Design Guide for Bridges for Service Life (2013)

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445 10.1 introduction Bearings are a critical element within overall bridge systems. Although they represent only a small part of the overall structure cost, they can potentially cause significant problems if they function improperly or if possible maintenance, retrofit, or replace- ment strategies are not envisioned and well planned at the design stage. Bridge super- structures experience translational movements and end rotations caused by traffic load- ing, thermal effects, creep and shrinkage, wind and seismic forces, initial construction tolerances, and other factors. Bridge bearings are designed and built to accommodate these movements and rotations while supporting required gravity loads, transmitting those loads to the substructure, and providing necessary restraint for the superstruc- ture. Proper functioning of bridge bearings is assumed in the analysis and design of overall bridge systems. Bearing failure or improper behavior can lead to significant changes in load distribution and overall structural behavior that are not accounted for in the design and can significantly affect the superstructure–substructure interaction. This chapter describes various bearing types and provides information concern- ing factors affecting and increasing their service life. Methods for design for service life are discussed along with needs for future inspection, maintenance, and possible replacement. 10.2 beAring tyPeS Many bearing types have been developed, primarily to provide efficient, economical ways to accommodate various levels of load and movement. Each type has certain ad- vantages and potential disadvantages. Table 10.1 identifies the commonly used bridge bearing types discussed in this chapter. 10 BRiDGE BEARiNGS

446 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 10.2.1 Elastomeric Bearings: Plain and Reinforced Elastomeric bearings have become the most common type of bearing in recent years because of their desirable performance characteristics, durability, low maintenance requirements, and relative economy. Elastomeric bearings have no moveable parts. They accommodate movement and rotation by deformation of an elastomeric pad, which can be neoprene or natural rubber. Lateral and longitudinal movements are accommodated by the pad’s ability to deform in shear. These bearings can accommo- date combined movements in both longitudinal and transverse directions, and circular elastomeric bearings have been used to accommodate multirotation requirements. Ex- isting bridges using elastomeric bearings with more than 50 years of very good service performance are reported. Plain, unreinforced elastomeric pads are used for short spans on which loads and movements can be accommodated by a single layer of elastomer. As vertical load and movement requirements increase, thin reinforcing plates are combined with multiple layers of elastomer to form a laminated reinforced elastomeric assembly (see Figure 10.1). Steel and fiberglass reinforcement layers have been used; however, fiberglass is weaker, more flexible, and does not bond as well to the elastomer as does steel reinforcement. As a result, the use of thin steel-plate reinforcement has become more common. Neoprene is the most widely used elastomer, but some states also use natural rubber (Stanton et al. 2004), particularly in colder climates, to meet AASHTO low- temperature requirements. Natural rubber generally stiffens less than neoprene at low temperatures. Neoprene has greater resistance to ozone and a wide range of chemicals than natural rubber, making it more suitable for some harsh chemical environments. The LRFD Bridge Design Specifications (LRFD specifications) (AASHTO 2012) currently provide two design methods for the design of elastomeric bearings: Method A, which is the simpler method and has fewer testing requirements; and Method B, which requires greater design effort and more extensive testing. Method A leads to viable tABLE 10.1. beAring tyPeS General Category Bearing Type Elastomeric bearings Plain elastomeric pads Steel-reinforced elastomeric pads Cotton duck pads Sliding bearings Polytetrafluorethylene Alternative sliding materials High-load multirotational bearings Pot bearings Disc bearings Spherical bearings (cylindrical for unidirectional) Fabricated steel mechanical bearings Fixed pin Rocker or roller expansion

447 Chapter 10. BRiDGE BEARiNGS Figure 10.1. Laminated elastomeric pad. Source: Courtesy (left) D.S. Brown and (right) National Steel Bridge Alliance. Source: Courtesy (left) D.S. Brown and (right) National Steel Bridge Alliance. Figure 10.1. Laminated elastomeric pad. designs for bridges up to 150 ft (Stanton et al. 2004), and Method B is generally used when a reasonable bearing cannot be designed using Method A. The majority of states use Method A (Stanton et al. 2004). 10.2.2 Cotton Duck Pads Cotton duck bearing pads are another type of elastomeric bearings that are occasion- ally used in some states, typically for precast concrete I-girder bridges with span lengths up to the 150- to 180-ft range. Cotton duck pads (CDPs) are preformed elastomeric pads consisting of very thin layers of elastomer (less than 0.4 mm [1/60 in.]) interlaid with cotton or polyester fabric. They are stiff and strong in compression, giving them much larger compressive load capacities than plain elastomeric pads; however, CDP shear deflection capability is very limited. The CDP bearings provide a high stiffness in the direction of applied compressive force and are helpful in limiting problems encoun- tered during construction of heavy girders because of rotational instability, generally observed with other elastomeric bearing types. For large shear strain, CDPs may split and crack or result in girder slip on the CDP. The limited shear deflection capacity is frequently overcome by the addition of a polytetrafluorethylene (PTFE) sliding surface to accommodate large movement. When PTFE surfaces are used, they are often com- bined with stainless steel sliding surfaces, similar to that shown in Figure 10.2. The overall capacities depend on the stiffness and deformation capacity of the CDP and vary from manufacturer to manufacturer. To assure adequate performance from CDP, quality control (QC) testing measures and design recommendations have been devel- oped and incorporated into the LRFD specifications (Lehman et al. 2003).

448 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 10.2.3 Sliding Bearings 10.2.3.1 Polytetrafluorethylene When horizontal movements become too large for elastomeric bearings to reason- ably accommodate in shear, PTFE sliding surfaces can be used to provide additional capacity (see Figure 10.2). They are commonly used to provide movement capability with CDPs, and they are also used to provide for horizontal movement in combina- tion with other bearing systems that internally provide for compression and rotation, such as high-load multirotation (HLMR) pot and disc bearings (Figures 10.3 and 10.4, respectively). They are also used to accommodate large translations and rotations when combined with spherical or cylindrical bearings. PTFE has low frictional characteristics, chemical inertness, and resistance to weathering and water absorption, making it an attractive material for bridge bearing applications. The sliding movement is typically provided by a very smooth stainless steel-plate sliding on a PTFE surface. The stainless steel surface is larger than the PTFE surface so that the full movement can be achieved without exposing the PTFE. The stainless steel is typically placed on top of the PTFE to prevent contamination with dirt or debris. PTFE sliding bearings may be guided, allowing movement in only one direc- tion, or nonguided, allowing multidirectional movement. When PTFE sliding surfaces are combined with elastomeric pads, the elastomeric pad must be designed to accom- modate the shear force that is needed to overcome the PTFE friction resistance. Figure 10.2. Elastomeric bearing with PTFE sliding surface. Source: Courtesy D.S. Brown.

449 Chapter 10. BRiDGE BEARiNGS Sliding surfaces develop a frictional force that acts on the superstructure, substruc- ture, and bearing. As a result, friction is an important design consideration, and the low frictional resistance of PTFE makes it very useful for this application. The coef- ficient of friction of PTFE increases with decreasing temperature and with decreasing contact pressure. It also increases if the mating surface is rough or contaminated with dust or dirt. Proper design, fabrication, and field installation are all essential for proper performance. Plain, unfilled PTFE is the most common material used for sliding bearings. Filled PTFE, with the addition of glass fibers, carbon fibers, or other chemically inert filler reinforcement, is sometimes used. Filled PTFE has significantly greater resistance to wear and creep, but it also has a higher friction coefficient by as much as 25% to 30%. Unfilled PTFE in the form of a woven fabric is occasionally used to provide higher bearing strength, longer wear, and increased creep resistance. Lubrication significantly reduces the coefficient of friction, and dimpling of the PTFE surface has been used as a means to facilitate lubrication. Dimples are spherical indentations (0.32 in. maximum diameter by 0.08 in. minimum depth and covering 20% to 30% of the surface area) that are machined into the PTFE surface to act as reservoirs for storing lubrication. Silicone greases are specified because they are effec- tive at low temperatures and do not attack the sliding material. Dimpled and lubri- cated PTFE has been used in Europe, but in the United States, it has been used only in special cases on large spherical bearings in which a very low coefficient of friction requirement is needed to reduce friction loads on substructures. Dimpled and lubri- cated PTFE demands a routine maintenance plan, as the coefficient of friction will significantly increase as the lubrication material is depleted. This increase in coefficient of friction can have an adverse effect on the service performance of other parts of the bridge system. 10.2.3.2 Alternatives to Plain PTFE Maurer sliding material (MSM) is an alternative sliding material developed in Germany as a better-performing substitute for current PTFE-based sliding material, mainly for high-speed rail applications (Maurer Söhne 2003). The new material is an ultra-high- molecular weight polyethylene that has performed well in recent field applications and experimental testing in Europe, where it is one of the most popular sliding sur- faces in use. MSM was primarily developed to accommodate bridge movements and related wear caused by high-speed trains, which induce high rates of movement due to girder end rotations that result in large accumulated movement over time. Initial specifica- tions required the bearing material to accommodate a rate of movement up to 15 mm/s and provide 80 years of service life. Experimental testing in Europe with dimpled and lubricated specimens subjected to high loading rates has shown MSM to outperform PTFE in regard to compressive strength, coefficient of friction, and rate of wear. But because this material is relatively new, there are no long-term data available. More recently, research conducted under SHRP 2 Project R19A compared coefficient of friction and wear between lubricated

450 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE and unlubricated MSM and plain PTFE specimens at high movement rates. Unlubri- cated specimen tests showed MSM to have significantly greater wear resistance than plain PTFE, but with a greater coefficient of friction. Project R19A testing also com- pared coefficient of friction and wear of a glass-reinforced PTFE, Fluorogold, with plain PTFE and MSM. Like MSM, the Fluorogold material had significantly greater wear resistance, but with a smaller increase in the coefficient of friction. 10.2.3.3 Service Life Design Method for Sliding Surfaces Appendix G provides further information regarding a potential service life design method for sliding surfaces that considers a pressure–velocity factor in determining an effective wear rate for the surface material. The method requires test data to establish material wear characteristics; therefore, its application as a design method will be subject to the availability of sufficient existing test data to establish reliable wear rate curves for different sliding materials. The proposed design provisions are based on research conducted by SHRP 2 Project R19A (Ala et al., submitted for publication). 10.2.4 High-Load multirotation Bearings When design loads and rotations exceed the reasonable limits for elastomeric bearings, HLMR bearings have typically been considered. HLMR situations often occur with longer spans, with curved or highly skewed bridges, or with complex framing, such as with straddle bents. In these cases the axis of rotation or the direction of movement, or both, are either not fixed or may be difficult to determine. HLMR bearings include pot, disc, and spherical bearings, each of which is unique in how it accommodates large loads and rotations. All are fabricated in fixed and expansion versions. The expansion versions accommodate translational movement by means of PTFE sliding elements. Expansion versions may be guided, allowing move- ment in only one direction, or nonguided, allowing multidirectional movement. The following sections describe and compare each HLMR bearing type. 10.2.4.1 Pot Bearings The pot bearing was first developed in Germany in the early 1960s, and its use began in the United States in the early 1970s (Fyfe et al. 2006). The main elements of these bearings include a shallow steel cylinder, or pot, which contains a tight-fitting elasto- meric disc that is thinner than the depth of the cylinder. A machined steel piston fits inside the cylinder and bears directly on the elastomeric disc. Brass rings are used to seal the elastomer between the piston and pot components (see Figure 10.3). Vertical load is carried through the piston of the bearing and is resisted by com- pressive stress in the elastomeric pad. The pad is deformable but almost incompressible in its confined condition and is often idealized as behaving hydrostatically. In practice, the elastomer has some shear stiffness and so this idealization is not completely jus- tified. Rotation can occur about any axis and is accommodated by deformation of the elastomeric pad. Horizontal loads on a pot bearing are resisted by direct contact between the pot wall and the piston.

451 Chapter 10. BRiDGE BEARiNGS Figure 10.3. Pot bearing components. Source: Courtesy D.S. Brown. To achieve satisfactory performance, pot bearings require a high degree of QC in the fabrication and field installation process and an accurate determination of design loads and displacements. Through the years, they have been the most economical and most common HLMR bearing. They have been implemented on bridges throughout the country. 10.2.4.2 Disc Bearings The disc bearing was developed and put into service in Canada in 1970 (Fyfe et al. 2006) and was a proprietary, patented device until recent times. It consists of a hard polyether urethane disc between upper and lower steel plates with a center shear pin device to resist horizontal load (Figures 10.4 and 10.5). The discs are stiff enough to support compressive load, yet can deform to permit rotation. However, rotational stiffness for a disc bearing is several times that of a pot bearing.

452 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Disc bearings are reasonably economical, but widespread use has been limited because of their originally patented and proprietary status, which made them available only from a single source. Now that there are additional bearing manufacturers that can supply disc bearings, their use has increased. Figure 10.4. Disc bearing components. Source: Courtesy R.J. Watson. Figure 10.5. Typical disc bearings. Source: Courtesy R.J. Watson.

