Elastomeric Bearing Pads Research Articles

Analytical solutions for rollover instability of concrete beams on elastomeric bearing pads

During construction, concrete beams are susceptible to rollover failure when unbraced and supported by elastomeric bearing pads. This failure occurs mainly due to the presence of initial horizontal curvature of the beam and the low torsional restraint provided by the supports. Besides, the uncertainties and variability about modulus of elasticity, initial imperfections and supports conditions can increase the risk of collapse of such elements. In contrast, there is a lack in analytical equations to predict the few published experimental results in this field that present different loading conditions from self-weight. Also, another lack is about recommendations of required pad stiffness for beam stability during construction. Thus, the current research focuses on presenting closed-form analytical solutions to predict the rollover load of beams supported by bearing pads and subjected to different loading conditions. Furthermore, this paper presents recommendations about required pad stiffness for the standard AASHTO bulb-tee beams subjected to self-weight. The variational Rayleigh-Ritz method enabled to determine the analytical solutions, and furthermore, the classical Southwell equation allowed to consider the effect of initial sweep on the beam stability. With this, the required pad stiffness was determined through Monte Carlo simulations. The proposed equations accurately predicted the experimental results available in the literature. The analytical and experimental rollover loads differed by 4.37% and 13.6% for the two studied cases. Besides, Monte Carlo simulations were performed to determine tolerances for initial imperfections.

Rollover stability of precast concrete beams supported by elastomeric bearing pads

Abstract During construction of precast girder bridges, there is the concern on rollover instability of the beams when exclusively supported by elastomeric bearing pads. This type of failure has been the focus of recent research due several accidents reported. However, these studies have not considered the nonlinear behavior of elastomeric bearing pads and the lift-off effect. Therefore, this paper presents a parametric study with a finite element model calibrated with experimental results, reported in the literature, through a nonlinear geometrical analysis and considering that concrete behaves linearly. Besides, the experimental results are compared to simplified approaches which account the pad nonlinear behavior and the lift-off effect by utilizing its secant rotational stiffness. From the results, the difference between the instability load from nonlinear geometrical analysis and experimental results was 8.7 %, and the simplified eigenvalue solution and experimental was 11.4 %, which was the best fit compared to the other existing analytical models. From the parametrical analysis, the instability load was considerably decreased by varying the initial lateral deflection, the skewed pad rotation, top flange width and the span of the beam.

Moment-Rotation Response of Beam-Column Connections in Precast Concrete Structures

This paper provides an experimental investigation on the moment-rotation response of typical moment resisting beam-column connections, employing continuous negative bars consolidated with cast in place concrete over the precast beam and passing through grouted corrugated sleeves into an intermediate column. According to [1], the relative beam-column rotation is highly dependent on the elongation mechanism of the negative bars related to both the embedment length into the grouted sleeves and the development length over the beam end, being also inversely dependent on the vertical distance between the position of the top bars and the centre of rotation at end beam section. The flexural secant stiffness of the moment-rotation response is caused by a sum of the joint opening mechanisms at the beam-column interface and crack propagation within the connection zone, wherein the bond-slip at crack positions occurs prior to the first yielding of the negative bars. Therefore, the semi-rigid behaviour of the beam-column connections is associated with deformation mechanisms that occur at the SLS, but which also affects the global behaviour and stability analysis of precast frames at the ULS.Cruciform tests of full scale beam-column connections were carried out at the Precast Research Centre of the Federal University of Sao Carlos (Brazil), where 6 prototypes were studied varying the detailing of the positive connectors over the concrete corbel. The first pair of connectors employed elastomeric bearing pads with 2 vertical dowel bars, the second pair of connectors employed horizontal joints filled with grout with 2 vertical dowel bars and the last pair of connectors employed positive welded plates. The comparison between the experimental results showed that the smallest secant stiffness, which was obtained from the connector with elastomeric bearing pad, was corresponded to 89% and to 82% of the highest secant stiffness obtained for the connectors with welded plates and grouted joint, respectively. Therefore, the experimental results indicate that the major deformation mechanism within the beam-column connections is mostly dependent on the elongation of the top bars. Finally, a simplified analytical equation has been calibrated against the experimental results of the studied beam-column connections.

