Design Guide for Bridges for Service Life (2013)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

602 G.1 introduction This appendix provides a procedure for designing sliding surfaces for service life that is applicable to various bearing devices that allow rotation and use sliding surfaces to allow horizontal movements. A key factor in the process is being able to predict the service life of sliding surfaces, which is based on the following essential parameters: 1. Wear rate of the sliding material, which can be obtained through experimental work; 2. Total accumulated movements, which can be approximated from loading demand (traffic and thermal loads) and analysis; and 3. The speed or velocity of movement, which can be determined from analysis de- pending on movement due to truck load or temperature change. The target service life of the bridge system is established by the owner. The designer must ensure that the bearing device incorporating a sliding surface can provide a ser- vice life exceeding the bridge system. If the service life of the sliding surface is less than the service life of the bridge system, steps must be taken to accommodate replacement of the sliding surface or the entire bearing. The following sections provide detailed descriptions of the parameters listed and the design steps. g DESiGN pROviSiON FOR SLiDiNG SURFACES USED iN BEARiNG DEviCES FOR SERviCE LiFE

603 Appendix G. DESiGN pROviSiON FOR SLiDiNG SURFACES USED iN BEARiNG DEviCES FOR SERviCE LiFE G.2 eLementS oF deSign ProviSionS G.2.1 Wear Rate Tests have shown that plain polytetrafluorethylene (PTFE) will wear over time, causing reduction in thickness, which ultimately affects service life. If the right type of slid- ing material is selected along with the right thickness, there is greater probability of achieving the desired service life. The rate of wear, which can be identified as the anticipated thickness reduction per length traveled, can be used to approximately predict service life. The rate of wear is affected by contact pressure, travel speed, temperature, and lubrication. Considering these factors, Equation G.1 can be used to estimate the wear rate for a sliding surface: wear rate = base wear rate (material, P, V) × CT × CL (G.1) where wear rate is defined in terms of mil thickness per mile of travel distance; base wear rate (material, P, V) is defined as a function of material type, contact pressure, and velocity, based on experimental tests; and CT = modification factor for the effects of low temperature (function of material type); CL = modification factor for the effects of lubrication (function of material type); P = contact pressure acting normal to the sliding surface; and V = travel speed of the sliding bearing (see Section G.2.3). The base wear rate defined in this procedure is the wear rate determined from tests conducted at various combinations of speed and contact pressure at room temperature, without lubrication. Stanton et al. (1999) showed that low temperature and lubrica- tion also contributed to wear rate. Low temperatures increased wear, but lubrication significantly reduced wear. The effects of these parameters can be seen in Table G.1. To account for these effects, the factors CT and CL are added to Equation G.1. These factors are a function of material type and must be determined from tests. At this time, there is insufficient data to develop these factors accurately for final service life design, but estimates can be drawn from Table G.1. Research by Campbell and Kong (1987) on wear of PTFE sliding surfaces indi- cated that the value of pressure times velocity, referred to as the PV factor, could be used as a base parameter to predict the corresponding rate of wear. Their research indi- cated that there was a PV threshold below which there would be a low-wear regime, and above which there would be a high-wear regime. Limited proof-of-concept testing in SHRP 2 Project R19A resulted in preliminary development of PV curves for two types of PTFE sliding materials and an alternate non-PTFE sliding material. These studies confirmed the concept of a PV factor affect- ing wear rate for PTFE-based materials, and further confirmed the concept of PV threshold. However, because of the limited amount of data, additional tests need to be

604 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE carried out to develop final PV versus wear rate curves that can be reliably used for actual service life design. Figure G.1 shows wear rate versus PV data for plain PTFE sliding surfaces. It com- bines data from the SHRP 2 R19A study with data from NCHRP Report 432 (Stanton et al. 1999) and shows the relative low-wear and high-wear regimes. Data shown in red are from NCHRP Report 432. As stated, further testing is required to develop more accurate curves within each of these regions. Figure G.2 shows a similar curve for glass-reinforced PTFE (Fluorogold) from SHRP 2 R19A tests. Rates of wear for reinforced PTFE are significantly reduced from plain PTFE and could be considered as an alternative for plain PTFE in conditions of high PV. Table G.1 presents wear data from NCHRP Report 432 (Stanton et al. 1999) and shows wear rates for various PTFE-based materials at constant pressure but with variations in sliding speed, temperature, and lubrication. These data can be helpful in providing input for parameters in Equation G.1, but as mentioned, further testing is required to establish final values. tABLE g.1. PtFe weAr rAteS Material Lubrication V (in./min) T (°F) PV (lb/in.2 ft/min) Wear Rate (mil/mi) Unfilled PTFE Dimpled, lubricated 2.5 68 625 0.3 25 68 6,250 0.5 Flat, unlubricated 2.5 68 625 0.7 25 68 6,250 189 2.5 –13 625 10 25 –13 6,250 259 Woven PTFE Flat, unlubricated 2.5 68 625 0.3 25 68 6,250 17 2.5 –13 625 27 25 –13 6,250 24 15% Glass filled Flat, unlubricated 2.5 68 625 –1 25 68 6,250 –0.5 2.5 –13 625 No result 25 –13 6,250 6 25% Glass filled Flat, unlubricated 2.5 68 625 –0.3 25 68 6,250 2 2.5 –13 625 4 25 –13 6,250 46 Source: Stanton et al. 1999. Note: Pressure is 3,000 psi.