453 Chapter 10. BRiDGE BEARiNGS 10.2.4.3 Spherical and Cylindrical Bearings Spherical bearings are used primarily for accommodating large rotations about mul- tiple or unknown axes. Sometimes referred to as curved sliding bearings, spherical bearings permit rotation about any axis; cylindrical bearings permit rotation about one axis. In these bearing types, rotation is developed by sliding a convex metal surface (lower element) against a concave PTFE surface (upper element) (see Figure 10.6). The rotation occurs about the center of the radius of the curved surface, and the maximum rotation is limited by the geometry and clearances of the bearing. Translational move- ment is accomplished by incorporating a flat PTFE sliding surface. Horizontal loads may be partially resisted by the curved geometry of the spherical head; however, large horizontal loads may require additional external restraint. Spherical bearings require highly machined fabrication and are more sensitive to the quality of the initial manufacture and installation than other HLMR bearings. Although they are generally the most expensive HLMR type, their advantage is their ability to accommodate higher gravity loads and rotations. 10.2.5 fabricated Steel Bearings Fabricated steel mechanical bearings have been used for both fixed and expansion conditions (see Figure 10.7) and are the longest-used of any other bearing type. Many existing bridges have these types of bearings, and some states still use them for new construction. When functioning properly, mechanical steel bearings generally provide the closest representation of assumed structural end conditions of all bearing types and transmit loads through direct metal-to-metal contact. Most fixed bearings rely on a pin or knuckle to allow rotation while restricting translational movement. Rockers, rollers, and sliding types are commonly used expansion bearings. Typically, steel bear- ings are expensive to fabricate, install, and maintain, which in part accounts for the popularity of elastomeric bearings. Further, steel bearings typically provide only uni- directional movement. These types of bearings are fully designed by the engineer to accommodate loads, movements, and rotations and can be developed to accommodate large requirements. Figure 10.6. Typical spherical bearing. Source: Courtesy D.S. Brown.

454 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Figure 10.7. Fabricated steel bearings with rocker expansion and fixed conditions. Source: Courtesy HDR Engineering, Inc. Bronze lubricated plate bearings have been used in conjunction with steel bearings to accommodate smaller amounts of movement at expansion ends, but they are not used much today. PTFE sliding surfaces have replaced bronze sliding plates because of a much lower coefficient of friction and lower cost. 10.3 FActorS inFLuencing Service LiFe oF beAringS This section discusses various factors influencing the service life of bearings by using a fault tree analysis approach that first identifies service life issues that generally per- tain to all bearing types. This analysis is followed by specific discussions of service life issues pertaining to individual bearing types. 10.3.1 factors Affecting Service Life of All Bearing types: fault tree Analysis A general description of the fault tree analysis approach for identifying factors af- fecting service life is given in Chapter 1 on general framework. Chapter 2, on system selection, further applies fault tree analysis to identify factors affecting the service life of the overall bridge system, which includes deck, superstructure, and substructure components. This section applies specific parts of the fault tree analysis to identify service life factors that apply to bearings. Figure 10.8 shows an overall fault tree diagram that identifies factors affecting service life for bridge bearings. The diagram identifies factors at descending levels that cause or contribute to service life reduction.

455 Chapter 10. BRiDGE BEARiNGS Figure 10.8. Fault tree analysis for factors affecting service life of bearings. Reduced Service Life of Bearings Of Bearing Element– Related Items Of Bridge System– Related Items Caused by Deficiency Due to Natural or Human-Caused Hazards Due to Loads Due to Production/ Operation Defects Figure 10.8. Fault tree analysis for factors affecting service life of bearings. As discussed in Chapter 2, the first level affecting the service life of a bridge sys- tem is either obsolescence (relating to function or operation) or deficiency (relating to deterioration or damage). In relation to bearings, however, service life issues are typi- cally caused by deterioration or damage, not by obsolescence, so the fault tree moves directly to deficiency. Deficiencies can be either system related (i.e., related to other items within the bridge system or to the layout of the system) or bearing element related (i.e., related directly to bearing element performance). Deficiencies can then be subcategorized to those caused by loads, natural or man-made hazards, or production or operation defects. 10.3.1.1 Deficiency of System-Related Items System-related items whose deficiencies can directly affect bearings are primarily at- tributed to system production or operation defects, as illustrated in Figure 10.9. These factors typically relate to deficiencies in elements, details, and the general layout of the overall bridge system that can adversely affect bearing performance. Other types of production and operation defects are discussed in Section 10.3.1.2.3 as they relate to individual bearing elements.

456 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Deficiency of Bridge System–Related Items Due to Production/ Operation Defects Leaking Deck Joints Inadequate Inspectability Improper Bearing Orientation Inadequate Replaceability Figure 10.9. Bridge system–related deficiencies. 10.3.1.1.1 Leaking Deck Joints Of all system-related deficiencies, leaking deck expansion joints can have the greatest negative impact on bridge bearings. This applies to both open and sealed joints. Open joints, such as finger dams or sliding plate dams, typically allow drainage to pass through and be collected in troughs and below-joint drainage systems. Failure or clogging of these drainage systems allows deck drainage and debris to spill on all bridge elements below, including bearings. Sealed joints, such as compression joints, strip seals, or large modular joints, are intended to prevent deck drainage from spilling through. However, failure or damage to these types of joints can also allow deck drainage to leak through to the bridge ele- ments below. Bearings located below leaking deck joints—particularly those in northern wet climates—are subject to deck drainage, deicing chemicals, and other deck debris, which is a leading cause of deterioration and reduced service life. Drainage and deicing chemicals cause corrosion of exposed steel elements, and debris buildup affects proper rotation and expansion movement. 10.3.1.1.2 Improper Bearing Orientation In skewed, curved, and wide bridges, bearings are subjected to multidirectional move- ments or rotations, or both. Improper bearing orientation and/or inadequate multi- directional movement capacity can lead to higher stresses, wear, and reduced service life.

457 Chapter 10. BRiDGE BEARiNGS Bridges wider than three lanes can experience significant transverse thermal move- ment. Guides and keeper assemblies should be limited to the interior portions of the bridge that do not experience large transverse movements. Bearing details for outer portions on wide bridges should be designed to accommodate transverse movement. In the case of skewed steel bridges, a phenomenon commonly referred to as lay- overs exists during construction. The layovers subject the bearings to rotation of the steel girder about the longitudinal axis of the girder (twisting of the section). This action subjects the bearing to a rotation that is not generally considered in design and subjects the bearing to multirotation. 10.3.1.1.3 Inadequate Inspectability Proper inspection of bearings during their service life is critical for an accurate evalu- ation of performance, wear, or deterioration. Early detection of problems can allow maintenance or repair before more serious conditions can develop. Shallower bearing types can be difficult to properly inspect, particularly when limited headroom prevents close access. Consideration should be given in overall bridge system design to allow access for proper inspection of bearings. 10.3.1.1.4 Inadequate Replaceability Regardless of expected service life, bearings are subjected to severe service conditions and have a high potential for unintended consequences related to improper design, manufacturing, installation, and maintenance that can lead to shorter service lives than other bridge elements. Consideration should be given in the overall bridge sys- tem design to allow for easy replacement of bearings with minimal traffic disrup- tion. AASHTO and the National Steel Bridge Alliance provide recommended bearing details that facilitate replacement (AASHTO/NSBA 2004). 10.3.1.2 Deficiency of Bearing-Related Items Reduced service life of bearings is often caused by deficiencies of individual bearing elements themselves. As illustrated in Figure 10.8, bearing deficiency can be caused by loads, natural or man-made hazards, or production and operation defects. 10.3.1.2.1 Bearing Deficiency Due to Loads Figure 10.10 illustrates load-related factors that affect bearing service life. Loads can be traffic loads (primarily truck loads) or system- dependent loads (primarily due to thermal activity). Each of these load types can result in element damage due to wear and fatigue or overload. Truck traffic applies direct vertical load to bearings, as well as superstructure end rotation and accompanying horizontal translation. Thermal activity applies horizontal and transverse translations and girder end rotation to bearings; however, depending on intended or unintended levels of restraint, thermal activity can also apply a significant horizontal load to bearings.

458 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Due to Loads Traffic-Induced Loads System- Dependent Loads Element Wear/ Fatigue Element Overload Figure 10.10. Load-related factors affecting service life. Truck traffic produces high-frequency, low-amplitude cyclic movement at expan- sion bearings combined with vertical load. Cyclic movement from girder end rotation results in a total cumulative movement that is significantly greater than total cumula- tive movement due to thermal activity. This behavior primarily affects wear of sliding surfaces. Other loads, such as those due to wind, longitudinal braking, or earthquakes, can also apply various vertical and horizontal loads to bearings that must be resisted. All of these loads can ultimately result in bearing element fatigue, wear, or over- load at various levels depending on the specific bearing type and makeup. For exam- ple, elastomeric bearings are subject to element fatigue, and PTFE sliding surfaces are subject to wear. Overload, either from heavy truck loads or large thermal movement, in which the bearings experience greater loads than assumed in design, can also lead to reduced ser- vice life. Overload can result in various forms of damage depending on bearing type. Incorrect assumptions during the design process can also significantly affect the service life of bearings because of system restraint. For example, as described in Chap- ter 8 on jointless bridges, in the case of curved girder bridges, there might not be a point within the structure that can be designated as a point of zero movement. Such an assumption can lead to the use of a fixed bearing type without any capability or allowance for transverse or longitudinal movements. The end result of such a wrong assumption is that the bearing is subjected to actions that cannot be accommodated by the bearing and the development of significant damage to the bearing in the process. Service life applications to specific bearing types are discussed later in this chapter.

459 Chapter 10. BRiDGE BEARiNGS 10.3.1.2.2 Bearing Deficiency Due to Natural or Man-Made Hazards Figure 10.11 illustrates factors affecting service life due to natural or man-made haz- ards. For bearings, hazard-related deficiencies typically pertain to thermal climates, coastal climates, or chemical environments. Thermal climates pertain to cold and wet climates accompanied by snow and ice that result in high usage of deicing chemicals on roadways and bridge decks. Bridge bearings are affected by roadway drainage and salts leaking through expansion joints, or by salt spray rising up from crossed roadways. Coastal climates are climates near the ocean or other saltwater bodies where bridge bearing elements can be affected by airborne salt spray. Chemical environments can include environments near chemical or industrial facilities where corrosive airborne chemicals can affect exposed bearing elements. Ultimately, these climates or environments can result in exposed steel element cor- rosion or degradation of other bearing materials at various levels, depending on the specific bearing type and make up. More specific environmental factors are discussed as they relate to individual bearing types later in this chapter. Figure 10.11. Factors affecting service life due to natural or man-made hazards. Due to Natural or Man- Made Hazards Thermal Climate Coastal Climate Chemical/ Atmospheric Conditions Steel Element Corrosion Bearing Element Degradation

460 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Figure 10.12. Production and operation defects affecting service life. Design/ Detail Fabrication/ Manufacturing Construction Maintenance Improper Placement Debris Buildup Improper Design Parameters Corrosion of Steel Elements Element Flaws Improper Design Clearances Element Damage Due to Production/ Operation Defects 10.3.1.2.3 Bearing Deficiency Due to Production or Operation Defects Figure 10.12 illustrates factors affecting the service life of bearings that are related to production or operation defects. For bearings, these defects can be in any one of four general subcategories: • Design and detail; • Fabrication and manufacturing; • Construction; or • Maintenance. 10.3.1.2.3a Design and Detail Improper bearing design and accompanying details can result in significantly reduced service life. Major base factors include improper design parameters and improper design clearances. Incorrect or improperly computed design values can affect vertical load, move- ment, and rotation, or combinations of these. All bearings must be designed for proper superstructure loads, movements, and rotations. Improper calculation and application

461 Chapter 10. BRiDGE BEARiNGS of these parameters at the design stage can result in bearings that are subject to exces- sive rotation, higher stresses, greater wear, and, ultimately, reduced service life. Clearances within bearing details should permit proper movement and/or rota- tion. Details with inadequate clearances can cause binding that limits proper move- ment or rotation, which results in higher stress, damage, wear, and reduced service life. 10.3.1.2.3b Fabrication and Manufacturing Material or fabrication flaws can lead to performance failure and reduced service life. Proper quality assurance (QA) and QC procedures in accordance with the cur- rent LRFD Bridge Construction Specifications (LRFD construction specifications) ( AASHTO 2010a) must be implemented in the fabrication or manufacturing of bridge bearings to ensure that the completed bearings meet specifications and provide re- quired levels of performance. See Section 10.4.1.2.3b for additional discussion. 10.3.1.2.3c Construction Production and operation defects at the construction stage can be caused by either element damage or improper placement. Protective care must be taken during field construction to prevent damage or contamination to sensitive bearing parts such as sliding surfaces or elastomeric pads or discs. Bearings must be set in the field at proper positions to accommodate installation temperatures and rotations. 10.3.1.2.3d Maintenance Lack of or inadequate bearing maintenance can also lead to reduced service life. Issues typically relate to lack of cleaning, which results in debris buildup below deck expan- sion joints, and steel element corrosion. Other types of maintenance-related issues will be discussed in the following subsections for specific bearing types. Dirt and debris buildup prevents proper bearing movement and rotation. Debris buildup also retains and holds moisture and salt against exposed steel elements, which leads to corrosion if the elements are not cleaned. 10.3.2 Factors Affecting Service Life Unique to Each Bearing Type This section looks at specific service life issues pertaining to the bearing types described in Section 10.2 and addressed within the general fault tree categories described in Sec- tion 10.3.1. 10.3.2.1 Steel-Reinforced Elastomeric Bearings Steel-reinforced elastomeric (SRE) bearings have been in service in the United States for over 50 years, and longer elsewhere in the world, with very good results. They are typically very robust, and of all bearing types they likely have the best chance for achieving a service life greater than 100 years. When properly designed, manufactured, and installed, there is very little that can go wrong, and long-term maintenance require- ments are minimal. In rare instances, certain problems have been observed, most often associated with production or operation defects relating to design and manufacturing.

