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Rotation Limits for Elastomeric Bearings (2008)

Chapter: Chapter 1 - Introduction and Research Approach

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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Rotation Limits for Elastomeric Bearings. Washington, DC: The National Academies Press. doi: 10.17226/23131.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Rotation Limits for Elastomeric Bearings. Washington, DC: The National Academies Press. doi: 10.17226/23131.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Rotation Limits for Elastomeric Bearings. Washington, DC: The National Academies Press. doi: 10.17226/23131.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Rotation Limits for Elastomeric Bearings. Washington, DC: The National Academies Press. doi: 10.17226/23131.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Rotation Limits for Elastomeric Bearings. Washington, DC: The National Academies Press. doi: 10.17226/23131.
×
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Rotation Limits for Elastomeric Bearings. Washington, DC: The National Academies Press. doi: 10.17226/23131.
×
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Rotation Limits for Elastomeric Bearings. Washington, DC: The National Academies Press. doi: 10.17226/23131.
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31.1 Bridge Bearings Elastomeric bearings have been used in bridges since the late 1950s, and have grown in popularity so they are now the most common type of bridge bearing in all regions of the United States. They are capable of resisting typical bridge loads and accommodating deformations without the use of machined or moving parts, which largely eliminates any need for mainte- nance. This characteristic, together with their economy, has for many years made them an attractive choice. The spread of seis- mic design requirements throughout the country and the fact that elastomeric bearings typically have better seismic perfor- mance than traditional bearing types only adds to their appeal. Elastomeric bearings are often thought of as having a lim- ited load capacity. When loads exceed a certain threshold, de- signers tend to use pot, disk, or spherical bearings instead. One reason is practical: the low stresses presently permitted on elastomeric bearings result in a large bearing for carrying a high load, and the space may not be available for it. Mold- ing a large elastomeric bearing also may pose problems. How- ever, there is no inherent reason why they should not be used for high loads. During the course of the research, the team encountered a laminated elastomeric bridge bearing designed for 16,000 kips service load. However, such large bearings are likely to have limited rotation capacity, and that may ultimately prove more of a limit than the load capacity. 1.2 Bearing Mechanics Elastomeric bridge bearings come in four principal types: • Plain elastomeric pads, used primarily in low-load situations, • Cotton duck pads, made from very closely spaced layers of elastomer and woven cotton. Relatively rigid, they often are equipped with a slider to accommodate horizontal displacements. • Fiberglass-reinforced pads, like cotton duck pads, have the advantage that they can be cut from a large sheet and do not need to be molded individually, but are seldom used today, • Steel-reinforced elastomeric bearings, used for the highest loads. The main focus of this research is on rectangular steel- reinforced elastomeric bearings, as shown in Figure 1.1. Steel plates are bonded with rubber, either natural or polychloro- prene, in alternating layers to form a sandwich. The finished product contains rubber cover on the top and bottom and around the edges, creating a sealed system in which the plates are protected against corrosion. The rubber and steel layers are bonded together by an adhesive that is activated when the rubber is cured. Curing, or vulcanization, is the process of subjecting the raw rubber compound to high temperature and pressure, which both change its chemical structure and cause it to take the shape of the mold. The concepts underlying behavior of laminated elastomeric bearings are quite different from those of conventional struc- tural components made from concrete, metal, or timber. They depend on the fact that elastomers can undergo reversible, elastic deformations that are enormous compared with those of conventional materials, but that fact also brings with it the need for special design procedures. The simplest concept for accommodating expansion or contraction of the girders is to do so through the elastic shear deformations of a plain rubber block. As the bridge expands, the block changes shape from a rectangle to a parallelogram as shown in Figure 1.2. To accommodate the necessary deformations the block would have to be thick, and so it would be unacceptably flex- ible in axial compression; trucks would experience steps in the roadway as they passed from the end of one girder to another. The challenge is thus to stiffen the bearing in compression C H A P T E R 1 Introduction and Research Approach