Evaluation of the lateral stability of precast beams on an elastic bearing support with a consideration of the initial sweep

This study assessed the lateral behavior and stability of precast concrete beams under elastomeric bearing pad conditions. The lateral instability failure of the beam occurs due to the lateral displacement induced by the self-weight and rotation along the beam combined with a rigid body rotation at the support. Therefore, accounting for all the components of the lateral displacement, an analytical equation that can calculate the critical weight of a beam related to the lateral instability of the beam was derived. Furthermore, this study evaluated the critical lateral movements, including the displacement and rotation, with a variation in the initial sweep in the beam. Finally, an analytical procedure was also developed to evaluate the safety of the precast concrete beam against the lateral instability of the beam with a consideration of the initial sweep. The critical values obtained from the proposed method would be used to establish control values for retaining the lateral and torsional stability of a precast concrete beam on the elastic bearing support.

Rollover instability of precast girders subjected to wind load

The use of longer and deeper precast concrete girders has created concern regarding their rollover instability, particularly during construction. Current design and construction specifications do not provide any specific guidelines that can be used to evaluate the rollover instability of a girder. Therefore, analytical and simplified numerical studies were performed to evaluate the critical wind load, lateral displacement and rotational angle that would induce rollover instability of a girder supported on an elastomeric bearing pad. The influence of the length and section properties of the girder on rollover instability was also investigated. The analytical method proposed in this study can be effectively used for evaluating the lateral behaviour and rollover instability of bridge girders and can also provide management values for securing the lateral stability of girders. The paper also provides a worked problem for a long-span typical European girder to determine critical values with application of the proposed method.

Some remarks on the influence of temperature-variations, non-linearities, repeatability and ageing on modal-analysis for structural health monitoring of real bridges

Structural Health Monitoring (SHM) intends to identify damage by changes of characteristics as for instance the modal parameters. The eigenfrequencies, mode-shapes and damping-values are either directly used as damage indicators or the changes of derived parameters are analysed, such as e.g. flexibilities or updated finite element models. One common way is a continuous monitoring under environmental excitation forces, such as wind or traffic, i.e. the so-called output-only modal analysis. Alternatively, a forced measured external excitation in distinct time-intervals may be used for input-output modal analysis. Both methods are limited by the precision or the repeatability under real-life conditions at site. The paper will summarize several field tests of artificially step- by-step damaged bridges prior to their final demolishment and it will show the changes of eigenfrequencies due to induced artificial damage. Additionally, some results of a monitoring campaign of a healthy bridge in Luxembourg are presented. Reinforced concrete shows non-linear behaviour in the sense that modal parameters depend on the excitation force amplitude, i.e. higher forces lead often to lower eigenfrequencies than smaller forces. Furthermore, the temperature of real bridges is neither constant in space nor in time, while for instance the stiffness of asphalt is strongly dependant on it. Finally, ageing as such can also change a bridge's stiffness and its modal parameters, e.g. because creep and shrinkage of concrete or ageing of elastomeric bearing pads influence their modulus of elasticity. These effects cannot be considered as damage, though they influence the measurement of modal parameters and hinder damage detection.

Collapse of a precast concrete beam for a light roof. Importance of elastomeric bearing pads in the element’s stability

The importance of second order effects has been widely studied in slender concrete structures subjected to compressive stresses, although their effects are not generally taken into account in other fields of structural engineering; this is the case of the overall stability of structures flexible supports, as elastomeric bearing pads. In this paper, the collapse in construction of a 24.9m span precast concrete Y-beam for the light roof of a sports hall is studied. The roof consisted of a series of these beams simply supported at each end by plain neoprene bearing pads. After a worker had climbed up to the last placed element, the element over-turned producing a partial collapse of the light roof. A simplified analysis, without considering second order effects, indicated that there was an over-turning safety factor (OSF) of 2.67. However, a more detailed analysis showed that with the bearing pad arrangement used, rotational stiffness was drastically reduced, causing the second order effects to gain importance. In spite of the OSF the safety of the element is shown not to be adequate. Upon considering the actual flexibility of the bearing and the large rotations, a reduction of the OSF to neutral equilibrium is explained. Sensitivity and probabilistic analyses are conducted in order to investigate the influence of each parameter and the most likely failure cause. A 90° plan rotation of the bearings would increase stiffness, remarkably reducing the second order effects and would achieve an OSF of 3.99 and a reliability index of 7.9.

