Elastomeric Pads Research Articles

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.

Direct Visualization of Particle Dynamics in Model CMP Geometries

ABSTRACTChemical-mechanical polishing (CMP) involves complex mechanical and chemical interactions between a rough elastomeric pad, the wafer being polished and an abrasive slurry containing sub-micron diameter silica particles. The interaction between these abrasive particles and the wafer surface and their impact upon material removal rates is still unclear. Particle dynamics in a model CMP geometry are studied experimentally in this work. Only the particles that actually interact with, and presumably polish, the model wafer surface are visualized using total internal reflection fluorescence (TIRF). The effect of process properties such as particle diameter, particle material, pad-wafer separation and shear rate upon the particle concentration immediately adjacent to the wafer surface is investigated.

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.

Increased Damping in Cantilevered Traffic Signal Structures

Dynamic characteristics of a traffic signal structure were determined experimentally. Various strategies were investigated to provide additional damping. By decreasing the magnitude and the number of fatigue cycles caused by wind-induced oscillations, the service life is expected to be extended. Several damping devices were investigated and tested. The free-vibration response was used to determine the in- and out-of-plane damping of a full-scale pole with and without dampers. The dampers included elastomeric pads located within the joints, several impact dampers, tuning-mass systems, and a mechanical shock absorber (with a strut) positioned between the luminaire extension and the mast arm. An elastomeric pad located at the pole foundation connection and the shock absorber exhibited the most damping and were considered viable and practical options. The construction characteristics of each damper were noted.

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.

The effects of rail support stiffness on railway rolling noise

Rolling noise from trains is radiated by both wheel and track vibrations, the track being dominant in many cases. The stiffness of the rail fastener system, especially the elastomeric rail pad usually inserted between the rails and the sleepers, has a significant influence on the noise emitted by the track. Railways are increasingly using softer pads to reduce potential damage to sleepers and ballast. Softer pads decouple the rail from the sleeper. This reduces the noise from the sleeper but also reduces the decay of vibration with distance along the rail and hence leads to an increase of the noise radiated by the rail. The paper describes experimental and theoretical work to investigate the influence of pad stiffness on the dynamic and acoustic behavior of track. The vibrational behavior has been measured on a dedicated 36-m section of railway track, with different types of rail pad installed. The results are compared with predictions using theoretical models of the track behavior. This allows the effective pad stiffness and damping to be determined, laboratory measurements also being available for comparison. The attenuation of vertical and lateral vibration along the track is measured in order to estimate the effect on radiated noise.

Impact Loads due to Wheel Flats and Shells

A Finite Element (FE) model of vehicle-track system is employed to duplicate the experiments carried out by British Rail and CP Rail System. The theoretical results of the wheel/rail contact forces, rail-pad forces and strains in the rail showed very good correlation to the experimental data. Extensive results are compared with experimental data in the time domain for through validation of the developed model. The characteristics of the impact loads due to wheel flats and shells are investigated based on the validated FE model. The study shows that the shape and size of flat or shell, axle load, vehicle speed and rail-pad stiffness mainly affect the impact loads. Adding elastomeric shear pads on the wheelset bearing does not reduce the wheel/rail dynamic contact force but it may reduce the dynamic force on the bearing. Reducing rail-pad stiffness to a certain level on a concrete-tie track may significantly reduce the dynamic load and the force transmitted to the concrete tie.

Elastomeric Materials Used for Vibration Isolation of Railway Lines

To attenuate disturbance in residential buildings located along metropolitan railway lines, special types of track bedding have been designed, based on fixation of the rail on floating slabs, resting on elastomeric materials. Laboratory tests on small elastomeric pads show that the energy dissipation, under cyclic loading of constant amplitude, can be suitably represented by a combination of a viscous and an hysteretic damping. A significant strain-rate sensitivity is evident. As a first approximation, the stiffness has been represented as an increasing function of frequency, characterized by an asymptotic value. The frequency range of 0–100 Hz has been investigated. Eight types of elastomeric materials have been tested. Comparison shows that the stiffness increases with the increase of the material density. The analytical model has been checked in two different experiences. (1) A prototype of a floating slab track, 14.8 m long, has been assembled in the laboratory and tested under fixed amplitude and harmonic loads. While the experiments on small pads were restricted to uniform loading conditions, the track prototype was excited by a concentrated load, thus inducing different deformations along the specimen, decreasing with the increase of the distance from the point of loading. (2) An experimental track, 100 m long, made up of the same set of elements of the prototype, has been inserted in a railway line. A series of vibration records has been collected during train passages. Comparisons between measured and computed vibrations were used to provide a specific notion of the relevance in a practical problem of the analytical assumptions. Errors in the evaluation of the peak displacement and of the root-mean-square value are limited to a few percent.

