of developing countries has motivated several investigations of low-cost seismic isolators in recent years. One of the most promising device is the Unbonded Fiber Reinforced Elastomeric Isolator (UFREI): in UFREIs no bonding or fastening is provided between the bearing and top–bottom supports and expensive thick steel plates are not required. The behavior of UFREIs is characterized by rollover and full-contact deformations under horizontal loads: the former decreases the effective stiffness of the isolation system, thus reducing the seismic demand, while the latter plays a key role in generating a hardening phase, thus limiting the horizontal displacement of the isolator under severe earthquakes. Such remarkable advantages highlight a great potential for world-wide UFREIs applications. However, until now there is no representative UFREI model available in structural analysis software codes. In this work, a comprehensive but simple UFREI model is implemented in an ABAQUS user element (UEL), taking into account non-linear, hardening and hysteretic behavior of the bearing: in addition, multiple DOFs are considered to simulate the complex 3D behavior of UFREIs, which is characterized by horizontal and vertical displacements, rotation and torsion. The effectiveness of the UEL model is evaluated performing 3D FE dynamic time history analyses on a rigid slab supported by four different types of UFREI. The results show that the UEL model can reasonably fit the behavior of the detailed FE model, significantly reducing the computational efforts of the analyses. The UEL model proposed in this study can be particularly suitable for 3D dynamic time history analyses of complex base isolated structures.

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.

Two-step advanced numerical approach for the design of low-cost unbonded fiber reinforced elastomeric seismic isolation systems in new masonry buildings

Abstract In developing countries, masonry is generally employed in the construction of residential buildings due to its relatively cheap cost. However, these structures are often provided with inadequate seismic protection. A low-cost base isolation aimed at decreasing the seismic vulnerability of masonry buildings is studied in this work from a numerical standpoint. The studied isolator is an unbonded fiber-reinforced elastomeric isolator (UFREI). With fewer rubber pads than conventional isolators, the UFREI is a cheaper option. A 3D finite element analysis is performed to predict the behavior of the UFREI under large displacements. The isolation system is then implemented into a two-story masonry building prototype, where the 3D model of a single UFREI is substituted by a nonlinear spring and a damper. This simplification decreases the computational costs of the analysis. Seven scaled ground motions are applied to the numerical model to investigate the seismic performance of the isolated masonry building subjected to a maximum considered earthquake. Nonlinear dynamic analyses are performed in Abaqus, taking into account the two horizontal components of the seismic motion. Numerical results show an excellent isolation performance of the system, with a significant reduction of the inter-story drift and a suitable deformation of the UFREIs, as well as demonstrate that the simplified numerical approach adopted is useful for practical design and quick safety assessments.

Seismic rehabilitation of cultural heritage masonry buildings with unbonded fiber reinforced elastomeric isolators (U-FREIs) – A case of study

In order to assess the structural behavior and to evaluate the seismic vulnerability of masonry structures of relevant historical and artistic significance, which is a widespread building type in Italy and in the world, an historical masonry church is analyzed under earthquake loading. Linear and non-linear analyses are performed on the finite element models of the structure. From these analyses it is pointed out that the structure does not behave elastically in its existing condition even when subjected to the frequent design earthquake (81% probability of being exceeded over 50 years). Two traditional rehabilitation methods are studied: the placement of a rigid diaphragm which connects the top of the masonry walls only enclosing the church entrance area and the placement of a rigid diaphragm which connects the tops of all masonry walls. None of the traditional method is sufficient for the structure to survive basic design earthquake (10% probability of being exceeded over 50 years). Hence an advanced seismic retrofit solution using innovative carbon fiber reinforced elastomeric isolators is proposed. The proposed intervention consists in the installation of six Unbonded Fiber-Reinforced Elastomeric Isolators (U-FREI) and six Flat Surface Sliders (FSS) as passive protective devices besides the placement of a rigid diaphragm which connects the tops of all masonry walls. The process of installation of the devices is illustrated. The use of the proposed solution leads to a remarkable enhancement of the seismic response capacities of the structure; indeed a general elastic response under the Basic Design Earthquake (BDE) is attained.

