Track Design Handbook for Light Rail Transit, Second Edition (2012) / Chapter Skim
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From page 140... ...
4-i Chapter 4 -- Track Structure Design Table of Contents 4.1 INTRODUCTION 4-1 4.2 TRACK AND WHEEL GAUGES AND FLANGEWAYS 4-1 4.2.1 Vehicle Truck Factors 4-1 4.2.2 Standard Track and Wheel Gauges 4-2 4.2.2.1 Railroad Gauge Practice 4-2 4.2.2.2 Transit Gauge Practice 4-3 4.2.2.3 Gauge Measurement Location 4-5 4.2.2.4 Gauge Issues -- Joint LRT and Railroad and Mixed Fleets 4-6 4.2.2.5 Gauge Issues for Embedded Track 4-8 4.2.2.6 Non-Standard Track Gauges 4-9 4.2.3 Track Gauge Variation -- General Discussion 4-9 4.2.4 Curved Track Gauge Analysis 4-11 4.2.4.1 Filkins-Wharton Flangeway Analysis 4-11 4.2.4.2 Nytram Plots -- Truck-Axle-Wheel Positioning on Curved Track 4-14 4.2.4.2.1 Nytram Plot -- Wheel Profile Sections 4-15 4.2.4.2.2 Nytram Plots -- Static Condition 4-17 4.2.4.2.3 Nytram Plots -- Dynamic Condition 4-18 4.2.4.2.4 Nytram Plots Considering Restraining Rail 4-20 4.2.5 Rail Cant and Wheel Taper -- Implications for Track Gauge 4-23 4.2.5.1 Tapered Wheel Tread Rationale 4-24 4.2.5.2 Rail Grinding 4-26 4.2.5.3 Asymmetrical Rail Grinding 4-27 4.2.5.4 Variation of Rail Cant as a Tool for Enhancing Truck Steering 4-27 4.2.6 Construction and Maintenance Tolerances -- Implications for Track Gauge 4-30 4.2.6.1 Tolerances -- General Discussion 4-30 4.2.6.2 Tolerances and Track Gauge 4-31 4.2.6.3 Suggested Track Construction Tolerances 4-31 4.3 GUARDED CURVES AND RESTRAINING RAILS 4-32 4.3.1 Functional Description 4-33 4.3.2 Theory 4-33 4.3.3 Application Criteria 4-35 4.3.3.1 Non-Quantifiable Considerations for Restraining Rail 4-35 4.3.3.2 Longitudinal Limits for Restraining Rail Installations 4-37 4.3.4 Curve Double Guarding 4-38 4.3.5 Restraining Rail Design 4-38 4.3.5.1 Restraining Rail Working Face Angle 4-39 4.3.5.2 Restraining Rail Height 4-39 4.3.5.3 ADAAG Considerations for Restraining Rail 4-40 4.3.6 Omitting Restraining Rails -- Pros and Cons 4-40 4.4 TRACK SUPPORT MODULUS 4-42
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From page 141... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-ii 4.4.1 Modulus of Elasticity 4-42 4.4.2 Track Stiffness and Modulus of Various Track Types 4-44 4.4.2.1 Ballasted Track 4-44 4.4.2.2 Direct Fixation Track 4-45 4.4.2.3 Embedded Track 4-47 4.4.3 Transition Zone Track Modulus 4-48 4.4.3.1 Interface between Track Types 4-49 4.4.3.2 Transition Zone Track Design Details 4-49 4.4.3.3 Transition Zone Conditions 4-51 4.4.3.3.1 Transition from Ballasted Track to Direct Fixation Track 4-51 4.4.3.3.2 Transition from Ballasted Track to Embedded Track 4-51 4.4.3.3.3 Design Recommendation 4-52 4.5 BALLASTED TRACK 4-53 4.5.1 Ballasted Track Defined 4-53 4.5.2 Ballasted Track Criteria 4-54 4.5.2.1 Ballasted Track Rail Section and Track Gauge 4-54 4.5.2.2 Ballasted Track with Restraining Rail 4-54 4.5.2.3 Ballasted Track Fastening 4-54 4.5.3 Ballasted Track Structure Types 4-54 4.5.3.1 Ballasted Track Resilience 4-55 4.5.3.2 Timber Cross Tie Ballasted Track 4-56 4.5.3.2.1 Timber Cross Tie Rail Fastenings 4-56 4.5.3.2.2 Timber Cross Ties 4-57 4.5.3.3 Concrete Cross Tie Ballasted Track 4-58 4.5.3.3.1 Concrete Cross Tie Rail Fastenings 4-58 4.5.3.3.2 Concrete Cross Ties 4-59 4.5.4 Cross Tie Spacing 4-59 4.5.4.1 Cross Tie Spacing -- Vertical Support Considerations 4-59 4.5.4.2 Cross Tie Spacing -- Lateral Stability Considerations 4-61 4.5.5 Special Trackwork Switch Ties 4-62 4.5.5.1 Timber Switch Ties 4-62 4.5.5.2 Concrete Switch Ties 4-63 4.5.6 Ballast and Subballast 4-64 4.5.6.1 Ballast Depth 4-64 4.5.6.2 Ballast Width 4-64 4.5.6.3 Subballast Depth and Width 4-65 4.5.6.4 Subgrade 4-66 4.5.7 Ballasted Track Drainage 4-66 4.5.8 Retained Ballasted Guideway 4-67 4.5.9 Stray Current Protection Requirements 4-67 4.5.10 Ballasted Special Trackwork 4-68 4.5.11 Noise and Vibration 4-68 4.5.12 Signal/Train Control System 4-68 4.5.13 Traction Power 4-69 4.5.14 Grade Crossings 4-69
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From page 142... ...