The dynamic behaviour of rail fasteners at high frequencies

The stiffness and damping of rail fasteners, and in particular the elastomeric pads inserted between the rail and sleeper, are important parameters determining the dynamic behaviour of railway track. For noise predictions, the frequency range from about 100 Hz up to 5000 Hz is important. Laboratory measurements are described in which the vertical and lateral dynamic stiffnesses of rail fastener systems have been measured under preload in the range 100–1000 Hz. Extension of the method to higher frequencies is also possible but would require a modified test arrangement. Field measurements are also described, which allow the stiffness and damping to be derived indirectly. Comparisons are made between the two sets of data.

Noise and vibration reduction at elevator cab hitch

An elevator cable hitch plate is isolated for the cab frame crosshead by an elastomeric pad and a mounting plate bolted to the crosshead is disposed between the pad and the crosshead. A stabilizer bracket is fixed to the mounting plate and to the hitch plate. The bracket allows limited vertical movement of the mounting plate and hitch plate relative to each other, but prevents lateral sliding movement therebetween. The bracket thus prevents the hoist cable thimble rods from contacting the edges of the holes in the plates through which the thimble rods pass. Minimal cable-induced vibration and noise is thus produced during operation of the elevator.

Prediction of impact forces using Hertzian contact theory and measured modal structural data

A method is presented for predicting the impact force between a mass of known velocity and a structure using measured frequency response functions to characterise the dynamics of the underlying structure and a Hertzian model of the local contact stiffness at the point of impact. The method is applied by obtaining Laplace domain parameters determined by curvefitting the measured frequency response functions and writing linear state equations to characterise the structure. The non-linear contact stiffness is represented by a two parameter Hertzian model, identified by matching a predicted force pulse with a measured force pulse. It is shown that the non-linear dynamic characteristics of an elastomeric pad at the point of contact are modeled very satisfactorily by static Hertz theory during the deformation period and reasonably well during the restitution period of impact.

Pilling, Abrasion, and Tensile Properties of Fabrics from Open-End and Ring Spun Yarns 1

This study investigates the flex abrasion, stress-strain, and pilling performance of test fabrics woven from open-end and ring spun yams. The propensity of test fabrics to pilling is evaluated after repeated exposure to random tumble pilling (RTP) and the elastomeric pad method (EPM), using scanning electron micrographs. The as sociation between the pilling and other performance properties of test fabrics is also investigated. The SEM results reveal that the degree of pilling and fiber fracture on test samples after specified test intervals is influenced by yarn types and the pilling machines themselves. The first test interval produces a few fiber pull-outs but little pilling. The second interval produces flaking and transverse cracking of fibers and fuzz assemblies. The last two test intervals produce more extensive fiber fractures and fiber entanglements. The mean abrasion, tensile strength, elongation, and pill ratings of the test samples woven from two kinds of yarns are compared using t-tests. In all groups, the test samples woven from ring spun yams for both fabric directions out- perform open-end yams in flex abrasion and filling tenacity. The association between pill ratings and other performance properties investigated indicates that RTP ratings are positively related and EPM pill ratings are negatively related to abrasion and tenacity.

Effect of Varying the Twist Multiplier of Open-End Yarn on Pilling and Other Fabric Properties

Pilling has been one of the major consumer problems of man-made staple yarn products. This study evaluated the pilling performance of 65/35% Dacron/cotton fabrics woven from open-end (OE) yarns by the Random Tumble Pilling Tester Method (ASTM Test Method D3512-82) and the Elastomeric Pad Method (ASTM Test Method D3514-81) at five specified test intervals. The effect of twist level of warp and filling yarns on pilling propensity, flex abrasion, tensile strength, and breaking elongation was also investigated. Pilling produced by the Random Tumble Pilling (RTP) procedure was mainly fiber fuzz and loosely anchored pills; that produced by the Elastomeric Pad Method (EPM) was large scale pills and fiber entanglement which did not wear off readily. ANOVA test revealed that the twist level of warp yarns had a significant influence on thepilling characteristics as evidenced in both pilling tests. Twist level of filling yarns influenced EPMpilling ratings only intermittently. The effect of twist level of warp yarns was highly significant for the warp direction on flex abrasion, tensile strength, and breaking elongation. On the other hand the twist level of filling yarns influencedfilling tensile and breaking elongation. The major findings of the study indicate that pilling performance was significantly influenced by pilling test method and testperiod. The low and intermediate twist levels of OE yarns investigated did not consistently affect the pilling and other fabric properties analyzed.

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

Angular vibration isolator for seat back

Angular vibration isolation is provided for a seat back member in combination with a mechanism for adjusting the seat back angle. Eccentrically mounted cylindrically shaped cam members located at each side of the seat back frame can be adjustably rotated within a guide channel carried by the seat back frame. The cams are positioned at a distance from a pivot axis on the seat frame around which the seat back can tilt in order to adjust the angle of tilt. One of the parallel, opposed flanges which define each of the guide channels is spaced from its respective cam member by a resilient means such as a thick pad of elastomeric material which fills the space between the cam member and said one flange. The elastomeric pads are compressed as the seat back is forced forwardly against the seat occupant's back due to vehicle vibration, thus permitting the seat back to be driven rearwardly while the occupant's position in space remains relatively fixed.

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.