462 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 10.3.2.1.1 Improper Design In rare occurrences, improper design has led to overloaded pads or pads subjected to excessive lateral movement, causing excessive bulging, splitting, or delamination. The following paragraphs describe various potential failure modes and other issues with elastomeric bearings and how they are addressed within current AASHTO design pro- visions. A significant amount of research has been performed on SRE bearings, and designs that follow recent provisions in the LRFD specifications should adequately avoid these issues as they would affect service life. 10.3.2.1.1a Shear Deformation Elastomeric bearings accommodate longitudinal and transverse expansion and con- traction by shear deformation within the elastomer itself (see Figure 10.13). If the shear displacements of the bearing are large, they may cause some rollover at the acute ends of the layer, which leads to cracking in the elastomer at the end of the top and bottom reinforcement plates. This condition is exacerbated by cyclic loading. Fatigue tests simulating temperature movement, which is low-cycle, high-amplitude move- ment, indicate that keeping the shear strain below 0.5 will prevent this cracking. This is not conservative, however, if the deformation is due to high-cycle loading caused by braking forces or end rotation. It was found that high-cycle fatigue was more damag- ing, and in those cases the maximum shear strain should be limited to 0.10 (Roeder et al. 1990). 10.3.2.1.1b Plate Delamination and Debonding Shear delamination between layers of elastomer and steel reinforcing plates is the most significant potential mode of failure. Large shear strains occur in elastomeric bearings as a result of combined axial load, rotation, and shear, as illustrated in Figure 10.14 (Stanton et al. 2008). Each of these shear strains reaches its maximum value at the same location, namely the very edge of the layer at which the elastomer is bonded to the reinforcing plate. Under severe loading, this condition leads to local detachment and debonding of the elastomer from the plate. Once debonding occurs, the elastomer starts to extrude from the bearing, which in turn causes significant vertical deflec- tion. Further, cyclic shear strain causes additional debonding damage than static shear strain of the same magnitude. Section 14.7.5.3.3 of the LRFD specifications provides Figure 10.13. Shear deformation in elastomeric pad. Source: Stanton et al. 2008.

463 Chapter 10. BRiDGE BEARiNGS updated (2010b) requirements for considering shear strain caused by combined axial load, rotation, and shear displacement and considers an amplification factor of 1.75 for combined shear strains due to cyclic loading caused by traffic. 10.3.2.1.1c Instability Large lateral movement combined with large end rotations result in thick bearing designs. If the bearing becomes thick enough, instability can affect its performance (Stanton et al. 2008). The layered construction and the very low shear modulus of the rubber combine to cause this potential problem. Quite often the bearing is made as wide as possible (transverse to the girder axis), and 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 because of the indirect influence of sta- bility require ments. Section 14.7.5.3.4 of the LRFD specifications provides updated (2010) requirements for stability and limits the average compression stress to half the predicted buckling stress. 10.3.2.1.1d Plate Fracture Lateral expansion of the elastomeric layers causes tension in the steel reinforcing plates (Stanton et al. 2008). At extreme loads the plates could fracture, typically splitting along the longitudinal axis of the bearing. However, in plates of the thickness cur- rently used, this behavior does not occur until the load has reached 5 to 10 times its design value, so plate fracture seldom controls design. Plates could actually be thinner, but they are typically sized in practice to keep them from bending during the molding Figure 10.14. Deformations of a laminated elastomeric bearing layer. Source: Stanton et al. 2008.

464 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE process. Section 14.7.5.3.5 of the LRFD specifications provides requirements for the thickness of steel reinforcement and considers both strength and fatigue. 10.3.2.1.1e Compressive Deflection Instantaneous compressive deflection is limited by the LRFD specifications to avoid damage to deck joints and seals and to avoid additional impact when traffic passes from one girder to the other across a joint. A maximum relative live load deflection across a joint of 1/8 in. is recommended. Section 14.7.5.3.6 of the LRFD specifications provides requirements for compressive deflection. 10.3.2.1.2 Improper Fabrication, Manufacturing, and Installation In addition to proper design, it is imperative that proper manufacturing and installa- tion also be performed, and effective QC during these processes is essential for suc- cessful performance. One of the most commonly reported field problems associated with elastomeric bearings, albeit in only a few instances, has been walking out, or slipping from the original position under the girder. Slippage has occurred primarily in natural rubber elastomeric pads that had paraffin added during the manufacturing process in order to meet ozone degradation requirements established by AASHTO. Neoprene pads have inherently greater ozone resistance and do not need wax additives. Over time, these waxes will bleed to the bearing surfaces and drastically reduce the coefficient of fric- tion between the bearing and its contact surface, which leads to slipping (Chen and Yura 1995). The use of positive attachment to the substructure or superstructure, such as bonding, is recommended to prevent this occurrence. However, caution needs to be exercised in bonding elastomeric pads and contact surfaces in highly skewed steel bridges because of layovers. 10.3.2.2 Cotton Duck Pads CDPs are used by only a few states so their performance history is limited. Like SRE bearings, potential service life issues for CDPs are most often associated with production and operation defects, and CDPs require proper design, manufacturing, installation, and maintenance. For SRE bearings, longitudinal and transverse bridge movements are accommo- dated by shear deformation of the elastomer, and the reinforcement has little influ- ence on the shear stiffness of the bearing. CDPs behave differently. The fabric layers of CDPs are many and closely spaced and result in significantly larger shear stiffness and smaller shear deformation capacity than in other elastomeric bearing types. As a consequence, CDPs tolerate large compressive stresses but limited shear deformation, because interlayer splitting occurs at much smaller shear strains. Also, the large shear stiffness of CDPs can result in slip of the girder on the CDP, which may result in abra- sion and deterioration of the CDP. As a consequence, translational shear strain is lim- ited to only about 10%. That is, the shear deflection is limited to only 1/10 of the total

465 Chapter 10. BRiDGE BEARiNGS CDP thickness. Larger movement requirements with CDPs must be accommodated by the addition of a sliding surface such as a PTFE sliding surface (Lehman et al. 2003). The stiffness and deformation capacity of CDP varies from manufacturer to manu- facturer. Proper QC testing is necessary to assure that the bearing pad provides ade- quate performance. Delamination of elastomer layers or secretion of oil and wax from the CDP are the common serviceability limit states for CDPs. To control delamination, compressive stress limits of 3,000 psi for total dead plus live load and 2,000 psi for live load are recommended. Dynamic or cyclic rotation, which induces uplift or partial separation of the pad from the load surface, may cause delamination and reduced service life. Uplift damage depends on the maximum total rotation, as well as the rotation range caused by the live load variation, and separate rotation limits are provided. The LRFD specifications now include design provisions to ensure the serviceability and durability of CDPs. 10.3.2.3 Primary Sliding Surfaces Factors affecting the service life of bearing sliding surfaces (primary sliding surfaces, not guide bars) most often relate to deficiencies caused by loads, including both traf- fic load from trucks and system-induced (thermal movement) loads. These loads can result in sliding element wear (from cyclic movement) and creep or cold flow (from compressive overload). Deficiencies caused by production or operation defects can also occur, primarily during manufacturing and construction. They typically relate to surface scratching and damage due to inadequate protection during shipping or installation. Surface damage leads to increased wear. These problems can be mitigated by proper protection and inspection during shipping and installation. Inadequate maintenance, particularly below open or leaking deck expansion joints, can allow buildup of dirt and other debris that can cause contamination of slid- ing surfaces and increased wear. Periodic maintenance and cleaning in these areas can mitigate these problems. 10.3.2.3.1 Wear of Sliding Surfaces Plain PTFE is most commonly used for bearing sliding surfaces. It wears under service conditions and may require replacement after a period of time. Low temperatures, fast sliding speeds, high contact pressures, rough mating surfaces, and contamination of the sliding interface increase the wear rate. However, fast sliding speed has been shown to be the more dominant parameter (Stanton et al. 1999). Movement due to tempera- ture change is low-cycle, high-amplitude movement, with a slow movement rate, and produces the least amount of wear. However, movement due to truck load and asso- ciated dynamic effects is high-cycle, low-amplitude movement; its sliding speed can be much faster, by as much as a factor of 10. Wear rates associated with high sliding speeds can be as much as 150+ times greater than wear rates at lower sliding speeds.

466 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Thus, plain PTFE should not be used as a sliding surface for bearings subject to rela- tively high sliding speeds and low temperatures. Relatively thin layers of PTFE, from 1/16 to 3/16 in., are commonly used in the United States, but engineers in other countries often use thicker PTFE layers to accom- modate wear. Woven or glass-filled PTFE surfaces provide much higher overall wear resistance, especially at higher sliding speeds, but these surfaces have higher friction coefficients that must be taken into account in the bridge system design. Dimpled and lubricated PTFE also provides exceptional wear resistance and low friction, but the long-term effectiveness of lubrication is questionable (Stanton et al. 1999). MSM has been shown to provide exceptional wear resistance, but when used in a dry condition (without lubrication), it has a much higher friction coefficient than plain PTFE. When used in a dimpled and lubricated condition, however, its friction coefficient reduces considerably and is more comparable to lubricated PTFE. 10.3.2.3.2 Creep or Cold Flow PTFE may creep (or cold flow) laterally when subjected to high compressive stress and shorten the life of the bearing. The reduction in PTFE thickness may also allow hard contact between metal components. Thus, although the compressive stress should be high to reduce friction, it must also be limited to control creep. PTFE is frequently recessed for one-half its thickness and bonded to control creep and permit larger com- pressive stress. Filled PTFE, which is reinforced with fiberglass or carbon fibers, has significantly greater resistance to creep and is sometimes used to resist creep or cold flow. 10.3.2.4 High-Load Multirotation Pot Bearings In past years, pot bearings have had service life problems most often associated with production or operation defects relating to design and manufacturing. These issues have ultimately resulted in broken internal seals, leakage or extrusion of the elastomer, abraded elastomeric pads, and metal-to-metal contact, which have led some state depart ments of transportation to recommend avoiding pot bearings altogether. How- ever, improved design specifications and tighter manufacturing tolerances developed in the late 1990s have greatly improved overall performance. Current LRFD specifica- tions (AASHTO 2012) now incorporate research findings reported in NCHRP Report 432: High-Load Multi-Rotational Bridge Bearings (Stanton et al. 1999) and address specific past service life issues. The following items have been recommended to miti- gate certain past deficiencies and are now part of standard design and manufacturing practice: • The rotational capacity of pot bearings is limited by the clearance between various elements of the pot, piston, sliding surface, guides, and restraints (Stanton et al. 1999). Inadequate clearances can cause binding between metal components (see Figure 10.15). Clearance requirements are now included in the specifications.

467 Chapter 10. BRiDGE BEARiNGS • The thickness of the elastomeric pad also affects rotational capacity. It is currently controlled by a 15% strain limit on the pad-edge deflection under rotation. This strain value is based more on past practice than research results, but it is believed to be a reasonable value. • Pot bearings are able to sustain many cycles of small rotation better than a smaller number of large rotation cycles. This is believed to be true because smaller ro- tations cause deformation of the elastomer but little slip. Slip caused by larger rotations abrades the surface of the elastomer. In an attempt to accommodate this behavior, pot bearings should be designed for larger minimum rotations that reduce the potential for overrotation. This minimum rotation should reflect in- creased rotation caused by construction tolerances expected in practice. Further, greater emphasis should be placed on calculation of rotations caused by service loads, construction loads, and environmental conditions. • Rotational resistance, wear, and abrasion are significantly reduced with a smooth surface finish inside the pot and on the piston. Metalizing of these interior surfaces for corrosion protection causes a rougher surface that leads to increased dam- age under cyclic rotation; such metalized surfaces should not be used unless they are buffed to a smooth finish. In extremely corrosive environments, stainless steel could be considered. • The failure of sealing rings causing escape of elastomer has been one of the major service life issues with pot bearings in past years. Solid circular cross-section brass rings and multiple flat brass rings have both been used, and each has advantages and limitations. Circular cross-section rings provide a tight seal, but they are sus- ceptible to wear under cyclic rotation. Flat rings appear to be more susceptible to leakage and ring fracture, but they experience less wear. Heavier flat brass rings have been suggested as a means of improving their performance. The performance of circular rings could also be improved if the friction and wear were reduced. Figure 10.15. Critical clearances affecting rotational capacity. Source: Courtesy D.S. Brown.