Calculation Method for Quantifying Axial and Roll Stiffnesses of Rectangular Steel-Reinforced Elastomeric Bridge Bearing Pads

Accurate estimates of bearing stiffnesses are often necessary for bridge design and construction calculations. In the case of steel-reinforced elastomeric bearing pads, the compression stiffness or roll stiffness is sometimes difficult to estimate because of complex deformations of the elastomer. A method of numerical analysis for estimating the axial and roll stiffnesses of bearing pads is presented. To validate the axial calculation method, experimental compression tests were performed on pad specimens constructed in accordance with Florida design standards. A method for calculating roll stiffness is also presented in which the pad is modeled as a grillage of compression-only axial springs. The grillage method was partially derived from roll stiffness data measured in a separate experimental study. The grillage method was demonstrated to accurately capture both the nonlinear moment–rotation behavior caused by liftoff of the girder from the pad and the observed sensitivity of roll stiffness to initial compressive loading caused by self-weight of the girder. Reductions in roll stiffness that are caused by girder slope relative to the pad were also examined.

Long-Term Wireless Structural Health Monitoring of the Ferriby Road Bridge

As part of an effective bridge management system, sensor networks can provide data to support both inspection and assessment. Wireless sensor networks (WSNs) have the potential to offer significant advantages over traditional wired monitoring systems in terms of sensor, cabling, and installation costs as well as expandability. However, there are drawbacks with WSNs relating to power, data bandwidth, and robustness. To evaluate the potential of WSNs for use in bridge management, a network of seven sensor nodes was installed on the Ferriby Road Bridge, a three-span reinforced concrete bridge. Three displacement transducer nodes were placed across cracks on the soffit of the bridge to measure the change in crack width. Three inclinometer sensor nodes were mounted on two of the elastomeric bearing pads to measure the change in inclination of the bearing pads while a final node monitored temperature in the box that contained the gateway. The installation of the WSN is discussed and data from this network is analyzed. Finally, the use of sensor networks to support inspection and assessment is discussed.

Experimental Study of Precast Bridge Columns with Built-In Elastomer

Two one-third-scale precast concrete segmental column models with energy-dissipating plastic hinges were designed and tested on a shake table at the University of Nevada, Reno. Conventional reinforced concrete was used in the plastic hinge of one column (SC-2). In the other model, a built-in elastomeric pad integrated with the footing and a concrete segment constituted the plastic hinge (SBR-1). The purpose of using the pad was to minimize damage while dissipating energy through yielding of the longitudinal bars and deformation of the pad. Both columns were subjected to the same ground motion as was recorded at the Sylmar station during the 1994 Northridge, California, earthquake with increasing amplitudes until failure. Compared with conventional precast concrete segmental columns, both SC-2 and SBR-1 showed superior performance, such as minimal residual displacement and larger energy dissipation. SBR-1 with the elastomeric bearing pad dissipated more energy and was free from damage in the plastic hinge zone.

On the behavior of sole plates in steel girders

Elastomeric bearing systems (elastomeric bearing pads or steel-reinforced elastomeric bearings) are used widely as a low cost option as well as for minimal maintenance. In elastomeric bearing components, the sole plates are important so as to resist not only compression but also flexural stresses due to bending of bridge girders and local stress concentration near the load-bearing diaphragm. In steel box and/or plate girders, the bottom flange and the sole plate may act together in dispersing concentrated stresses. In concrete girders, however, a major role of the sole plate is to spread the stresses into a finite area to make sure that the direct bearing stress acting on the concrete is within the allowable value specified. Although the sole plate is an important component of a bearing system, past studies have been focused on the elastomer or steel reinforcement in the elastomeric bearing system. No specific requirement is provided for the determination of the sole plate thickness in any standard specifications throughout the world. The behavior of the sole plates in the elastomeric bearing system has been investigated by a series of finite element analyses. Based on the analysis results, a simple design equation to determine the sole plate thickness is proposed. Finally, the reliability and rationality of the proposed equation is demonstrated through examples.

