NDM-526: INVESTIGATION OF STABLE AND UNSTABLE FIBER-REINFORCED ELASTOMERIC ISOLATORS

Fiber-reinforced elastomeric isolators (FREIs) are a potentially low-cost alternative to conventional steel-reinforced elastomeric isolators. FREIs can exhibit a non-linear horizontal force-displacement relationship characterized by a softening and stiffening phase, similar to other adaptive isolation devices such as the triple friction pendulum. This non-linear relationship is a consequence of unique deformations that occur during horizontal displacement denoted as rollover, which causes softening, and full rollover, which causes stiffening. The magnitude of the softening due to rollover is primarily governed by the width-to-total height aspect ratio of the FREI. If the aspect ratio is low, below about 2.5, the isolator may be susceptible to horizontal instability where the tangential stiffness becomes negative before increasing due to full rollover. Design codes prevent the use of an isolation system susceptible to horizontal instability within the design displacement. In this paper, experimental testing is used to calibrate a numerical model of a base isolated structure using horizontally unstable and stable FREIs. The performance of the structure is evaluated based on peak displacement of the isolation layer and peak acceleration of the base isolated structure. For the isolators considered, it is shown that the horizontal instability does not have a negative impact on the performance of the structure. It is postulated that some level of horizontal instability may be allowed in the design of unbonded FREIs

Evaluation of Design Equations for Critical Properties of Reinforced Elastomeric Bearings and Recommended Revisions

Global interest in seismic isolation technology continues to increase due to the satisfactory performance of isolated structures in recent earthquake events. In addition to the overall response of the structure, the design of the isolation devices is an important consideration. Reinforced elastomeric bearings are commonly used in seismic isolation applications and as bridge bearings. Relevant standards have been developed to guide designers on several critical design properties, such as the compression and bending modulus and the maximum shear strain due to compression and rotation. These standards commonly assume that the elastomer is incompressible and that the reinforcement is rigid and inextensible. In this paper, inconsistencies in design equations within and between selected standards are identified and discussed. It is shown that the aforementioned assumptions can result in significant error depending on the design and geometry of the bearing. To address these inconsistencies and to minimize the error, new design equations are derived and proposed. The proposed design equations include the compressibility of the elastomer and the extensibility of the reinforcement. The derivation of the proposed design equations is presented and the equations are compared against the analytical solutions and current design standards. It is shown that the proposed design equations are accurate, simple to use, and can address current inconsistencies in critical design properties

Rotation in rectangular and circular reinforced elastomeric bearings resulting in lift-off

Rotation is an important design consideration in reinforced elastomeric bearings (a composite of an elastomer and steel or fiber reinforcement). In certain circumstances, an applied rotation may overcome the hydrostatic compressive pressure due to the applied axial load. If this occurs, a tensile stress will develop in bonded bearings, or lift-off (i.e. the loss of contact between the bearing and the supports) will occur in unbonded bearings. In this paper, analytical solutions are derived to predict the initiation of lift-off or development of tensile stress in infinite strip, rectangular and circular pads including the compressibility of the elastomer and the extensibility of the reinforcement – parameters which are often ignored. The unbonded infinite strip and rectangular pad geometries are further investigated for the impact of rotations exceeding lift-off on the instantaneous geometry and moment-rotation relationship of the bearing. Due to the complexity of the analytical solutions, simplified geometry-specific approximations and graphs are derived that are appropriate for design applications. It is shown that the current code methodology may contain significant error in predicting the initiation of lift-off in unbonded applications or the introduction of a tensile stress in bonded applications

Fiber-reinforced elastomeric isolators: A review

The concept of seismic isolation is well established. Provisions for seismic isolation are gradually being included in building and design codes around the world. Consequently, the quantity of base isolated structures continues to grow globally with favorable performance observed after every significant seismic event. The growth in seismic isolation technology has led to the development of numerous innovative and unique base isolation devices. Although many different types of devices exist, most can be classified as either an elastomeric or sliding isolator. Within the elastomeric classification, steel-reinforced elastomeric isolators are the most common. Fiber-reinforced elastomeric isolators (FREIs) have been proposed as a type of reinforced elastomeric isolators that have distinct performance characteristics. The original intent of FREIs centered on developing a low-cost device appropriate for wide-spread application, particularly in developing countries where the devastation of earthquakes is often more severe. For this reason, the concept has gained significant attention within the research community. This review summarizes the development and current state-of-knowledge of FREIs