Modeling and Evaluation of a Seismically Isolated Bridge Using Unbonded Fiber-Reinforced Elastomeric Isolators

An unbonded fiber-reinforced elastomeric isolator (U-FREI) is a relatively new type of elastomeric bearing that can be implemented as a seismic isolator for bridges. U-FREIs possess beneficial characteristics, including their potentially low-cost and light-weight construction. Individual U-FREIs can be rapidly produced as they can be easily cut from large sheets to the required size and shape, which is an attractive feature for accelerated bridge construction. The objectives of this paper are to introduce a noniterative analytical model to simulate the lateral response of U-FREI, and then to utilize the model to investigate the seismic response of a typical highway bridge isolated using U-FREI and compare it with a traditional non-isolated bridge. The isolated bridge demonstrated a resilient seismic behavior where all the bridge components remained elastic with no damage or residual deformation. The analytical results indicate that the seismic demand of the isolated bridge is reduced by up to 77% and 84% in terms of base shear and accelerations, respectively, as compared with the non-isolated bridge.

Mitigation of Seismic Vulnerability of Prototype Low-Rise Masonry Building Using U-FREIs

AbstractSeismic vulnerability of a two-story stone masonry building supported on U-FREIs (unbonded fiber-reinforced elastomeric isolators) is evaluated. The prototype building is located in Tawang,...

Vertical and Lateral Behavior of Unbonded Fiber-Reinforced Elastomeric Isolators

AbstractFiber-reinforced elastomeric isolators (FREIs) are relatively lightweight, can be cut to the required size from larger pads, and can be placed unbonded (i.e., unfastened) between the suppor...

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.

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.

Variation of the vertical stiffness of strip-shaped fiber-reinforced elastomeric isolators under lateral loading

A fiber-reinforced elastomeric isolator (FREI) is a relatively new type of isolator that utilizes fiber material for the reinforcing layers. FREIs can be installed in a bonded or unbonded application. In this study, finite element analysis (FEA) is carried out on bonded and unbonded strip-shaped FREIs to investigate the variation in vertical stiffness as the isolator undergoes lateral displacement. The vertical stiffness of the isolators under pure compression obtained by FEA was in good agreement with the predictions of two available closed-form solutions. As the lateral displacement increases, it was observed that for bonded FREIs the vertical stiffness decreases monotonically; whereas, for unbonded FREIs, the vertical stiffness decreased up to 175% shear deformation, where an increase in vertical stiffness was observed. FEA results confirmed that the effective overlap area method provides reasonable estimates for the vertical stiffness of bonded FREIs. It is observed that as the applied vertical stress increases, the vertical stiffness of bonded and unbonded FREIs increases. Finally, the paper shows that under large lateral displacements, bonded FREIs develop large tensile stresses in the regions outside the overlapping areas, while the tensile stresses that develop in unbonded FREIs are very low and confined in a small region.

Shake table testing of un‐reinforced brick masonry building test model isolated by U‐FREI

SummaryThis paper presents a detailed study on feasibility of un‐bonded fiber reinforced elastomeric isolator (U‐FREI) as an alternative to steel reinforced elastomeric isolator (SREI) for seismic isolation of un‐reinforced masonry buildings. Un‐reinforced masonry buildings are inherently vulnerable under seismic excitation, and U‐FREIs are used for seismic isolation of such buildings in the present study. Shake table testing of a base isolated two storey un‐reinforced masonry building model subjected to four prescribed input excitations is carried out to ascertain its effectiveness in controlling seismic response. To compare the performance of U‐FREI, same building is placed directly on the shake table without isolator, and fixed base (FB) condition is simulated by restraining the base of the building with the shake table. Dynamic response characteristic of base isolated (BI) masonry building subjected to different intensities of input earthquakes is compared with the response of the same building without base isolation system. Acceleration response amplification and peak response values of test model with and without base isolation system are compared for different intensities of table acceleration. Distribution of shear forces and moment along the height of the structure and response time histories indicates significant reduction of dynamic responses of the structure with U‐FREI system. This study clearly demonstrates the improved seismic performance of un‐reinforced masonry building model supported on U‐FREIs under the action of considered ground motions. Copyright © 2015 John Wiley & Sons, Ltd.

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.