468 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Currently, both circular cross-section and flat brass rings are permitted in the specifications. Current design and manufacturing in accordance with the LRFD specifications have greatly resolved past issues with sealing rings and leakage of elastomer. • Silicon grease lubrication appears to reduce the wear noted on rings, pot walls, and pistons and is recommended for use. This lubrication does not reduce the ultimate wear of the elastomeric disc. • A relatively small lateral load (5% of gravity load) combined with cyclic rota- tion can dramatically increase the rotational resistance, wear, and leakage of pot bearings. This damage is caused by the piston rim dragging against the pot wall during rotation. Alternative methods of external restraint are recommended for large lateral loads to mitigate this damage potential. • Dirt or contamination in the pot increases wear and abrasion of the rings, pot, and elastomeric disc and increases the potential of elastomer leakage. To mitigate this potential, pot bearings need to be sealed and protected during shipping and installation. Although newer design and manufacturing criteria have improved the overall per- formance of pot bearings in recent years, it is still recognized that this bearing type has internal moving parts subject to wear and abrasion that can lead to reduced service life. Newer requirements for long-term deterioration testing in accordance with the current LRFD construction specifications provide greater assurance of proper performance. 10.3.2.5 High-Load Multirotation Disc Bearings Since their first use in the early 1970s, HLMR bearings have had good performance and few reported field problems. Potential (albeit few) service life problems would most likely be associated with production or operation defects relating to design and manufacturing. Current specifications are minimal regarding design and are generally performance related. Therefore, performance testing and manufacturing QC measures are necessary to verify compliance. Previous research testing of disc bearings for combined load and rotation (Stanton et al. 1999) made several performance conclusions: • Tests showed that rotation of disc bearings is partly accommodated by uplift of the steel plates from the urethane disc, especially if the compressive load is light. This uplift should not result in any problems with fixed bearings, but it could be a concern with sliding bearings, because uplift of the disc produces edge loading on the PTFE sliding surface. To mitigate this potential problem, LRFD specifica- tions limit the edge contact stress on PTFE surfaces, and the LRFD construction specifications require proof load testing. • Tests showed the urethane disc to be somewhat deformed and abraded by cyclic rotations, but the damage was not severe enough to affect the performance of the bearing. LRFD construction specifications require cyclic deterioration testing to confirm long-term performance.

469 Chapter 10. BRiDGE BEARiNGS 10.3.2.6 High-Load Multirotation Spherical Bearings Although a very robust system, HLMR spherical bearings have had service life prob- lems, most often associated with production or operation defects relating to design and manufacturing. The difficulties appear to be attributable to faulty fabrication by manufacturers that were not necessarily first-tier suppliers. These bearing types re- quire very precise manufacturing tolerances to assure proper fit of the curved mating surfaces. Rotational capacity can be set at almost any desired level provided adequate clear- ances are provided. PTFE surfaces may also eventually wear out. Variations in friction with different types of PTFE and under different temperature and load conditions cause variations in behavior that can lead to performance issues. Woven PTFE has often been used with spherical bearings in the United States; dimpled and lubricated PTFE is often used in Canada and Europe. These types of bearings are typically larger than other types and generally require additional space. Spherical bearings are traditionally considered to be the most expen- sive HLMR bearing type, but they are also traditionally considered the most reliable. 10.3.2.7 Fabricated Mechanical Steel Bearings Fabricated mechanical steel bearings have been used for the longest time of any bear- ing type and have the potential for extended service life if properly protected and maintained. Factors affecting service life relate to several categories, including loads, primarily overload, which results in binding or overrotation of rocker bearings; natu- ral or man-made hazards, which result in steel element corrosion; and production or operation defects, specifically due to lack of maintenance. 10.3.2.7.1 Loads Rocker expansion bearings can be designed for wide ranges of movement, but they can have limited potential for accommodating overload. Excessive translation can lead to rockers tipping over, which has been reported in a few instances. 10.3.2.7.2 Natural or Man-Made Hazards Corrosion of steel bearings, particularly those located below open or leaking deck joints in thermal environments, is the highest cause of reduced service life for these bearing types. In these locations, steel bearings are highly susceptible to corrosion from roadway drainage with deicing salts and other dirt accumulation. Coastal cli- mates and other chemical environments are also catalysts for steel element corrosion. Pin, roller, and rocker bearings have direct metal-to-metal contact, which creates an environment of high stress concentration and accumulation of moisture between surfaces. In the areas of contact, any protective coating on the steel is inevitably dam- aged by the relative movement. All of these conditions contribute to corrosion.

470 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Corroded expansion bearings can lock up or freeze, subjecting beams and the substructure elements below to additional load and potential damage. Fyfe et al. (2006) reported that steel rocker and roller bearings, along with older type metal slid- ing plates, are the most susceptible to freezing in position. When bearings are frozen, bridges develop their own provisions for contraction and expansion, such as pier cap cracking or rocking of piers or abutments. Dirt and debris accumulation has caused some rocker-type expansion bearings to move beyond their limits and actually roll over, causing the superstructure to drop sev- eral inches or more. Figure 10.16 illustrates a ratcheting effect reported by Modjeski and Masters (2008) in which rocker bearings at one pier on the Birmingham Bridge in Pittsburgh, Pennsylvania, tipped over because of corrosion and debris accumulation. The rockers had an initial lean, and a leaking expansion joint above caused corro- sion and increasing accumulation of debris on the bearings and under one side of the rockers. A ratcheting effect followed in which additional debris and corrosion material kept accumulating under the rockers, causing additional and increasing tilt. This pro- cess continued until a lateral force or kick developed against the pier cap that caused the pier to move just enough to initiate the final tipping of the rocker bearings. Steel rocker bearings have also performed poorly in seismic events and have been replaced as part of seismic retrofit in many instances. Figure 10.16. Ratcheting effect caused by debris accumulation and corrosion. Source: Modjeski and Masters 2008.

471 Chapter 10. BRiDGE BEARiNGS 10.3.2.7.3 Production and Operation Defects Lack of maintenance has been a contributing factor to reduced service life for steel bearings in many instances. Coating loss, surface corrosion, and debris buildup, all of which lead to reduced service life, can be effectively mitigated by periodic mainte- nance. In locations below open deck expansion joints, particularly in thermal environ- ments in which bearings are highly susceptible to roadway drainage, salt, and debris, periodic cleaning to prevent buildup of deleterious materials is essential. 10.4 oPtionS For enhAncing Service LiFe oF beAringS This section presents available options for mitigating the various bearing service life issues that were identified in Section 10.3. A procedure to select the optimal bear- ing type for given loads, movements, and environmental conditions is presented in Section 10.5. The options for enhancing the service life of bearings address the issues discussed in the fault tree analysis and the specific categories and subcategories of deficiencies that were shown to cause damage or deterioration. Bearing deficiencies can be either related to bridge system deficiencies or related directly to bearing element deficiencies. Deficiencies can then be attributed to those caused by loads, natural or man-made hazards, or production and operation defects. In the discussion of available options for enhancing the service life of individual bearing types, the following general solution categories are included and are addressed as applicable: • Avoidance eliminates or bypasses a particular service life issue. This is a primary consideration and should be incorporated when possible. • Mitigation improves performance through enhanced materials, greater protection, or effective maintenance. When avoidance is not practicable, mitigation techniques should be implemented to improve service life. • Acceptance considers that a particular bearing or bearing component may not be able to achieve a service life equal to the bridge system design service life, even after mitigation to improve its service life, and may eventually need to be replaced. In many instances, the need to provide capabilities and details for easy bearing replacement may still be a necessary consideration because of uncertainties regarding loads, hazards, or production and operation defects that can cause premature service life issues. 10.4.1 general options for All Bearing types 10.4.1.1 Solutions for Deficiencies of System-Related Items Table 10.2 summarizes solutions for various service life issues identified in Sec- tion 10.3.1.1 and briefly identifies what each solution provides and what other con- siderations may still be needed. System-related issues are primarily due to production

472 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE tABLE 10.2. SoLutionS For Service LiFe ProbLemS cAuSed by bridge SyStem–reLAted deFiciencieS: ALL beAring tyPeS Service Life Problem Solution Advantages Disadvantages Leaking deck joints (Bridge systems with deck joints can have deterioration of bridge elements below deck caused by drainage through leaking deck joints carrying deicing salts and other debris.) Avoid by using integral or semi-integral construction at abutments. Fully integral abutment eliminates joints and bearings. Semi-integral eliminates joints. Prevents deterioration due to drainage and chlorides to elements below deck. Requires an understanding of integral abutment behavior and limitations. See Chapter 8 on jointless bridges for detail design provisions. Avoid by using integral construction between superstructure and substructure at piers. Eliminates deck joints, bearings, and the need for future bearing maintenance. At piers, can also avoid sharp skews, or longer spans, or higher profiles. Requires system design approach to accommodate bridge movements. Some systems require posttensioning for integral cap design and construction. May be more costly than isolated construction with bearings. Avoid by using continuous superstructure over piers. Eliminates deck joints and protects bearings below. None. Mitigate by protecting bearings with coatings and maintenance. Coatings on steel surfaces protect against corrosion. Maintenance improves coating life and prevents debris buildup. Requires continual maintenance. Mitigate by repair or replacing leaking joints. Cost-effective solution for existing conditions. Requires continual maintenance. Misorientation of expansion bearings on complex alignments Align bearings along girder chords for curved bridges, in keeping with current recommendations. Align bearings along long diagonal line on deck for skewed bridges. See Chapter 8 on jointless bridges for additional detail discussions and recommendations. Proper orientation of guided expansion bearings prevents wear and binding against restraint mechanisms. Requires understanding of bridge system behavior. Limited access for inspection Provide details and clearances that facilitate bearing inspection and maintenance. Early detection of problems avoids major deterioration or distress. Reduces risk. Possible additional initial cost. Difficulty in replacing deteriorated bearings Provide details that facilitate jacking and replacement under traffic. Minimizes impact to traveling public during rehabilitation. Facilitates speed of replacement. Additional initial cost is higher but saves greater future cost when replacing bearings.

473 Chapter 10. BRiDGE BEARiNGS or operational defects and, more specifically, due to design and details of components within the overall bridge system. Further discussion relating to each issue follows this table. 10.4.1.1.1 Leaking Deck Joints The following are possible solutions for avoiding or mitigating bearing damage or deterioration caused by leaking deck joints. In considering options for improving ser- vice life, avoiding deck expansion joints by using continuous systems and/or integral systems offers the best opportunity. When possible, integral systems that eliminate bearings entirely should also be considered, but in most cases this will not be feasible or practical, and the use of bearings cannot be avoided. • Integral abutment systems that avoid deck joints and bearings. Refer to Chapter 8 on jointless bridges for general design guidelines. This has become a popular ap- proach by many states for improving bridge service life. Fully integral abutment construction eliminates both deck joints and bearings at abutments and is also lower in initial cost. Semi-integral construction eliminates deck joints. • Proper use of integral abutments requires an understanding of pile and cap be- havior and limitations regarding bridge length and geometrics such as curvature and skew. • Integral pier systems that avoid deck joints and bearings. See Chapter 2 on system selection for a more detailed discussion of integral pier options. • Bridge systems using integral girder–pier cap construction have been used occa- sionally by some states to accommodate vertical clearance issues, to avoid sharp skews, or to develop frame action for seismic design. But they can also serve to eliminate joints, bearings, and associated future maintenance. Although there is a higher initial structure cost, savings are realized with integral pier caps in lower approach fills and lower long-term maintenance. • Cast-in-place, posttensioned integral bent caps have been used for many years, but they are not widely employed. The Tennessee Department of Transportation constructed its first application in 1978, with several others since, and all have per- formed well. The concept allows main longitudinal girders to pass directly through the pier’s cap rather than over the top in the traditional manner. • Integral caps can also maximize column efficiency. Frame action in the longitudinal direction reduces column design moments at column bases compared with conven- tional cantilever columns and can also enhance seismic performance. However, it must also be considered that integral pier columns may need to resist additional longitudinal moments due to live load, and thus a system analysis is required. • When multiple piers are made integral with the superstructure, expansion must be accommodated by flexure of the pier columns. Tall, slender columns are best suited for this type of construction because their greater flexibility can accommo- date temperature movement with less force developed.