without losing its shear flexibility. This is achieved by adding internal horizontal reinforcing plates known as shims in the bearing. Behavior under compressive load is illustrated in Figure 1.3. A plain elastomeric pad responds to vertical load by ex- panding laterally and slipping against the supporting surface as shown in Figure 1.3a. The rubber at the top and bottom surfaces of the pad is partially restrained against outward move- ment by friction against the support, but the rubber at mid- thickness is not. This results in some bulging at the edges. The lateral expansion leads to significant vertical deflections. By contrast, the rubber in the laminated pad is largely prevented from such expansion by its bond to the steel plates, and the layers only form small bulges, as shown in Figure 1.3b. Rubber is almost incompressible, so the volume of rubber remains almost constant under load, and the small lateral expansion leads to only a small vertical deflection. The lami- nated bearing is much stiffer and stronger in compression than a plain pad. However, the steel plates do not inhibit the shear deformations of the rubber, so the bearing is still able to undergo the same shear deformations as the plain pad for the purpose of accommodating changes in length of the girders. 1.3 Failure Modes, Analysis, and Design Criteria The elastomer in a bearing experiences large shear strains. These occur for axial load, rotation and shear, and are illus- trated in Figure 1.4. The shear strains can be envisaged by considering regions or elements of the rubber that are rec- tangular when unstressed, but are forced to become parallel- ograms when the bearing is under load. The shear strains caused by axial load and rotation reach their maxima at the same place, namely the very edge of the layer where the rubber is bonded to the plate. Imposed shear deformations cause shear strains that are relatively constant over the whole layer, including the critical end of the plate. If the steel reinforcing plates have square edges and if no cover exists, the shear stress at the corner of the rubber layer is theoretically singular. (The shear stress must be zero on the vertical free surface of the rubber, but most methods of analy- sis predict non-zero shear stress along the horizontal interface between steel and rubber. Yet, for equilibrium, the vertical and horizontal shear stresses must be equal.) In practice, these plates have slightly rounded edges because they are deburred, and rubber cover exists, so the stress is concentrated there rather than being truly singular. Under severe loading, this stress leads to local detachment of the elastomer from the steel. This starts with tensile debonding of the cover from the ends of the plates as shown in Figure 1.5 then propagates inwards along the surface as shear delamination. In this report, the terms debonding and delamination are used to indicate these two different behaviors. Debonding is easy to see on the surface because two bulges merge into one larger bulge, as shown in Figure 1.5. How- ever, distinguishing between local tensile debonding of the cover and shear delamination of the internal layers is diffi- cult. This is unfortunate, because the former has little imme- diate effect on the bearing’s performance, whereas the latter has significant adverse consequences. The shear delamina- 4 Figure 1.1. Cross-section of a steel-reinforced elastomeric bridge bearing. Figure 1.2. Elastomeric plain pad shearing to accommodate girder expansion. (a) (b) Figure 1.3. Bulges without (a) and with (b) steel plates.

tion always is preceded by the tensile debonding, so the one acts as a precursor of the other. The lateral expansion of the rubber layers causes tension in the steel plates. At extreme loads, the plates may fracture, typ- ically splitting along the longitudinal axis of the bearing, as shown in Figure 1.6. With plates of the thickness used in practice, this behavior does not occur until the load has reached five to 10 times its design value, so shim fracture seldom con- trols design. Theoretically the plates could be made thinner and they still would be strong enough in tension, but they would be more flexible in bending. Keeping them plane during molding would then become problematic. Shear delamination is the most important potential mode of failure. If it were to occur in practice, the elastomer would start to extrude from the bearing, which would in turn cause significant vertical deflection, horizontal forces of unpredictable magnitude, and possible hard contact between the girder and the support. While none of these necessarily creates collapse conditions in the bridge itself, any one of them will seriously degrade the serviceability and reduce the performance of the bridge. It is necessary to relate the delamination to a local strain measure, so its onset can be predicted. Prediction of any local deformations is difficult, because elastomeric bearings have three characteristics that make conventional stress analysis invalid: • Rubber is almost incompressible (Poisson’s ratio ≈0.50), • Parts of the bearing undergo very large displacements, so the geometry of the system changes significantly during the loading, and • Rubber obeys a nonlinear stress-strain law. Further difficulties are caused by stress-strain behavior that is often somewhat rate-dependent (for example, visco-elastic) and it changes with cycling as the internal crystal structure changes with loading. The only viable way of predicting inter- nal deformations throughout the entire rubber mass is by using FEA, but even that tool has its own additional challenges, 5 Figure 1.4. Deformations of a laminated elastomeric bearing layer. Tension debonding Shear delamination Figure 1.5. Tension debonding at the shim end and shear delamination at the shim surface. Figure 1.6. Fractured steel plates in bearing.