605 Appendix G. DESiGN pROviSiON FOR SLiDiNG SURFACES USED iN BEARiNG DEviCES FOR SERviCE LiFE NCHRP 432 Tests SHRP 2 Tests Dimpled unlubricated SHRP 2 Tests Plain ♦ NCHRP 432 Tests Glass Filled SHRP 2 Tests Fluorogold Wear Rate – Reinforced PTFE Figure G.1. Wear rate versus PV factor for plain PTFE. Figure G.2. Wear rate versus PV factor for glass-reinforced PTFE.

606 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE G.2.2 Estimating total Accumulated movements The total travel demand is the total accumulated distance that the sliding surface will be traveling throughout the service life of the bridge system. This total travel demand can be estimated using (1) specified bridge system service life, (2) traffic and thermal loading demands, and (3) calculated horizontal movements related to applied traffic and thermal loadings. Sliding surfaces are a means to accommodate horizontal movements associated with traffic load and daily and seasonal bridge superstructure expansion and contrac- tion. The total bridge movement at the bearing [(TD)Demand] in miles is produced by the following three mechanisms: 1. Traffic-induced horizontal movement, (TD)Tr. The total accumulated travel with this type of movement can be considerably greater than that associated with Mechanisms 2 and 3. 2. Daily temperature–induced horizontal movement, (TD)DT. 3. Seasonal temperature-induced horizontal movement, (TD)ST. The total movement due to temperature is the combination of daily and seasonal movements. G.2.2.1 Traffic-Induced Horizontal Movement (TD)Tr Equation G.2 estimates total horizontal movement of the sliding surface, in miles due to traffic movement, for the designed service life [(SL)B] in years: ( ) ( )= × × θ × × × × × ×TD A D n SL2 1.33 (ADTT) 365 63,360Tr SL B1 (G.2) where (TD)Tr = traffic-induced horizontal movement (mi); A = 1, if each end of the girder is free to move in horizontal direction, or 2, if all horizontal movements are accommodated at one end, with the other end pinned against horizontal movement; (ADTT)SL = single-lane ADTT (average daily truck traffic); q = rotation of girder end with sliding bearing (rad); D1 = depth of neutral axis measured from the bottom flange (in.); (SL)B = design service life (years); 1.33 = impact factor for truck load; and n = number of equivalent full-amplitude horizontal movement cycles per truck passage (due to free vibration) initially taken equal to 1.0 for this procedure.

607 Appendix G. DESiGN pROviSiON FOR SLiDiNG SURFACES USED iN BEARiNG DEviCES FOR SERviCE LiFE In Equation G.2, A is a parameter that accounts for boundary conditions at both ends of the span. The term q × D1 is the horizontal movement due to girder end rota- tion. The factor 2 accounts for the full cycle of movement, which includes deflection and rebound. When a truck passes over a span, the girders deflect to a maximum amount as the truck approaches midspan and then recover as the truck moves toward the end of the span. However, because of dynamic behavior, the girders may continue to vibrate until the girder deflection is damped out. The cycles produced after truck passage have successively smaller amplitudes, and the decay is dependent on the damping ratios. This characteristic is represented by the term n, which is the equivalent number of cycles with full amplitude that corresponds to the total number of cycles with decreas- ingly smaller amplitude. For the purposes of this procedure, however, this term can be neglected (using n = 1). Although it is recognized that this behavior occurs, its true magnitude as it applies to bearing movement requires further study and field verification. In Equation G.2, ADTT is the average daily truck traffic, and (SL)B is the owner- specified service life of the bridge system. The constant terms in Equation G.2 are conversion factors. G.2.2.2 Daily Temperature–Induced Horizontal Movement, (TD)DT Equation G.3 estimates total horizontal movement (in miles) of the sliding surface due to daily temperature fluctuation over the designed service life of the bridge (in years): (TD)DT = DLDaily × (SL)B × 365/5,280 (G.3) where DLDaily = 2aLDTDaily; a = coefficient of thermal expansion; L = maximum span length (ft) or length contributing to expansion in the case of multiple spans; and DTDaily = maximum daily temperature fluctuation. G.2.2.3 Seasonal Temperature–Induced Horizontal Movement, (TD)ST Equation G.4 estimates the total horizontal movement (in miles) of the sliding surface due to yearly temperature fluctuation over the designed service life (in years): ( ) ( )= ∆ × ×TD L SL 1 5,280ST BAnnual (G.4) where DLAnnual is 2aLDTAnnual, and DTAnnual is the maximum annual temperature fluctuation.