474 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE • Continuous superstructure systems that avoid or minimize the number of deck joints include fully continuous, continuous for live load, or continuous deck slabs (link slabs). These continuous systems can be combined with either integral abut- ments or conventional abutments. Bearings are still used at piers, but they are protected by continuous superstructure. • Mitigate for bearings below deck joints. When bearings must be located below deck expansion joints, mitigation procedures should be implemented. Depending on the environmental severity, all exposed steel bearing parts, which include sole plates, masonry plates, guides, and anchor bolts, should be stainless steel, galva- nized, or metalized. These areas should also be cleaned periodically as part of a regular maintenance program to prevent salt and debris buildup. • Mitigate by repairing or replacing leaking joints. In existing bridges with deck joints, a cost-effective strategy is to repair or replace joints when they start to leak, which would prevent any deterioration below the joint. This proactive approach to preventative maintenance requires periodic inspection with immediate repair or replacement when necessary. 10.4.1.1.2 Improper Expansion-Bearing Orientation Proper orientation of bearings in curved and skewed bridges, which are subjected to multidirectional movements and/or rotations, should be provided. Improper bear- ing orientation and/or multidirectional movement demand can lead to higher stresses, damage, wear, and reduced service life. • Curved girder bridges do not expand and contract along girder lines. A typical approach is to assume movement to occur along chord lines from the fixed point to expansion points (see Figure 10.17). See Chapter 8 on jointless bridges for ad- ditional discussion on determining the point of zero movement or fixed point in integral abutment bridges. Figure 10.17. Recommended bearing orientation on a horizontally curved alignment. Source: Courtesy National Steel Bridge Alliance.

475 Chapter 10. BRiDGE BEARiNGS • For large skew bridges, one approximate solution is to consider the major axis of thermal movement along the diagonal from the acute deck corners caused by thermal movement of the bridge deck. Bearing alignment should be parallel to that axis (see Figure 10.18). It is necessary for expansion bearings to have multidirec- tional capabilities. • Bridges wider than three lanes can experience significant transverse thermal move- ment. Guides and keeper assemblies should be limited to the interior portions of the bridge that do not experience large transverse movements. Bearing details for outer portions on wide bridges should be designed to accommodate transverse movement combined with longitudinal movement. 10.4.1.1.3 Proper Access for Bearing Inspection Details, particularly sufficient room and access at the tops of piers and abutments to permit proper bearing inspection, should be provided. Early detection of bearing-re- lated issues can allow maintenance or repair before major service life–reducing events can develop. When possible, shallow bearings should be placed on pedestals at the tops of piers and abutments in order to provide greater clearance for inspection. Use of thicker sole plates can also help provide greater clearance. 10.4.1.1.4 Proper Capability for Bearing Replacement Details that will accommodate the potential need for replacement of all or part of the bearing system should be provided with the following considerations: • Jacking locations should be provided at every girder. An alternative is to provide for jacking under a diaphragm that lifts adjacent girders simultaneously. • Bearing attachment details should allow ease of replacement. If the bearing is un- attached, it can easily be pulled from its position when the load is removed. Welds Figure 10.18. Recommended bearing orientation for large skew bridges. Source: Courtesy National Steel Bridge Alliance.

476 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE can be cut, but doing so requires equipment that may be cumbersome in the space available. Grinding may also be needed to produce a surface flat enough for in- stalling the new bearing. Any anchor bolts should be placed so that they do not impede the removal of the bearing. • Bearings should be detailed so they can be replaced with only ¼ in. of jacking to avoid causing a bump at the top of the deck that would affect traffic. The details also need to provide adequate vertical clearance for jacking. Jacking points on the structure should be designed to accommodate both dead load and live load in order to be able to maintain traffic during bearing replacement. (Actual jack- ing loads may have to include a factor applied to the design load to break loose the component before lifting.) Jacking for elastomeric bearings needs to consider the effect of compressive dead-load strain in determining the amount of lift. 10.4.1.2 Solutions for Bearing-Related Deficiencies 10.4.1.2.1 Deficiencies Due to Traffic- and System-Dependent Loads Service life issues can be avoided by determining truck traffic–induced cyclic move- ment for consideration in particular bearing type designs or by determining proper levels and combinations of load, movement, and rotation. 10.4.1.2.2 Deficiencies Due to Natural or Man-Made Hazards The potential for steel element corrosion should be avoided by using stainless steel or mitigated by high-performance protective coatings (galvanizing or metalizing). 10.4.1.2.3 Deficiencies Due to Production or Operation Defects 10.4.1.2.3a Design- and Detail-Related Deficiencies Design should take into account proper levels and possible combinations of load, movement, and rotation in order to avoid problems due to excessive stress, transla- tion, or overrotation. A considerable number of bearing failures are attributed to im- proper allowance for displacements. Construction tolerances and possible construction loadings and how they might affect bearing loads and movements should also be considered. It is recommended that the worst possible combination of loads and displacements due to construction toler- ances be considered. In other words, design displacements should include the largest sum of displacements caused by the worst out-of-level placement of piers and abut- ments, the largest camber and deflection, and the most adverse tolerances permissible in the construction of the bridge and bearing. In Steel Bridge Bearing Design and Detailing Guidelines (AASHTO/NSBA 2004), Appendix A provides recommendations for calculating beam rotations for dead load and live load conditions. There is great variation in the methods used in the industry

477 Chapter 10. BRiDGE BEARiNGS for calculating live load rotations, and the Guide was developed according to the methods used in several states. A realistic approach for computing beam rotations is presented in the NSBA guidelines. Adequate clearances should be provided for horizontal movement and rotation to prevent binding, wear, or damage to restraining devices, anchor bolts, or internal bearing elements. Adequate widths and clearances at pier and abutment bridge seats should be provided to allow for required movement and rotation. 10.4.1.2.3b Fabrication- and Manufacturing-Related Deficiencies Proper QA-QC procedures in the fabrication or manufacturing of bridge bearings are essential to assure that the completed bearings meet specifications and provide the required levels of performance. The current LRFD construction specifications provide minimum requirements for packaging, handling, and storage; fabrication tolerances; materials; and general performance. Testing requirements include material certification tests, material friction tests, dimension checks, clearance tests, bearing friction tests, long-term deterioration tests, proof load tests, and horizontal force capacity tests. The specifications require that manufacturers provide certification that each bearing satis- fies the requirements of the contract drawings and construction specifications. 10.4.1.2.3c Construction-Related Deficiencies Care and protection during field construction should be provided to prevent damage to protective coatings or damage to or contamination of sensitive bearing parts, such as sliding surfaces or elastomeric pads or discs. Proper QA-QC procedures should be provided during construction to assure that bearings are initially set with specified position, clearances, and rotation. 10.4.1.2.3d Maintenance-Related Deficiencies Periodic maintenance cleaning should be provided depending on the location within the bridge system and environmental hazard conditions to avoid debris buildup that could affect movement and rotation performance. Steel surface cleaning and coating maintenance should be provided to prevent cor- rosion of exposed steel elements. 10.4.2 options Related to Specific Bearing types 10.4.2.1 Elastomeric Bearings: Plain and Steel Reinforced Table 10.3 summarizes solutions for various service life issues identified in Sec- tion 10.3.2.1 for elastomeric bearings and briefly identifies the advantages and dis- advantages of each solution. Problems with elastomeric bearings, albeit rare, have been asso ciated with production or operational defects relating to design, manufactur- ing, and installation. Following this table is further discussion relating to each issue.

478 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 10.4.2.1.1 Improper Element Design Resulting in Excessive Shear Deformation and Excessive Bulging, Splitting, or Delamination Excessive bulging, splitting, or delamination can be avoided by following recent pro- visions (2010) in the LRFD specifications for either Method A or B. As discussed, improper design has, in some instances, resulted in deficiencies. Extensive research has identified various failure modes when pads are subjected to excessive shear deforma- tion or overload. See Section 10.3.2.1.1 for further discussion of failure modes. 10.4.2.1.2 Need for Greater Longitudinal Movement Capacity When there is a need to accommodate large displacements along with smaller axial loads, such as might be found at the end spans of a long continuous bridge, the use of a low-profile elastomeric bearing combined with a PTFE or higher-performing sliding tABLE 10.3. SoLutionS For Service LiFe ProbLemS: PLAin And SteeL-reinForced eLAStomeric beAringS Service Life Problem Solution Advantages Disadvantages Improper element design resulting in excessive shear deformation and excessive bulging, splitting, or delamination Avoid by following current AASHTO LRFD design criteria for Methods A or B. Very robust bearing with minimal problems and need for maintenance when properly designed, manufactured, and installed. AASHTO design criteria have incorporated recent research to avoid failure modes, including combined axial load, shear, and rotation. None. Need for greater longitudinal movement capacity Combine with high- performing sliding surfaces. Can provide unlimited movement capability and longer service life. Additional initial cost. Need to accommodate multidirectional complex movement (skewed or curved bridges) Use circular bearing pads. Accommodates multidirectional movement and rotation. Well suited for cases in which transverse movements are uncertain. May require wider bridge seat. Not as efficient as rectangular pad with proper orientation. Slipping or walking out (This has been more of a problem with plain pads.) Control the use of wax additives in elastomer. Prevents reduction in coefficient of friction between pad and bearing surface. Less resistance to ozone degradation, but should not be an issue with neoprene. Provide positive attachment to sole plates. Restrains pad from walking out. Small additional initial cost. Improper fabrication or manufacturing Provide effective QA-QC. Assures adherence to quality and performance requirements. None.

479 Chapter 10. BRiDGE BEARiNGS surface offers a practical solution. In these cases, cyclic end movement due to girder ro- tation caused by truck load can cause PTFE surface wear. This wear can be avoided by designing the elastomeric pad to accommodate rotation and the smaller cyclic move- ment due to truck loads and by designing the sliding surface to accommodate the larger longitudinal movement due to thermal load. 10.4.2.1.3 Need to Accommodate Multidirectional Complex Movement When movement direction is not readily determined, such as with curved or skewed bridges, circular pads can more easily accommodate multidirectional requirements. Circular pads typically offer an advantage by requiring narrower bridge seats than rectangular pads when placed on a skew; however, circular pads are not as efficient from a design standpoint as properly oriented rectangular pads. 10.4.2.1.4 Slipping or Walking Out Slipping can be avoided in two ways. First, limiting the use of wax additives in the manu- facturing of the elastomer prevents or reduces future bleeding of the wax to the contact surface, which results in a much lower coefficient of friction and bearing slipping. Slip- ping can also be avoided by providing positive attachment of the elastomeric pad to the bearing sole plates. 10.4.2.1.5 Improper Fabrication or Manufacturing Effective QC during manufacture is essential for successful performance. Proper clean- liness and care during fabrication helps prevent debonding of reinforcing plates with elastomer. 10.4.2.2 Cotton Duck Pads Table 10.4 summarizes solutions for various service life issues identified in Sec- tion 10.3.2.2 for CDPs. These bearing types have performed well, but they have had limited usage. Research testing has identified potential failure modes that could result from improper design and/or manufacturing and installation. Further discussion relat- ing to each issue follows. 10.4.2.2.1 Improper Pad Design Resulting in Interlayer Splitting and Delamination Splitting and delamination damage is caused by excessive horizontal movement, axial load, and rotation and can be avoided by following the current LRFD specifications. Key criteria include limiting shear strain to 10% of total pad thickness and limiting compressive stress to 3,000 psi for total load and 2,000 psi for live load. 10.4.2.2.2 Improper Pad Design Resulting in Overrotation Overrotation can be avoided by implementing the rotation limits provided in the cur- rent LRFD specifications. The use of narrow pads in the direction of movement and rotation can improve expansion and rotational capacity.

480 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE tABLE 10.4. SoLutionS For Service LiFe ProbLemS: cdPS Service Life Problem Solution Advantages Disadvantages Improper pad design resulting in interlayer splitting and delamination Avoid with proper design, manufacturing, and installation. CDPs are economical and very stiff in supporting vertical load. Provide greater stability during construction. High stiffness reduces capacity for rotation and horizontal translation. Service life experience and use is limited. Limit shear strain to 10% of total pad thickness. Limit compressive stress to 3,000 psi for total load and 2,000 psi for live load. Improper pad design resulting in over rotation Use narrow pads in direction of movement. Improves expansion and rotation capacity. Need for greater longitudinal movement capacity Combine with high- performing sliding surfaces. Can provide unlimited movement capability. Use of PTFE surfaces can limit service life due to wear. Additional initial cost. Improper fabrication or manufacturing Provide effective QA-QC. Assures adherence to quality and performance requirements. None. 10.4.2.2.3 Need for Greater Longitudinal Movement Capacity The high shear stiffness of these bearing types reduces their capacity for accommodat- ing horizontal translation. This lower capacity can be accommodated by combining CDPs with PTFE sliding surfaces. PTFE can be subject to wear, which can reduce ser- vice life. See further discussion of solutions for sliding surface wear in Section 10.4.2.3. 10.4.2.2.4 Improper Fabrication or Manufacturing Proper QA-QC and testing during manufacturing is necessary to confirm required performance. Stiffness and other performance characteristics can vary between manu- facturers, so QC testing measures have been developed. 10.4.2.3 Sliding Surface Bearings Table 10.5 summarizes solutions for various service life issues identified in Sec- tion 10.3.2.3 for sliding surfaces. PTFE is most commonly used for sliding surfaces. Its service life is affected by deficiencies caused by loads, which result in wear and creep or cold flow; and by production or operation defects, which result in damage due to contamination. It is recommended to use higher-performing sliding surfaces when pos- sible. The higher initial cost of these surfaces is a very small fraction of total bridge cost, and cost by itself is not a justifiable reason for not using higher-performing sliding surfaces in place of PTFE, which is used exclusively in United States. Further discus- sion relating to each issue is provided following the table.