Seismic retrofit of spread footings supporting bridge columns with short dowels

A study was undertaken to develop and test a seismic retrofit method for bridge spread footings. Two 0.4-scale footing specimens representing the as-built and retrofitted footings were tested under cyclic lateral loads. The soil flexibility was simulated by elastomeric bearing pads. The test of the as-built specimen showed that the footing failed before the column flexural capacity could be reached. The retrofit included the enlargement of the plan view dimensions and the thickness, addition of top steel, and placement of dowels to connect the existing and new concrete. The column to footing connection was retrofitted with a steel jacket connected to the footing by bolts. The experimental and analytical results indicated that the retrofit reduced footing damage to a negligible level while it increased the flexural strength and deformation capacity of the footing.

Neoprene Bearing Pad Slippage at Louisiana Bridges

Elastomeric bearing pads provide a medium to transfer girder loads to the supporting substructure. Low cost and low maintenance, in comparison with mechanical-type bearings, make elastomeric bearing pads attractive to use. However, some problems have developed with their use. Failure modes for bearing pads include crushing, delamination, and slippage. The most notable of these failure modes is slippage or “walking” out of position. The objective in this paper is to determine the cause of neoprene bearing pad slippage and recommend practical guidelines to remedy the problem. The Louisiana Department of Transportation and Development has experienced bearing pad slippage at some of their prestressed concrete beam bridges. The methods used to investigate this phenomenon included measuring thermal movements and erecting video equipment to monitor bearing pads and traffic simultaneously. In addition, material testing was performed to determine the composition of problematic elastomeric bearing pads. By using ti...

Effect of Bearing Stiffness and Skew Angle on Performance of Precast Concrete Bridge

Reinforced elastomeric bearing pads generally support precast concrete bridge girders. The condition of these bearing pads and the pad-bridge interface defines the support boundary conditions of the bridge super-structure. The interface between bearing pads and bridge girders affects the performance of the superstructure and the substructure. The pads are designed to carry vertical loads and to accommodate horizontal movements of the bridge girders. A study was conducted to determine the effects of various parameters on bridge superstructure performance and to compare the girder performance with simple support conditions corresponding to the current Florida Department of Transportation design. A parametric study on the interaction of the support boundary conditions and bridge girders was performed. The parameters studied included the skewness of the bridge and the effect of the bearing stiffness on the bridge system. A finite element model of a bridge superstructure containing Florida bulb tee 78 girders was created using ANSYS software. The model was subjected to an HL93 truck load according to the AASHTO load and resistance factor design specifications. The results indicate that pads with higher bearing stiffness are beneficial to bridges with higher skew angles. However, the reductions in deflections and stresses are smaller for increasing skew angles.

Effect of Bearing Pads on Precast Prestressed Concrete Bridges

Precast AASHTO concrete bridge I-beams are often supported at the ends by elastomeric bearing pads. The bearing pad-bridge beam interface defines support boundary conditions that may affect the performance of the bridge. In this study, finite-element modeling was used to validate AASHTO bearing stiffness specifications. Stiffness characteristics of the Florida DOT bearing pads were theoretically determined under varying elastomer shear modulus values. Finite-element models of AASHTO Types III and V beams were subjected to simulated static truckloads. Vertical and horizontal spring elements simulating new bearing pads were incorporated at the ends of the beam models. A full section of a bridge on U.S. Route 27 was also modeled, and the results were compared with field tests. In general, the restraint effects of the bearing pads are beneficial to the performance of the beams and the bridge. The beneficial effect, however, is small for new bearing pads and more pronounced under a drastic increase in bearing stiffness due to aging and colder temperatures. Such a dramatic increase in bearing stiffness must be justified if the beneficial elements are to be utilized. Current Florida DOT bearing pads are serving the main purpose of their application, which is to provide minimum horizontal restraint force to the beams while allowing horizontal movement.