Common bearings have S in the range 3 < S < 8. The shape factor also provides a useful basis for normalizing the com- pressive stress, since the shear strain caused by compression is, according to small displacement theory, directly proportional to σ/GS. Increasing S therefore increases the axial stiffness and strength, but it reduces the ability of the bearing to accom- modate rotation. These opposite tendencies may cause a dilemma in design. A larger bearing with a higher shape factor would carry the axial load better, but it would reduce the bearing’s ability to accommodate rotations. It is worth noting that such design involves the use of a mixture of force and displacement load- ings and this combination presents challenges. The axial load is a force yet the rotation is a displacement. Designing for both simultaneously requires that the bearing be stiff in compres- sion yet flexible in rotation. That may be difficult, because the features (size, shape factor) that make it stiff in compression also tend to make it stiff in rotation. Rubber is not completely incompressible, and for high shape factor bearings the slight compressibility affects the stiffness in response to both axial load and rotation. It has the greater effect on axial stiffness. The effect can be captured in the small- displacement, linear theory by means of the compressibility index, λ, developed in Stanton and Lund (2006). It charac- terizes the extent to which the slight compressibility of the material affects the stiffnesses, and appears naturally in the closed form equations for them. It is defined by where K = bulk modulus of the rubber, and G = shear modulus of the rubber. Gent’s linear, small-deflection theory has another major at- traction. The assumption of linear behavior implies that the Principle of Superposition is valid, which allows strains from different load cases to be added directly. This vastly simplifies the calculations. Many bearings, such as those in Figure 1.1, support the girder by direct bearing and do nothing to prevent it from lifting off should upward load occur. Such separation of the girder from the bearing is referred to in this report as lift-off. This configuration is common in concrete bridges, for which elastomeric bearings have been used for many years. It may be due to the difficulty of making a tension connection between the concrete and the rubber. Other bearings are fabricated with bonded external plates that permit bolting or welding to the support or girder. These are commonly used in steel bridges, in which the bearings have traditionally been made of steel and for which a tension attachment has been obligatory. With such bearings the girder cannot lose partial contact with λ = S G K 3 (1-3) such as element instability and difficulties with convergence of the iterative calculations. Furthermore, FEA is not an ap- propriate design tool for a component that may cost as little as $100. Gent and Lindley (1959a) pioneered a simplified analysis of laminated bearings that uses small-deflection theory and depends on an assumed displacement field. It provides a re- markably accurate estimate of the strains in the rubber layers up to a point near the edge. It breaks down in the region at the very edge of the shims where the local shear stresses become singular, and it does not address the cover rubber at all, but its relative simplicity does provide the basis for estimating the shear strains near the critical region and the fact that it provides closed-form analytical solutions is beneficial. It has formed the basis for the computations in nearly every specification to date, and was used extensively in this research. The method was originally developed for a completely in- compressible material, for which conventional analysis fails. It treats the lateral deformation of the rubber layers as distrib- uted parabolically through the layer thickness, and depends on calculating the shear stresses in the material from that as- sumed displacement field. It is discussed briefly in Appendix F, and in detail in Gent and Lindley (1959a), Conversy (1967), Gent and Meinecke (1970), and Stanton and Lund (2006). The latter paper provides numerical values for all the coeffi- cients needed to evaluate stiffnesses and strains, for rectangu- lar bearings of all aspect ratios. The behavior can be characterized in terms of the shape factor of the layer, defined by For a rectangular bearing, where L = length of the bearing, parallel to the span of the bridge W = width of the bearing, measured perpendicular to the length. The definition of shape factor given in Equation (1-1) is ex- pressed in terms of the gross bearing dimensions. As described in Section 2.2.4.1, a definition based on effective dimensions was found to correlate better with computed strains, and is used in the design procedures. The shape factor defines the thinness of the layer compared with its lateral dimensions. For an infinitely wide strip bear- ing, W is infinite and S = L/2t. For a square, S = L/4t, and for other rectangular shapes, S lies between those two bounds. S LW t L W = +( )2 (1-2) S shape factor of the elastomer layer load = = ed area perimeter area free to bulge 1-1( ) 6