481 Chapter 10. BRiDGE BEARiNGS 10.4.2.3.1 PTFE Wear Due to Cyclic Truck Load As mentioned throughout this section, the use of higher-wear-resistant sliding surface materials is recommended, when possible, when surfaces are subject to the fast sliding speeds associated with cyclic truck load. The reference to PTFE in this section reflects its widespread use in practice and providing solutions where they are needed. How- ever, this approach is not meant to recommend the use of PTFE as a material of choice for sliding surfaces, except in instances in which wear is not a factor. PTFE use can be improved with the following solutions: • Thicker PTFE than that used in the United States is used in other countries, but plain PTFE wears very rapidly, and very thick surfaces would be required to achieve long service life when subjected to cyclic truck load. tABLE 10.5. SoLutionS For Service LiFe ProbLemS: PtFe SLiding SurFAceS Service Life Problem Solution Advantages Disadvantages Wear caused by cyclic truck load Possible thicker PTFE surface. Increases time for material to wear out. Thickness may be limited by other factors. Lower service life than higher-performing sliding surfaces. Dimpled and lubricated PTFE. Improves wear resistance and reduces coefficient of friction. Long-term effectiveness of lubrication is uncertain. PTFE has much lower service life than higher-performing sliding surfaces. Woven or glass-reinforced PTFE. Greatly improves wear resistance. Has slightly increased coefficient of friction. Improved sliding material, MSM. Greatly improves wear resistance. Proprietary product. Possibly higher initial cost. Has significantly increased coefficient of friction without lubrication. Combined performance of high-performing sliding surfaces with elastomeric pad. Elastomeric pad can be designed to accommodate cyclic truck load. Requires special design considerations. Creep or cold flow caused by high compressive load Compressive overload prevention. PTFE material set in recess, and epoxy bond. Recess and bonding holds PTFE in place. Has a possibly higher initial cost but prevents higher future maintenance cost. Surface damage caused by contamination Protection during shipment and installation. Periodic maintenance cleaning. Prevents damage to sliding surface that increases coefficient of friction and wear. Maintenance needs to be scheduled and actually performed.

482 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE • Dimpled and lubricated PTFE has been shown to provide both improved wear resistance and reduced coefficient of friction; however, the long-term effectiveness of lubrication is uncertain. • Woven or glass-filled PTFE greatly improves wear resistance, but it has a higher coefficient of friction. • MSM is an improved sliding material that has greatly improved wear resistance and has a reduced coefficient of friction when used in the dimpled and lubricated condition. The coefficient of friction is greatly increased when unlubricated. This material is a proprietary product. • The design of combined elastomeric–PTFE bearings considers the combined per- formance of both materials. The elastomeric pad can be designed to accommo- date rotation along with the low-amplitude, high-cycle movement caused by truck load, and the PTFE can be designed to accommodate the high-amplitude, low- cycle movement caused by temperature load, which produces the least amount of wear. These accommodations require some additional design considerations. 10.4.2.3.2 Creep or Cold Flow Due to High Compressive Stress Creep or cold flow can be avoided by limiting compressive stress, which prevents cold flow, but lower compressive stress results in a higher coefficient of friction. These problems can also be avoided by recessing the PTFE for one-half its thickness, which provides the better solution because it prevents cold flow and also allows higher com- pressive stress for a lower coefficient of friction. 10.4.2.3.3 PTFE Wear Due to Surface Contamination Surface contamination can be avoided by protection during shipment and installation. Bearing assemblies should be shipped together and protected to avoid PTFE surface damage. In addition, periodic maintenance and cleaning can mitigate the potential for sliding surface damage by preventing buildup of dirt and other debris. 10.4.2.4 High-Load Multirotation Pot Bearings Table 10.6 summarizes solutions for various service life issues identified in Section 10.3.2.4 for HLMR pot bearings. Past issues have most often been related to produc- tion or operation defects relating to design and manufacturing. Expansion pot bear- ings use PTFE sliding surfaces, which are affected by loads resulting in wear. External steel surfaces are exposed to environmental hazards, which can cause corrosion; how- ever, corrosion of external surfaces typically does not affect operation. Current design and manufacturing procedures have greatly reduced past issues. Further discussion relating to each issue follows.

483 Chapter 10. BRiDGE BEARiNGS 10.4.2.4.1 Improper Design and Manufacturing Causing Various Deficiencies Various recommendations by Stanton et al. (1999) to mitigate or avoid previous defi- ciencies have been incorporated into the current LRFD specifications. These recom- mendations include • Providing proper clearances between various elements of the pot, piston, sliding surface, guides, and restraints to avoid binding between metal components. • Providing proper rotational capacity with proper elastomeric pad thickness design, controlled by a 15% strain limit on the pad-edge deflection under rotation. • Protecting against overrotation by designing for larger rotations that include pos- sible rotation due to construction tolerances and by placing greater emphasis on calculation of rotations due to service loads, construction loads, and environmen- tal conditions. • Providing smooth surfaces on the piston and inside the pot to reduce rotational resistance, wear, and abrasion. Metalizing of these interior surfaces for corrosion protection should be avoided because it produces a rough surface that leads to tABLE 10.6. SoLutionS For Service LiFe ProbLemS: hLmr Pot beAringS Service Life Problem Solution Advantages Disadvantages Improper design and manufacturing resulting in leakage of elastomer, broken sealing rings, abraded elastomeric pads, and internal metal- to-metal contact Proper clearances between pot elements. This has been the most common and economical HLMR bearing for many years. Recent sealing ring and pot manufacturing improvements have eliminated most of the earlier failures. Long-term deterioration testing assures greater performance. This has internal moving parts and requires a high degree of QC in manufacturing. Internal sealing-ring wear could always be a concern. This should provide for bearing replacement in design details. Proper pad rotational capacity. Proper calculation of movements and rotations. Smooth surfaces on pot rim and inside piston. Improved sealing ring design and manufacturing tolerances. Silicon grease lubrication. Alternate methods of external restraint for lateral loads. Sealing and protecting pot bearings during shipping and installation. Long-term deterioration testing per current AASHTO procedures. Load-induced PTFE sliding surface wear See Table 10.5 on PTFE sliding surfaces.

484 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE increased damage under cyclic rotation. In highly corrosive environments, stain- less steel should be considered, but this affects cost greatly. • Providing tight control on design and manufacturing of sealing rings to avoid escape of elastomer. Circular cross-section rings provide a tight seal but are sus- ceptible to wear under cyclic rotation. Flat rings appear to be more susceptible to leakage and ring fracture, but they experience less wear. Heavier flat brass rings have been suggested as a means of improving performance. The performance of circular rings could also be improved if the internal friction and wear were re- duced. Multiple flat brass sealing rings have been the most frequently used system since the mid to late 1990s and have had good results. • Using silicon grease lubrication to reduce potential wear on rings, pot walls, and pistons. • Providing alternate methods of external restraint for lateral loads to avoid having these loads resisted by the piston rim bearing against the pot wall. Relatively small lateral load (5% of gravity load) when combined with cyclic rotation can dramati- cally increase the rotational resistance caused by the piston rim dragging against the pot wall during rotation and cause wear. • Sealing and protecting pot bearings during shipping and installation to prevent dirt or contamination from getting inside the pot, which can lead to increased wear and abrasion of the rings, pot, and elastomeric disc. • Providing long-term deterioration testing per current LRFD construction specifica- tions to assure required performance. Even with recent improvements in serviceability, this bearing type still has internal moving parts that are subject to wear and abrasion. This behavior can still lead to reduced element service life and the potential need for bearing replacement before the service life of the bridge system is realized. 10.4.2.4.2 Load-Induced Sliding Surface Wear Expansion bearings using PTFE sliding surfaces are subject to wear from truck loads or thermal loads. See further discussion of solutions for sliding surface wear in Sec- tion 10.4.2.3. 10.4.2.5 High-Load Multirotation Disc Bearings Table 10.7 summarizes solutions for various service life issues identified in Sec- tion 10.3.2.5 for HLMR disc bearings. Research testing has identified potential issues that would most often be related to production or operation defects relating to design and manufacturing. In addition, expansion disc bearings use PTFE sliding surfaces, which are subject to load-induced wear. External steel surfaces are exposed to environ- mental hazards, which can cause corrosion. Further discussion relating to each issue is provided.

485 Chapter 10. BRiDGE BEARiNGS 10.4.2.5.1 Improper Design and Manufacturing Causing Fatigue, Abrasion, and Overrotation Various performance conclusions and recommendations were made by Stanton et al. (1999) on the basis of tests of disc bearings subjected to combined axial load and rota- tion. These include the following: • The LRFD construction specifications requirements for cyclic load testing to con- firm long-term performance should be followed. Cyclic rotation tests have shown slight disc deformation and abrasion, but these changes did not affect performance. • The LRFD specifications requirements limiting edge contact stress on PTFE sur- faces in sliding expansion bearings should be followed. Tests showed that rotation of disc bearings is partly accompanied by uplift, which can produce high edge loading on sliding surfaces that affects service life. • Overrotation also causes binding on the center shear pin that can be mitigated by limiting rotation or providing adequate clearance. 10.4.2.5.2 Load-Induced Sliding Surface Wear Expansion bearings using PTFE sliding surfaces are subject to wear from truck loads or thermal loads. See further discussion of solutions for sliding surface wear in Sec- tion 10.4.2.3. tABLE 10.7. SoLutionS For Service LiFe ProbLemS: hLmr diSc beAringS Service Life Problem Solution Advantages Disadvantages Improper design and manufacturing causing fatigue deformation and abrasion of urethane disc Proper design, manufacturing, and installation. AASHTO construction specifications require cyclic testing to confirm performance. This is a simple concept using a polyether urethane disc, which provides multidirectional rotation capability. Good performance has been experienced over the last 40 years without any known field problems. They have not been extensively used because of proprietary status until recent years. Service life experience and fatigue testing experience are limited. AASHTO design specifications are limited. There is concern that rotational stiffness can cause high stresses on sliding surfaces. Additional research and experience are required to determine if 100-year service life is achievable. Improper design causing overrotation and high edge pressure and damage to PTFE surfaces on expansion bearings (Overrotation can cause binding on center pin.) Proper analysis and design to identify rotations. AASHTO design specifications limit edge contact stress on PTFE surfaces. Load-induced PTFE sliding surface wear See Table 10.5 for PTFE sliding surfaces.

486 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 10.4.2.6 High-Load Multirotation Spherical and Cylindrical Bearings Table 10.8 summarizes solutions for various service life issues identified in Sec- tion 10.3.2.6 for HLMR spherical and cylindrical bearings. Issues have most often been related to production or operation defects relating to design and manufacturing. Further discussion relating to each issue is provided following the table. 10.4.2.6.1 Improper Design and Manufacturing Causing Binding of Steel Components Adequate clearances between moving bearing components should be provided to ac- commodate full rotation demand without premature binding. 10.4.2.6.2 Improper Design Causing Overload of Spherical Surface from High Lateral Loads An additional external restraint system should be provided to accommodate large lat- eral loads to avoid generating excessive localized bearing stresses that can damage PTFE surfaces. 10.4.2.6.3 Improper Manufacturing Causing Surfaces to Not Mate Properly Tight manufacturing tolerances on curved mating surfaces should be maintained to avoid excessive localized stresses that can damage PTFE surfaces. tABLE 10.8. SoLutionS For Service LiFe ProbLemS: hLmr SPhericAL And cyLindricAL beAringS Service Life Problem Solution Advantages Disadvantages Improper design and manufacturing causing binding of steel components and reducing rotational capacity Ensure proper clearances. This is a robust bearing system that is traditionally considered to be the most reliable HLMR type. It can be designed to accommodate large loads and rotations. Behavior is close to what is assumed in design. This is the most expensive of the HLMR types. It requires a high degree of manufacturing QC. Improper design causing overload of spherical surface from high lateral loads Provide external restraint system. Improper manufacturing causing surfaces to not mate properly, causing excessive localized stress Ensure proper manufacturing tolerances. Load-induced PTFE sliding surface wear See Table 10.5 for PTFE sliding surfaces.