608 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE G.2.2.4 Total Induced Horizontal Movement, (TD)Demand Finally, Equation G.5 estimates the total bridge movement at the bearing: (TD)Demand = (TD)Tr + (TD)DT + (TD)ST (G.5) G.2.3 Estimating Speed of Sliding Surface movement The service life calculation described previously involves the use of PV curves that are specific for the material used for the sliding surface. The term V is the speed at which the sliding surface moves, which depends on whether the movement is caused by truck load or temperature load. This section provides a procedure for calculating V in the PV expression. G.2.3.1 Speed of Movement per Truck Passage The speed of travel (V) for the sliding bearing for movement caused by truck passage can be determined from the general equation shown in Equation G.6: average travel speed total horizontal movement per truck passage travel time = (G.6) The total horizontal movement of the sliding surface per truck passage can be determined from Equation G.7: total horizontal movement per truck passage = 2 × 1.33 × A × q × D1 × n (G.7) where A = 1, if each end of the girder is free to move in the horizontal direction, or 2, if all horizontal movements are accommodated at one end with the other end pinned against horizontal movement; q = rotation of the girder end with sliding surface (rad); D1 = depth of neutral axis measured from the bottom flange (in.); 1.33 = impact factor for truck load; and n = number of equivalent full-amplitude horizontal movement cycles per truck passage (due to free vibration) initially taken equal to 1.0 for this procedure. The total travel time is the time that it will take for the accumulated horizontal movement caused by the passage of one truck to occur. It is the time for the first cycle and for all succeeding dynamic vibration cycles to take place. If the component of the time due to dynamic vibration cycles as described in Section G.2.2 is neglected, the resulting time for the movement can be determined from Equation G.8: t bridge span length truck speed = (G.8)

609 Appendix G. DESiGN pROviSiON FOR SLiDiNG SURFACES USED iN BEARiNG DEviCES FOR SERviCE LiFE G.2.3.2 Speed of Movement per Temperature Variation The speed of travel (V) for the sliding bearing for movement caused by daily and seasonal temperature change is a much slower velocity that can be estimated from Equation G.9: average travel speed total horizontal movement per temperature change travel time = (G.9) The total horizontal movement of the sliding surface due to temperature move- ment is estimated by determining the total yearly temperature movement caused by daily temperature change and seasonal temperature change, as shown in Equa- tions G.10 to G.12: total movement due to daily temperature change = DLDaily × 365 (G.10) total movement due to seasonal temperature change = DLAnnual (G.11) total temperature movement = (DLDaily × 365) + DLAnnual (G.12) The total travel time for the total temperature movement as defined in Equation G.12 is 365 days, which can be converted into consistent units. G.3 deSign ProceSS For SLiding SurFAceS G.3.1 Steps in Design Process The following steps could be used to select the type of sliding material and its required thickness to meet service life requirements: Step 1. Calculate the total travel distance demand [(TD)Demand, in miles] using the procedures in Section G.2.2. Step 2. Determine the velocity of movement on the basis of traffic load or tempera- ture movement using the procedures in Section G.2.3. Step 3. Select a trial sliding surface type and determine the corresponding wear rate, based on PV curves for the type of material (in inches per mile), using the proce- dures in Section G.2.1. Step 4. Calculate the thickness demand, which is the total predicted wear or reduc- tion in thickness for the sliding surface, by using Equations G.13 and G.14: (thickness)Demand = (TD)Demand × wear rate × a (G.13) gross thickness = (thickness)Demand + thickness of recess (G.14)

610 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE where (TD)Demand = total induced horizontal movement (see Equation G.4), and a = factor to assure that the thickness will not be zero at the end of the service life to prevent undesirable metal-to-metal contact (>1.0). The (thickness)Demand is the thickness that is subject to wear. Accordingly, the gross thickness is the thickness subject to wear plus the recessed thickness that is used to positively connect the sliding surface to the backing plate. Step 5. Establish the gross thickness of the material to be specified in the design plan. The thickness of commercially available sliding surfaces must be larger than the gross thickness calculated in Step 4. Step 6. If the commercially available thicknesses are less than the required gross thickness, then there are two available approaches: (1) select another material, such as a reinforced or braided PTFE or other sliding material type that could meet the demand by repeating Steps 3 through 5; or (2) calculate the service life of the commer- cially available thicknesses and develop a replacement strategy accordingly. G.3.2 Design Process Application The design process has application to all bearing types that use sliding surfaces to per- mit horizontal movement and where horizontal movement is caused by truck load or temperature load. It can also be applied to evaluate the service life of sliding surfaces that are used in combination with elastomeric pads. In these cases, the elastomeric pad is designed to accommodate the high-cycle, low-amplitude horizontal movement due to truck load, and the sliding surface is designed to accommodate the larger-amplitude, low-cycle movement due to temperature. This approach has advantages for the design of expansion bearings at the end of a series of continuous spans where the temperature movement is large and the superstructure reactions are low. Combining elastomeric pads with sliding surfaces reduces the required thickness of the elastomeric pads and permits the use of more durable elastomeric bearings in cases in which high-load multi rotation (HLMR) types would have been required because of the excessive height required for the elastomeric pads. Further advantages of the reduced elastomeric pad thickness include better stability during construction and operation and reduced in- stantaneous and long-term compressive deflection.