the bearing. If large rotations occur, one side of the sole plate experiences net upward movement, the bearing necessarily follows and the elastomer experiences direct tension. That is referred to here as uplift. The two configurations give rise to two distinct behaviors. When no tension connection exists between the bearing and girder, the pattern of deformations in the rubber changes after lift-off starts. Prior to lift-off, the system is approximately linear. After lift-off, it becomes a contact problem in which the boundary conditions at the loaded surface change as the loading progresses. The problem is thus inherently nonlinear. From an analysis viewpoint, this creates complications that require the use of approximations if reasonably simple design equations are to be developed. In a bearing with externally bonded steel plates, the bearing is forced to follow the rotation angle dictated by the girder or sole plate, even if it results in local tension stress and strain in the rubber on the tension side. The local tension stress may re- sult in hydrostatic tension, which in extreme cases can cause the rubber to suffer sudden, brittle, internal rupture (Gent and Lindley, 1959b). The process is somewhat analogous to the brittle fracture of a weld if the surrounding metal provides 3-dimensional restraint. Such rupture is likely only under axial uplift loading or when the loading consists of a light com- pressive load and a large rotation. Axial uplift is rare, occur- ring primarily in continuous bridges or in some skew or curved bridges. Light axial load plus large rotation is more common and may occur when the girder is first set and the full camber is still present. That problem is most likely to be encountered in steel bridges, for which the girder self-weight is a relatively small fraction of the final load, and for which a bearing with external bonded plates is more likely to be specified. Prevention of internal rupture should be a design consid- eration. In the existing AASHTO Design Specifications inter- nal rupture is prevented by the provisions that completely disallow uplift or lift-off, so no explicit calculation of the hydro- static tension stress is required. One more potential failure mode exists. The bearing may become thick enough that instability affects their performance. By the standards of steel columns, bearings are very short and squat, and the possibility of buckling appears remote. How- ever, as with other aspects of their behavior, the layered construction and the very low shear modulus of the rubber combine to render conventional thinking invalid. Stability is a potential problem, although only for relatively thick bearings. Analysis is complicated by the shear-flexibility of the layered system, which dramatically reduces the buckling load below the conventional Euler Load value, and because the bearing be- comes thinner so the length of the column reduces as the load increases. The early analysis for shear-flexible systems was per- formed by Haryngx (1948), was considered by Timoshenko and Gere (1961) for helical springs, specialized for linear analy- sis of laminated bearings by Gent (1964), and modified for nonlinear behavior in bearings by Stanton et al. (1990). Stability criteria come into play if design rotations are large. Then the bearing needs to be thick in order to reduce the ro- tation per layer and the corresponding shear strain due to ro- tation. However a bearing that is too thick will become unsta- ble. It also is relevant to the selection of a minimum length for the bearing. Quite often the bearing is made as wide as possi- ble (transverse to the bridge axis) to prevent lateral-torsional buckling of the girder during construction (for example, Mast 1989). Then only a short length is needed to provide sufficient bearing area for supporting the axial load. However, too short a length would again risk instability. In such bearings, the axial stress may therefore be significantly lower than the limit be- cause of the indirect influence of stability requirements. 1.4 Current Design Specifications The AASHTO LRFD Bridge Design Specifications, 4th Edition, Section 14 pertains to bridge joints and bearings and contains two design methods for elastomeric bridge bear- ings: Method A and Method B. Method A is very simple and has tighter limits on stresses, while Method B requires a more detailed design but allows for greater loads. It also is associ- ated with more stringent testing procedures. 1.4.1 Method A Method A specifies that shear modulus of the elastomer should be between 0.080 ksi and 0.250 ksi, and nominal hard- ness should be between 50 and 70 on the Shore A scale, and all other physical properties should conform to ASTM D 4014. The service level axial stress is limited by and where σa = the average axial stress, and G = the shear modulus of the elastomer. (The terminology used here is intended to be consistent within the report, and in some cases differs slightly from that used in AASHTO.) This stress can be increased by 10% if the bearing is fixed against shear displacement. The shear deflec- tion is governed by where hrt is the total thickness of elastomer. To ensure that lift-off is prevented, the rotation and axial stress must satisfy Δ s rt h ≤ 2 (1-6) σa ≤ 1 0. ksi (1-5) σa GS≤ 1 0. (1-4) 7