487 Chapter 10. BRiDGE BEARiNGS 10.4.2.6.4 Load-Induced Sliding Surface Wear PTFE sliding surfaces are subject to wear from truck loads or thermal loads. See fur- ther discussion of solutions for sliding surface wear in Section 10.4.2.3. 10.4.2.7 Fabricated Mechanical Steel Bearings Table 10.9 summarizes solutions for various service life issues identified in Sec- tion 10.3.2.7 for fabricated mechanical steel bearings. As discussed, factors affecting service life relate to several categories including loads, primarily overload, which results in binding or overrotation of rocker bearings; natural or man-made hazards, which re- sult in steel element corrosion; and production or operation defects, specifically caused by lack of maintenance. Further discussion relating to each issue is provided. 10.4.2.7.1 Overload Special emphasis should be placed on determining load, rotation, and movement de- mands to avoid overrotation, which can lead to excessive tilting and possible tipping. Adequate clearances between moving components should be provided to avoid prema- ture binding. Set rocker bearings for proper temperature alignment. tABLE 10.9. SoLutionS For Service LiFe ProbLemS: FAbricAted mechAnicAL SteeL beAringS Service Life Problem Solution Advantages Disadvantages Overload causing binding or excessive rotation or tipping Use proper demand parameters, proper clearances, and proper initial setting. This has been the longest- used bearing type. This bearing type has potential for 100+ years of service life if maintained and protected from corrosion, freezing, and overrotation. It is expensive to fabricate and install. In comparison with other bearings, it requires additional maintenance.Corrosive environment causing steel surface corrosion Use stainless steel in extreme environments. Use galvanizing or metalizing on bearing assembly components (fixed or rocker) and accompanying plates and anchor bolts. Improper maintenance causing debris buildup, affecting movement capacity Establish and follow maintenance procedures to prevent debris buildup. Improper maintenance causing freezing, affecting movement and/or rotation capacity Establish and follow procedures to prevent or inhibit corrosion of steel surfaces.

488 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 10.4.2.7.2 Corrosive Environment For bearings in corrosive environments, the use of stainless steel can avoid the poten- tial of corrosion on exposed surfaces and on contact surfaces. Galvanizing, metalizing, or high-performance paint systems can serve to mitigate the potential for surface cor- rosion, but a maintenance plan for the longer term will still be required. 10.4.2.7.3 Improper Maintenance Causing Debris Buildup Proper periodic maintenance is required for bearings located below deck expansion joints to clean bearing areas and prevent excessive buildup of debris, which can affect horizontal movement and rotation. Dirt and debris buildup against steel surfaces can also hold moisture and accelerate coating deterioration and steel corrosion. 10.4.2.7.4 Improper Maintenance Causing Freezing Proper maintenance is also required in corrosive environments to prevent or inhibit corrosion of steel surfaces, which can cause freezing and reduce rotation and move- ment capacity. Proper field cleaning and recoating of existing steel bearings with zinc- rich paint systems, followed by maintenance touch-up as required, will provide ex- tended service life. Lubricating pins and knuckles that have metal-to-metal contact can also help prevent freezing. 10.5 StrAtegieS For beAring SeLection And deSign 10.5.1 Available Service Life Design Philosophies Currently there are no deterioration models for predicting the service life of bearings. Although service life cannot be accurately predicted, preventive measures should be taken to avoid deterioration and extend service life. Previous bearing research has studied the behavior of various bearing types under static and cyclic loading and has identified potential damage and deterioration modes that have been addressed by improved AASHTO design and construction specifica- tions. This research has been performed primarily to address observed field problems or to improve understanding and design methodology. However, these studies have focused primarily on developing criteria that will avoid observed problems or improve performance; models have not been developed for determining how various bearing types will deteriorate over time under given loading and environmental conditions. Research on sliding surfaces has determined the potential for a deterioration model for wear based on various factors, including pressure and sliding velocity, but this model has not yet been fully developed. With the lack of deterioration models, experience and expert opinion are the only methods for predicting service life. However, previous experience with certain bearing types may not be a good indicator of newer design performance, particularly with recently updated and improved design and construction requirements. Future

489 Chapter 10. BRiDGE BEARiNGS long-term data collection regarding bearing performance will be necessary for devel- oping more accurate service life predictions. 10.5.2 Bearing Selection and Design for Service Life Selecting the proper bearing should always be done as part of the overall bridge system development and should consider the expected bridge system behavior and optimal superstructure–substructure interaction. Bearings should be selected and designed as an integral part of the overall system and should not be designed as an afterthought, which increases chances for problems. It is recommended to use elastomeric bearing and higher-performing sliding surfaces when possible. A combination of rectangular or circular elastomeric bearing pad and higher-performing sliding surfaces can meet the demands of most bridges and result in a very long service life. 10.5.2.1 Selection Process The process for selecting the proper bearing type involves four main steps: Step 1. Determine demand by identifying operational and service life requirements that the bearing must accommodate. Step 2. Determine suitable options by identifying bearing types that have the potential to accommodate demand requirements, and perform preliminary design(s) to confirm. Step 3. Evaluate service life mitigation and replacement requirements and evaluate life-cycle costs. Step 4. Select optimal bearing type considering all factors. When considering service life, Steps 1 (demand) and 2 (options) need to address additional issues beyond loads and movements, and consider all hazards that can have adverse effects. Figure 10.19 illustrates the overall selection process. When considering multiple options, the selection process should take into account the various levels of bearing performance, the initial cost and maintenance requirements, and the reliability of the bearings and their potential to achieve a long service life. 10.5.2.2 Detailed Steps in Selection Process Step 1. Identify Demand Requirements The demand step identifies the desired service life and what requirements the bearings must accommodate from an operational and environmental standpoint throughout their service life. Operational requirements have typically included proper determination of gravity loads, rotations, and translational movements. However, for service life, this step needs to further consider cyclic movements and cumulative movement due to truck load, which in some cases can have a severe effect on service life. Environmental demand has typically involved determination of thermal climate and corresponding temperature ranges. But for service life, this determination also needs to identify specific local environ- mental hazards and their consequences that need to be avoided or mitigated in the design.

490 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Figure 10.19. Process flowchart for bearing selection and design for service life. Identify DEMAND requirements. Compare with SUPPLY parameters for bearing alternatives. Identify potential bearing alternative(s) that accommodate loads and movements. Perform preliminary design to confirm suitability of potential bearing type. No Perform final bearing selection. Yes Determine expected frequency of bearing replacement. Identify factors affecting service life and mitigation requirements. Compare life-cycle costs considering initial maintenance and replacement costs. Is bearing suitable? Evaluate potential service life of alternatives and determine if bearing has potential for achieving desired life. No Yes Is bearing suitable?

491 Chapter 10. BRiDGE BEARiNGS Table 10.10 provides a format for identifying various demand requirements, and Steps 1.1 to 1.4 summarize the process. Step 1.1. Determine operational and service life requirements: • Review the targeted bridge design service life, which should have been identified at the bridge system selection stage. See Chapter 2 on bridge system selection. • Identify total traffic and truck volumes. • Identify local environmental factors that can affect bearing service life or performance: – Thermal movement: design temperature ranges, and – Environmental hazards, including ¤ Severe corrosive environment—location below deck expansion joints in northern wet climates; ¤ Coastal environment—potential for salt spray; and ¤ Chemical environment—potential for other deleterious atmospheric or cor- rosive activity. Step 1.2. Determine general bearing requirements on the basis of bridge system alternatives and superstructure–substructure interaction: • Consider integral system options that eliminate bearings at abutments and/or at piers. • Consider continuous system options that eliminate deck joints at interior piers. • Determine optimal fixity and expansion options at piers and abutments on the ba- sis of bridge system evaluation. Consider flexibility of piers in determining options for multiple pier fixity. • For curved or skewed bridge systems, determine proper direction of movement; for curved systems, determine point of fixity. Step 1.3. Determine superstructure loads and movements for the given bridge system(s) being considered: • Gravity loads from dead and live loads; • End rotations due to all sources, including construction tolerances; • Longitudinal movements – Maximum movement due to temperature change, – Cyclic movement due to truck load, and – Movement due to posttensioning, creep, and shrinkage; • Requirements for multidirectional movement; and • Longitudinal and transverse loads to be resisted by bearings. Step 1.4. Ensure that loads are distributed to bearings in accordance with system analysis.

492 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE tA B LE 1 0 .1 0 . de m An d re Qu ir em en tS F or b eA ri n g Se Le ct io n D em an d D es cr ip ti o n Q u al it at iv e V al u e: Lo w , M ed iu m , H ig h Q u an ti ta ti ve V al u e: If A p p li ca b le H o w V al u es A re U se d G en er al R eq u ir em en ts Br id ge im po rt an ce a nd s ys te m d es ig n se rv ic e lif e Ex am pl e: H ig h Ex am pl e: 1 00 y ea rs To id en tif y be ar in g de si gn s er vi ce li fe a nd re pl ac em en t ne ed s Tr af fic Ex am pl e: H ig h In di ca te t ot al t ra ffi c vo lu m es a nd t ru ck vo lu m es To d et er m in e cy cl ic tr an sl at io n du e to tr uc k lo ad En vi ro nm en ta l fa ct or s Te m pe ra tu re r an ge fo r th er m al c lim at e an d su pe rs tr uc tu re t yp e M od er at e or c ol d In di ca te d es ig n te m pe ra tu re r an ge To c om pu te su pe rs tr uc tu re th er m al m ov em en t En vi ro nm en ta l ha za rd s C ol d cl im at e w ith d ec k de ic in g ch em ic al s an d su bj ec t to o pe n de ck jo in ts Ex am pl e: S ev er e na To d et er m in e se ve rit y of e nv iro nm en ta l ha za rd s to w hi ch be ar in gs w ill b e su bj ec te d, a nd t o de te rm in e m iti ga tio n ne ed s to a ch ie ve op tim al s er vi ce li fe C oa st al c lim at e su bj ec t to sa lt sp ra y Ex am pl e: H ig h na O th er c he m ic al ly c or ro si ve en vi ro nm en t Ex am pl e: n a Id en tif y ty pe if ap pl ic ab le O th er a tm os ph er ic en vi ro nm en ta l f ac to rs Ex am pl e: n a Id en tif y ty pe if ap pl ic ab le (c on tin ue d)

493 Chapter 10. BRiDGE BEARiNGS D em an d D es cr ip ti o n Q u an ti ta ti ve V al u e: If A p p li ca b le H o w V al u es A re U se d B ri d g e Sy st em Lo ad s an d M o ve m en ts D es ig n lo ad s (k ip ) Ve rt ic al M ax im um Fo r fix ed a nd ex pa ns io n be ar in g de si gn Pe rm an en t M in im um Tr an sv er se Lo ng itu di na l Ro ta tio n (r ad ) Lo ng itu di na l Pe rm an en t C yc lic — M ax im um C yc lic — M in im um Tr an sv er se Pe rm an en t C yc lic Tr an sl at io n (I n) Lo ng itu di na l C yc lic t he rm al Fo r ex pa ns io n be ar in g de si gn C yc lic t ru ck Ir re ve rs ib le Tr an sv er se C yc lic t he rm al C yc lic t ru ck Ir re ve rs ib le N ot e: n a = no t ap pl ic ab le . tA B LE 1 0 .1 0 . de m An d re Qu ir em en tS F or b eA ri n g Se Le ct io n (c on tin ue d)

494 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Step 2. Identify Suitable Bearing Options The options step involves comparing demand requirements with supply parameters for various bearing types and determining which types are suitable. Often, there may be more than one option that can meet load and movement performance requirements. For shorter spans with lighter loads, which represent the largest population of bridges, plain or reinforced elastomeric pads will typically provide the best overall service. CDPs with greater load capacity can be a suitable alternative for plain elastomeric pads when movement demand is light; however, when movement demand increases, CDPs will require sliding surfaces, which affect service life. When loads and rotational demands increase, SRE bearings will have to be eval- uated against HLMR pot or disc bearings for accommodating the required loads. As long as an SRE bearing can be designed for the combined load, movement, and rotation, it will also have the greatest potential for achieving the desired service life. Greater movement capacity can be provided with SRE bearings by combining them with sliding surfaces, but this combination typically would be considered only when movement demand is large and vertical loads are relatively light, such as at the ends of a long, multispan continuous unit. When load demand is beyond the capacity of SRE bearings, other bearings such as HLMR pot, disc, or spherical must be considered. Fabricated steel bearings can also be considered, but cost and service life mitigation issues have to be weighed. The preliminary design step involves performing preliminary design in accordance with AASHTO specifications to determine if potential bearing type(s) can actually pro- vide the required capacities depending on actual bridge layout and to evaluate further size and geometric requirements. The following steps summarize the process for identifying viable options: Step 2.1. Using Table 10.11, other applicable agency or industry guidelines, or professional experience, identify potential bearing alternatives that accommodate load and movement requirements. Step 2.2. Perform preliminary design on potential bearing types following AASHTO design requirements. Determine whether selected alternatives can actually accommodate load and movement demand. Determine bearing size requirements. Step 3. Evaluate Service Life Factors and Mitigation Requirements After suitable options are identified in Step 2 based on satisfying vertical load and movement requirements, Step 3 evaluates the bearing’s ability to resist various factors that affect service life and considers what mitigation requirements will be necessary. This step includes an evaluation of whether the bearing type or types have the poten- tial for achieving the desired service life and identifies possible replacement needs and maintenance requirements. Table 10.11 summarizes various supply parameters for individual bearing types related to service life. It lists relative durability factors for each bearing type and also identifies key avoidance or mitigation requirements. In addition, relative qualitative initial costs and maintenance requirements as part of a qualitative life-cycle cost com- parison are listed. The table also indicates relative qualitative service life potential.