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

Comparison of prediction and measurement techniques for pedestrian-induced vibrations of a low-frequency floor

Pedestrian footfalls are often the governing source of vibration on the upper floors of structures. These vibrations may cause discomfort for occupants or interfere with the operation of sensitive equipment. Numerous design guidelines are available to assist structural engineers in achieving vibration serviceability objectives. The current methodology is largely deterministic and does not accurately represent the stochastic nature of pedestrian-induced vibrations or the probability that walkers will generate the predicted response. Statistical analysis is applied to compare long-term vibration measurements in a commercial structure against predictions from relevant guidelines and short-term controlled walking tests. The comparison is used to evaluate the probability that vibration levels predicted by the guidelines and induced through controlled walking tests will be exceeded by typical day-to-day vibrations. The results are important for the development of future fully encompassing probabilistic models of pedestrian-induced vibrations and for aiding architects and engineers in deciding the necessity of potentially costly mitigation measures

Correcting for the influence of bulk compressibility on the design properties of elastomeric bearings

The compression and bending modulus of elastomeric bearings, commonly applied as vibration isolators, are important design considerations. Analytical solutions have demonstrated that the sensitivity to the compressibility of the elastomer can be significant and begins at relatively low shape factors. These analytical solutions, which include the effects of compressibility on the compression and bending modulus, are often complex and not suitable for design purposes. Alternatively, an ad hoc approximation has been recommended that expresses the compression and bending modulus, including the compressibility of the elastomer, by assuming incompressibility and correcting with the bulk modulus. It is demonstrated that the ad hoc approximation provides an unconservatively large value of the compression and bending modulus for infinite strip, square, circular, and annular pad geometries. A correction factor to the ad hoc approximation is determined by expanding and simplifying the analytical solutions. The proposed approximations typically reduce the magnitude of the error while also providing a conservatively lower estimate of the compression and bending modulus

Fiber-reinforced elastomeric bearings for vibration isolation

Fiber-reinforced elastomeric bearings were originally proposed as an alternative to conventional steel-reinforced elastomeric bearings for seismic isolation applications. The flexible fiber reinforcement is a light-weight and potentially cost saving alternative to steel reinforcement which is assumed rigid in the design process. The variety of fiber materials available also serves as an additional parameter for designers to tailor the vertical stiffness of the bearing. In this paper, the analytical solution for the vertical compression modulus of a rectangular elastomeric pad including the effects of bulk compressibility and extensibility of the fiber reinforcement is used to investigate the achievable decrease in vertical frequency. It is shown by an example that the extensibility of the fiber reinforcement can be used to significantly reduce the vertical stiffness in comparison to an equivalent steel-reinforced elastomeric bearing. The resulting decrease in the vertical frequency means that fiber-reinforced elastomeric bearings may have an advantage over steel-reinforced bearings in the vibration isolation of buildings

Retest of Neoprene seismic isolation bearings after 30 years

This paper describes the retest of a set of Neoprene (Polychloroprene) steel reinforced elastomeric isolator bearings that were originally tested as part of an experimental program in 1981. The original bearings were tested on a shake table as a demonstration of the feasibility of base isolation for the seismic protection of buildings. Two types of bearings were provided, and not all were used. The unused bearings were stored unloaded at room temperature for over 30 years. Although there has been a very substantial improvement in the number and quality of the instrumentation available to the test program over the span of 30 years, it is possible to compare the results of relatively similar shake table tests on the bearings. The shake table tests are used to assess the changes in the horizontal stiffness and damping values for the bearings over this period

A review of base isolation systems with adaptive characteristics

Base isolation systems are widely used as an effective and practical solution to protect the structure and non-structural elements from seismic hazards. However, the excessive displacement under severe events may cause damage to the bearing as well as the structure. The growth in seismic isolation technology has led to the development of innovative base isolation systems which exhibit adaptive behavior. The behavior is denoted adaptive when the properties of the device change substantially depending on the loading level. Thus, the response can be tailored to the hazard level based on the softening and subsequent stiffening response and/or changing damping ratio as displacement increases. Recently, the concept of adaptive behavior has gained significant attention within the research community. This review presents the development and current knowledge base of adaptive devices in the absence of active control means. Some types of adaptive devices have the remarkable ability to dissipate the input energy at severe events, which leads to seismic mitigation of the floor acceleration and interstory drift of the superstructure at all hazard levels. Others can effectively reduce the responses at low to moderate earthquake ground motions while limiting the displacement at extreme events