where θx = the rotation applied to the bearing about the x-axis, hri = the thickness of one rubber layer, and n = number of internal rubber layers Lastly, the length and width of the bearing must each be greater than three times the total thickness to prevent instability. 1.4.2 Method B Method B specifies that shear modulus of the elastomer should be between 0.080 ksi and 0.175 ksi, and nominal hard- ness should be between 50 and 60 on the Shore A scale. All other physical properties should conform to ASTM D 4014. For bearings subject to shear deformations, total axial stress is governed by and The live load stress also is required to be less than 0.66GS. Total load stress limits are increased to 2.0 GS and 1.75 ksi if shear displacement is prevented. The limit for shear displace- ment, Δs, is identical to that in Method A. Combinations of axial load and rotation are governed by the need to prevent lift-off and to avoid excessive shear strain on the compressive side of the bearing. The requirement for prevention of lift-off is based on previous studies. Caldwell et al. (1940) studied fatigue of small rubber coupons, and the results suggest that the rubber fatigues much more readily if the strain changes direction during cycles of load. Roeder et al. (1987) conducted fatigue tests on bearings loaded (separately) in shear and compression, and the present AASHTO Design Specifications are based on those results, at least partly. How- ever, prior to this study, no extensive investigation had been made of the effects of cyclic rotation on elastomeric bearings. The governing equation for prevention of lift-off is and for preventing excessive shear strain on the compres- sive side These two equations bound the axial stress. Any stress/ rotation pair lying between them will neither lift off nor cause excessive local compression. σ θ a ri xGS L h n ≤ − ⎛⎝⎜ ⎞ ⎠⎟ ⎡ ⎣⎢ ⎤ ⎦⎥ 1 875 1 0 2 2 . . (1-11) σ θ a ri xGS L h n ≥ ⎛⎝⎜ ⎞ ⎠⎟1 0 2 . (1-10) σa ≤ 1 60. ksi (1-9) σa GS≤ 1 66. (1-8) σ θ a ri xGS L h n ≥ ⎛⎝⎜ ⎞ ⎠⎟0 5 2 . (1-7) These two equations do not distinguish explicitly between bearings with and without external plates. They do not need to, because the no-uplift condition mandated by Equation (1-10) ensures not only that the stresses on the compressive side can be computed without the need to consider the geometric non- linearity caused by lift-off, but also that hydrostatic tension will never occur in a bearing with external plates. This is a simple and effective solution to avoiding those problems, but it proves to be unduly restrictive in some cases, in which it prevents the engineer from finding a design that meets all the requirements simultaneously. The only options are then to turn a blind eye to some code provisions or to use a different type of bearing. Method B also includes detailed requirements for stability of the bearing. 1.5 Motivation for this Study The present bearing design rules were introduced in the 1st edition of the AASHTO LRFD Specifications, in 1994. They have been modified slightly in the intervening years, but never subjected to wholesale review. During that time, sev- eral drawbacks have come to light: • The equations governing combined loading in Method B prove to be unrealistically restrictive. In particular, the no uplift requirement of Equation (1-10) introduces difficul- ties for construction conditions. In a steel bridge, girder self-weight plus a large camber might result in an axial compressive stress of only 100 psi, plus a rotation of 0.04 radians. To prevent lift-off under these circumstances, the bearing has to be very thick, but it may then violate the sta- bility requirements. Many engineers believe that tempo- rary lift-off under these circumstances is unlikely to dam- age the bearing, and that a bearing with no external plates would in reality provide a good solution, even though it would be disallowed by the present specifications. • In some cases the requirements of Method A allow a bear- ing that would not satisfy the Method B requirements. • In some cases, a design that has been used by a state for many years does not satisfy the AASHTO specification re- quirements, yet it has given good service without problems. • The absolute stress limit of 1.6 ksi and 1.75 ksi and the combined stress equations in Method B impose restrictions that are perceived as unnecessarily severe for design of un- usual or high-load bearings. The existing design rules for combined loadings were de- veloped without the benefit of a rotation testing program, for which resources were unavailable at the time. Investigating the effects of fatigue, and the need to avoid lift-off or uplift in bear- ings, was not possible, so the conservatism in the rules is un- derstandable and appropriate. Because of the cost-effectiveness of elastomeric bearings, and their good service over many 8