495 Chapter 10. BRiDGE BEARiNGS tA B LE 1 0 .1 1 . Su PP Ly P Ar Am et er S Fo r be Ar in g Se Le ct io n B ea ri n g Ty p e Lo ad a n d M o ve m en t P er fo rm an ce V al u es D u ra b il it y Fa ct o rs Avoidance or Mitigation Requirements Li fe -C yc le C o st s Service Life Potential Load (kips) Rotation (radians) Movement (in.) Multidirectional Rotation/Movement Capability Relative Ability to Accommodate Cyclic Truck Movement Resistance to Corrosive Environment Resistance to Production/ Operation Defects Relative Initial Cost A = Lowest Relative Maintenance P la in el as to m er ic p ad Lo w 0 to 1 00 Lo w 0. 01 Lo w 0. 5 Ye s H ig h H ig h — pa d no t af fe ct ed H ig h Av oi d pa d sp lit tin g by pr op er d es ig n A Lo w H ig h — po te nt ia l f or 10 0+ y ea rs St ee l- re in fo rc ed el as to m er ic p ad Lo w t o m ed iu m 50 t o 75 0 M ed iu m 0. 02 Lo w t o m ed iu m 4 Ye s— ci rc ul ar pa ds H ig h H ig h — SR E pa d no t af fe ct ed H ig h — SR E pa d sp lit tin g or de la m in at io n Av oi d pa d sp lit tin g by pr op er d es ig n B Lo w H ig h — po te nt ia l f or 10 0+ y ea rs El as to m er ic p ad w it h sl id in g su rf ac e Lo w t o m ed iu m 50 t o 75 0 M ed iu m 0. 02 H ig h N o lim it Ye s H ig h w ith h ig h pe rf or m in g sl id in g su rf ac e. Lo w w ith PT FE . H ig h H ig h M iti ga te P TF E w ea r w ith im pr ov ed sl id in g su rf ac e B Lo w t o m od er at e Lo w t o m od er at e (m od er at e w ith im pr ov ed sl id in g su rf ac e) C D P Lo w t o m ed iu m 0 to 3 00 Lo w 0. 00 5 m ax im um Lo w 0. 25 N o Lo w H ig h — pa d no t af fe ct ed H ig h — C D P pa d sp lit tin g or de la m in at io n Av oi d pa d sp lit tin g by pr op er d es ig n B Lo w U nc er ta in d ue to la ck o f d at a (c on tin ue d)

496 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE B ea ri n g Ty p e Lo ad a n d M o ve m en t P er fo rm an ce V al u es D u ra b il it y Fa ct o rs Avoidance or Mitigation Requirements Li fe -C yc le C o st s Service Life Potential Load (kips) Rotation (radians) Movement (in.) Multidirectional Rotation/Movement Capability Relative Ability to Accommodate Cyclic Truck Movement Resistance to Corrosive Environment Resistance to Production/ Operation Defects Relative Initial Cost A = Lowest Relative Maintenance C D P w it h sl id in g su rf ac e Lo w t o m ed iu m 0 to 3 00 Lo w 0. 00 5 m ax im um H ig h N o lim it Ye s Lo w d ue to w ea r of s lid in g su rf ac e H ig h — pa d no t af fe ct ed H ig h M iti ga te P TF E w ea r w ith im pr ov ed sl id in g su rf ac e B Lo w t o m od er at e Lo w (m od er at e w ith im pr ov ed sl id in g su rf ac e) H LM R p o t (e xc ep t w it h s li d in g su rf ac e) H ig h 25 0 to 2, 50 0+ H ig h 0. 02 t o 0. 04 H ig h N o lim it w ith sl id er Ye s Lo w d ue to w ea r of s lid in g su rf ac e M od er at e fo r ex po se d si de s of po t an d pi st on M od er at e— in te rn al se al in g rin g w ea r an d el as to m er le ak ag e En su re p ro pe r de si gn a nd co ns tr uc tio n. Im pr ov ed sl id in g su rf ac es . M et al iz ed su rf ac es . E M od er at e M od er at e H LM R d is c (e xc ep t w it h s li d in g su rf ac e) H ig h 25 0 to 2, 50 0+ H ig h 0. 02 t o 0. 03 H ig h N o lim it w ith sl id er Ye s Lo w d ue to w ea r of s lid in g su rf ac e H ig h di sc n ot af fe ct ed M od er at e PT FE w ea r Pr op er de si gn a nd co ns tr uc tio n. Im pr ov ed sl id in g su rf ac es . E M od er at e M od er at e tA B LE 1 0 .1 1 . Su PP Ly P Ar Am et er S Fo r be Ar in g Se Le ct io n (c on tin ue d) (c on tin ue d)

497 Chapter 10. BRiDGE BEARiNGS B ea ri n g Ty p e Lo ad a n d M o ve m en t P er fo rm an ce V al u es D u ra b il it y Fa ct o rs Avoidance or Mitigation Requirements Li fe -C yc le C o st s Service Life Potential Load (kips) Rotation (radians) Movement (in.) Multidirectional Rotation/Movement Capability Relative Ability to Accommodate Cyclic Truck Movement Resistance to Corrosive Environment Resistance to Production/ Operation Defects Relative Initial Cost A = Lowest Relative Maintenance H LM R sp h er ic al (c u rv ed sl id in g su rf ac e; ex ce p t w it h sl id in g su rf ac e) H ig h N o lim it H ig h N o lim it H ig h N o lim it w ith sl id er Ye s Lo w d ue to w ea r of s lid in g su rf ac e M od er at e fo r ex po se d si de s of s te el el em en ts M od er at e. PT FE w ea r. Su rf ac es no t m at in g pr op er ly . Pr op er de si gn a nd co ns tr uc tio n Im pr ov ed sl id in g su rf ac es . M et al iz ed su rf ac es . F M od er at e M od er at e Fa b ri ca te d st ee l (p in fi xe d , ro ck er o r ro ll er ex p an si o n ) Lo w t o m ed iu m 50 t o 75 0+ H ig h N o lim it H ig h N o lim it N o H ig h Lo w fo r al l el em en ts H ig h U se s ta in le ss st ee l o r m iti ga te w ith ga lv an iz in g or m et al iz in g D M od er at e Lo w w ith SS H ig h w ith m iti ga tio n fo r co rr os io n po te nt ia l St ee l so le p la te s, b as e p la te s, a n d an ch o r b o lt s na na na na na Lo w H ig h U se s ta in le ss st ee l o r m iti ga te w ith ga lv an iz in g or m et al iz in g na M od er at e Lo w w ith SS H ig h w ith m iti ga tio n fo r co rr os io n po te nt ia l So ur ce : S ta nt on e t al . 2 00 8. N ot e: P er fo rm an ce c on di tio n lim its a re a pp ro xi m at e an d m ay n ot o cc ur s im ul ta ne ou sl y. S S = st ai nl es s st ee l; na = n ot a pp lic ab le . tA B LE 1 0 .1 1 . Su PP Ly P Ar Am et er S Fo r be Ar in g Se Le ct io n (c on tin ue d)

498 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The following steps summarize the process for evaluating service life requirements: Step 3.1. For potential bearing alternatives, evaluate factors affecting service life and identify required avoidance, mitigation, or acceptance options. Consider primary service life–reduction categories, including reductions • Due to loads (primarily cyclic truck load), • Due to environmental hazards (primarily corrosive or deleterious environment), and • Due to production or operational defects (primarily element damage or wear). Step 3.2. Evaluate potential service life of identified bearing alternatives with con- sideration of any required mitigation and maintenance. • Use deterioration models (currently not available for bearings) – Potential deterioration model for sliding surface resistance to wear. • Use experience or expert opinion – Bearing system and/or material resistance to wear or other deterioration, and – Steel element resistance to and protection from corrosion. Step 3.3. Relate potential service life of identified bearing alternatives to the target design life of bridge system. If service life of bearing alternative is less than target bridge system design life, consider the need to replace bearing after service life is exhausted. Step 3.4. Evaluate life-cycle cost of bearing options considering initial cost, long- term maintenance cost, and potential replacement cost. This evaluation can be done qualitatively at this stage. Step 4. Select Optimal Bearing Type After determining suitable options according to load and movement requirements, evaluating service life mitigation and replacement requirements, and considering and comparing all parameters, the optimal bearing type for the given application can be determined. The selection process should summarize final mitigation, maintenance, and replacement requirements that will need to be incorporated into the final design. The final bearing design process should fine-tune the preliminary bearing design after the final bridge system analysis and final load distribution. Final design details would then be developed to accommodate required clearances and to include any ser- vice life mitigation measures or details for required bearing replacement. 10.6 bridge mAnAgement reLAted to beAringS This section provides guidance related to inspection and maintenance of bearings and needs for future data collection.

499 Chapter 10. BRiDGE BEARiNGS It is recommended that bridge inspections and inspection data collection for bear- ings be expanded to identify bearing types, specific conditions, and other relevant data. These recommendations for more detailed bearing data collection can be used within the FHWA Long-Term Bridge Performance program, which is intended to study the deterioration and durability of bridges and the impacts of maintenance and repair. These recommendations can also be used to supplement the types of data collected for use within bridge management systems such as Pontis. Improved data collection for bearings can be useful in determining and scheduling required maintenance and for developing more accurate deterioration models. 10.6.1 Bridge owner’s manual Chapter 1 provided a detailed description of a bridge Owner’s Manual, which must be provided when requested by the owner or if service life design of unique bridges is involved. The following data regarding bearing design should be included in the bridge Owner’s Manual. This information will be helpful in evaluating future bearing performance. • Bearing type(s); • Design movements considering temperature and truck loads; • Design rotations considering dead load, and expected construction rotation and live load; • Expected bearing service life and expected replacement schedule; • Summary of major factors considered that could affect bearing service life; • Summary of mitigations included in design for factors affecting service life; • Types and frequency of recommended maintenance; • Basis for designed-in details and capabilities for future bearing replacement; and • Special features that should be monitored with future National Bridge Inspection Standards inspections (see Section 10.6.2). 10.6.2 Recommendations for inspection The following list provides inspection recommendations that can provide early indi- cations of bearing problems for various bearing types. Early indication can facilitate maintenance scheduling, which can prevent more serious problems leading to bearing replacement. • All bearings should be checked for misalignment. All guided sliding bearings should be checked for binding against the guides. All expansion bearings should be checked for movement position at respective temperatures and compared against initial settings as identified in the Owner’s Manual.

500 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE • Elastomeric bearings should be checked for – Overrotation; – Excessive shear deformation; – Splitting or tearing; – Excessive bulging; and – Sliding or walking out. • PTFE and stainless steel surfaces should be checked for – PTFE fragments indicating wear; – Migration of PTFE surface; – Scratching, paint, or other contamination (exposed stainless steel surfaces); and – Proper position of stainless steel surface on PTFE surface. • Pot bearings should be checked for – Leakage of elastomer from pot; – PTFE and stainless steel surfaces for expansion bearings; – Steel surface corrosion on exposed surfaces of pot, piston and plates; and – Adequate rotational clearances or binding of pot elements due to rotation. • Disc bearings should be checked for – Splitting, cracking, or bulging of urethane disc; – PTFE and stainless steel surfaces for expansion bearings; and – Steel surface corrosion on plates. • Mechanical steel bearings should be checked for – Surface corrosion; – Debris buildup that could prevent movement and rotation; – Overrotation; and – Freezing. • Miscellaneous items to check for – Bent or misaligned anchor bolts; – Loose or missing nuts on anchor bolts; – Voids under bearing plates; and – General debris buildup. 10.6.3 future Data needs There is a need to study the performance of bridge bearings in actual service conditions and to accumulate data that would be helpful in determining more accurate life predic- tions for various bearing types given those service conditions. For example, measuring

501 Chapter 10. BRiDGE BEARiNGS actual longitudinal bearing movements caused by girder end rotations under traffic load could be useful in understanding and predicting the service life of sliding surfaces. This type of data should be collected by the FHWA Long-Term Bridge Performance program and other research programs for enhancing the service life prediction capabil- ity and development of deterioration models for various bearing types.