Slippage of Neoprene Bridge Bearings

Elastomeric bearing pads are more appropriate than mechanical bearings to support bridge loads for many reasons. Two of the most significant reasons are cost and maintenance. Failure modes for bearing pads include crushing, delamination, and slippage. The most notable of these failure modes is slippage or “walking” out of place. The writers' objective in this paper is to establish a methodology to determine why some neoprene bearings are slipping under bridge girders. The methods used to investigate this phenomenon are discussed. A general survey of state DOTs was performed. The results are presented to show how some states have remedied this problem. During the investigation period, numerous bridge engineers and researchers gave their opinions as to why bearings slip. These theories are presented. This investigation has discovered that bearing slippage occurs on a daily basis.

Validation of AASHTO Bearing Stiffness for Standard Precast Concrete Bridge Girders

Precast American Association of State Highway and Transportation Officials (AASHTO) concrete bridge girders are often supported at the ends of elastomeric bearing pads. The bearing pad-bridge girder interface defines support boundary conditions that may affect the performance of the bridge. In this study, finite element (FE) modeling was used to validate AASHTO bearing stiffness specifications. Stiffness characteristics of the Florida Department of Transportation (FDOT) type bearing pads were theoretically determined under varying elastomer shear modulus values. FE models of AASHTO Type III and V girders were subjected to simulated static truck loads. Vertical and horizontal spring elements, simulating new bearing pads, were incorporated at the ends of the girder models. A full section of a bridge on U.S. Highway 27 was also modeled, and the results were compared with field tests. In general, the restraint effects of the bearing pads were beneficial to the performance of the girders and the bridge. The beneficial effect, however, was small for new bearing pads, and more pronounced under drastic increases in bearing stiffness because of aging and colder temperatures. Such dramatic increase in bearing stiffness may be considered in design. Current FDOT type bearing pads are serving the main purpose of their application, which is to provide vertical support with minimum horizontal restraint force to the girders, thus allowing horizontal movement and rotational movements.

Behavior and Design of Selected Elastomeric Bearing Pads

e general objective of this project was the determination of the behavior of elastomeric bearing pads under monotonic or cyclic compression, cyclic shear and rotation, or combinations of these loading conditions. Two types of bearing pads were tested, random oriented fiber (ROF) pads and Neoprene pads of AASHTO grade. A summary of the test reports supplied with these pads has been included in Appendix A. These pads are intended for general use under stemmed members or beams in the precast concrete industry. The scope of the project can best be defined by describing the parameters

Seismic design of South Brighton bridge

The seismic design of a proposed Christchurch bridge is discussed. Initial design was based on supporting the superstructure on elastomeric bearing pads and steel-cantilever dampers. Dynamic analyses showed that the dampers were relatively ineffective, and that the seismic response was dictated by the characteristics of the bearing pads. Final design, based on a conventional ductile approach was found to be more economical without significant increase to seismic risk. It is shown that the decision to opt for a design incorporating mechanical energy dissipators as opposed to a monolithic pier/superstructure design will not necessarily result in a reduction in seismic response.

Shurun-Urgun Highway Composite Steel Bridge Elastomeric Bearings Under Seismic Force

This study examines the parameters of Elastomeric Bearing (EB) for affecting the seismic design and behavior for Shurun-Urgun Composite-Steel Bridge is located in Paktia province in eastern part of Afghanistan. Laminated elastomeric bearings used in addition to provide connection between bridge superstructure and substructure, reduce the response of bridge during an earthquake by increasing fundamental period of vibration of bridge, and might reduce the acceleration of superstructure and inertia force passed to the substructure. As an analysis has been done using computer models for two type of piers (tall & short) for similar bridge with or without elastomeric bearing pads and a manually calculation for this numerical investigation to determine the magnitude of forces, displacements and deformations. USGS seismic hazard maps for Afghanistan, and referred to AASHTO, FHWA, and Caltrans manuals where needed.