years, AASHTO determined that rotation of those bearings warranted further study, with the intent of proposing up- dated design provisions that would address the problems outlined above. 1.6 Previous Studies Elastomeric bearings were developed for commercial use in the 1950s, although references exist to earlier developments, and a good summary of general research and practice up to 1980 is provided in NCHRP Report 248. Further work, on low temperature and experimental studies of fatigue loading under compression, was reported in NCHRP 298 and 325. More re- cently, studies were conducted on low-temperature behavior and on the effects of various materials tests, and were reported in NCHRP 449. Many experimental studies have reported on elastomeric bearing behavior and design. The great majority have ad- dressed compression with or without shear (for example, for seismic isolation applications), but very few have been devoted to rotation. The main reason is believed to be that designing, developing, and building a suitable test appara- tus is difficult and expensive. (The researchers know of one only other machine, in Germany, but it automatically im- poses a shear displacement simultaneously with the rota- tion.) One of the few investigations devoted to rotation was conducted by Lund (2003), who conducted analytical and experimental studies on the rotation stiffness of bearings for the purpose of determining its effect on the lateral sta- bility of long bridge girders during transportation and erec- tion. However, cyclic loading, damage, and fatigue in the bearings were not studied. Considerable effort has been devoted to developing FEA approaches that can address the many difficulties posed by analysis of rubber. The Ogden family of constitutive models (Ogden 1984) provides a versatile basis for modeling rubber, and many authors have conducted analyses of bearings, par- ticularly under compressive loading (for example, Simo and Kelly, 1986; Seki and Fukahori, 1989; Bradley, Taylor, and Chang, 1997; Imbimbo and De Luca, 1998, and so forth). Herr- mann and his research team also developed some composite models that average the stiffness of the rubber and steel rein- forcement over the whole domain in order to permit more economical analyses (for example, Herrmann et al., 1988). 1.7 Survey of Practice At the start of this research study, a telephone survey of states and manufacturers was conducted. It is reported in de- tail in Appendix B and is summarized here. Follow-up visits were conducted to states or manufacturers that had provided responses that were unusual or particularly enlightening. The questions focused on: • How often elastomeric bearings were used, • What design methods were used, • What type of rubber was used (neoprene or natural rubber), • The manufacturer(s) from which bearings were usually purchased, • Any field issues that have occurred, and • Any design problems caused by rotation requirements. Responses were eventually received from 46 states. They re- vealed that elastomeric bearings are the bearings of choice in the overwhelming majority of applications and states. Engi- neers appreciate their economy, but also like the bearings and regard them as forgiving. The majority of states use Method A for design. One common reason is that many bearings are located in freeway overpasses, for which prestressed concrete girders are widely used. The bearing is made (nearly) the same width as the bottom flange of the girder to promote lateral sta- bility of the girder during erection. This criterion essentially decides the dimensions of the bearing, which prove to be large enough to keep the stress relatively low. Method B is unneces- sary for such bearings. The most common design problem reported by the states was with rotation, and particularly with load combinations that include light axial load and large rotation. Some agencies also reported field problems with bearings slipping out of place. Four manufacturers dominate the U.S. market for lami- nated bearings: DS Brown (Ohio), Dynamic Rubber/Cosmec (Texas), Scougal Rubber (Washington), and Seismic Energy Products (Texas). Considerable geographic diversity was found in their sales patterns. Some states did not know which man- ufacturer provided most of their bearings because the supplier is chosen by the main bridge contractor. 1.8 Goals, Scope, and Organization of Report The primary goal of this research was to propose improved design provisions for rotation to be considered for use in the AASHTO LRFD Specifications. To reach this goal, it was nec- essary to conduct physical tests and numerical simulations of bearings, from which a better understanding of their behavior could be obtained. Secondary goals included a review of qual- ity assurance/quality control (QA/QC) procedures for man- ufacturing and installing elastomeric bearings. The details are in Appendices A-G. The main chapters of the report provide summaries of the work described in the appendices. Chapter 2 contains the primary findings of the re- port, the interpretation of them is in Chapter 3, and the con- clusions are found in Chapter 4. 9

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 Rotation Limits for Elastomeric Bearings
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TRB's National Cooperative Highway Research Program (NCHRP) Report 596: Rotation Limits for Elastomeric Bearings explores the elastomeric bearing design procedures suitable for adoption in the American Association of State Highway and Transportation Officials' load and resistance factor design (LRFD) bridge design specifications.

The appendixes to the report include the following:

Appendix A Test Data

Appendix B Survey of Current Practice

Appendix C Test Apparatus and Procedures

Appendix D Test Results Overview

Appendix E Finite Element Analysis

Appendix F Development of Design Procedures

Appendix G Proposed Design Specifications

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