Track Structure Design 4-iii 4.6 DIRECT FIXATION TRACK (BALLASTLESS OPEN TRACK) 4-70 4.6.1 Direct Fixation Track Defined 4-70 4.6.2 Direct Fixation Track Criteria 4-71 4.6.2.1 Direct Fixation Track Rail Section and Track Gauge 4-71 4.6.2.2 Direct Fixation Track with Restraining Rail 4-71 4.6.2.3 Direct Fixation Track Rail Fasteners 4-71 4.6.2.4 Track Modulus 4-71 4.6.3 Direct Fixation Track Structure Types 4-71 4.6.3.1 Reinforced Concrete Plinths 4-73 4.6.3.1.1 Concrete Plinth in Tangent Track 4-74 4.6.3.1.2 Concrete Plinth in Superelevated Curved Track 4-75 4.6.3.1.3 Concrete Plinths with Restraining or Emergency Guard Rail 4-75 4.6.3.1.4 Concrete Plinth Lengths 4-77 4.6.3.1.5 Concrete Plinth Height 4-78 4.6.3.1.6 Plinths on Decks Twisted for Superelevation 4-79 4.6.3.1.7 Direct Fixation Vertical Tolerances 4-79 4.6.3.1.8 Concrete Plinth Reinforcing Bar Design 4-79 4.6.3.2 Cementitious Grout Pads 4-82 4.6.3.2.1 Cementitious Grout Pad on Concrete Surface 4-83 4.6.3.2.2 Cementitious Grout Pad in Concrete Recess 4-84 4.6.3.2.3 Cementitious Grout Material 4-84 4.6.3.3 Direct Fixation "Ballastless" Concrete Tie Block Track 4-85 4.6.3.4 Plinthless Direct Fixation Track 4-86 4.6.4 Direct Fixation Fastener Details at the Rail 4-87 4.6.5 Direct Fixation Track Drainage 4-88 4.6.6 Direct Fixation Stray Current Protection Requirements 4-89 4.6.7 Direct Fixation Special Trackwork 4-90 4.6.8 Noise and Vibration 4-90 4.6.9 Direct Fixation Track Communication and Signal Interfaces 4-90 4.6.10 Overhead Contact System -- Traction Power 4-91 4.7 EMBEDDED TRACK DESIGN 4-91 4.7.1 Embedded Track Defined 4-92 4.7.2 Embedded Rail and Flangeway Criteria 4-93 4.7.2.1 Embedded Rail Details at the Rail Head 4-94 4.7.2.2 Wheel/Rail Embedment Interference 4-95 4.7.3 Embedded Track Types 4-96 4.7.3.1 Non-Resilient Embedded Track 4-97 4.7.3.2 Resilient Embedded Track 4-98 4.7.3.3 Floating Slab Embedded Track 4-99 4.7.3.4 Proprietary Resilient Embedded Rail Designs 4-100 4.7.4 Concrete Slab Track Structure 4-100 4.7.4.1 Embedded Rail Installation 4-102 4.7.4.1.1 Top-Down Construction -- Rail Support and Gauge Restraint 4-102 4.7.4.1.2 Floating Rail Installation 4-105 4.7.4.1.3 Alignment Control in Top-Down Construction 4-105
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From page 143... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-iv 4.7.4.1.4 Bottom-Up Embedded Rail Installation 4-106 4.7.4.2 Stray Current Protection Requirements 4-107 4.7.4.3 Rail Insulating Materials 4-109 4.7.4.3.1 Extruded Elastomeric Rail Boot and Trough Components 4-110 4.7.4.3.2 Resilient Polyurethane 4-111 4.7.4.3.3 Elastomer Pads for Rail Base 4-112 4.7.4.3.4 Elastomeric Fastenings (Direct Fixation Fasteners) 4-112 4.7.4.3.5 Concrete and Bituminous Asphalt Trough Fillers 4-112 4.7.4.4 Embedded Track Drainage 4-112 4.7.4.4.1 Surface Drainage 4-114 4.7.5 Ballasted Track Structure with Embedment 4-116 4.7.6 Embedded Special Trackwork 4-119 4.7.7 Noise and Vibration 4-121 4.7.8 Transit Signal Work 4-122 4.7.9 Traction Power 4-122 4.7.10 Turf Track 4-122 4.8 LRT TRACK ON BRIDGES 4-125 4.9 REFERENCES 4-125 List of Figures Figure 4.2.1 AAR-1B narrow flange wheel 4-3 Figure 4.2.2 Suggested standard wheel gauge -- transit system 4-5 Figure 4.2.3 Gauge line locations on 115 RE rail head 4-6 Figure 4.2.4 Filkins-Wharton diagram for determining flangeway widths 4-13 Figure 4.2.5 Filkins-Wharton plot to establish flangeways 4-14 Figure 4.2.6 Wheel sections for Nytram plot -- oblique view 4-15 Figure 4.2.7 Wheel sections for Nytram plot -- modified AAR-1B transit wheel 4-16 Figure 4.2.8 Static Nytram plot 4-18 Figure 4.2.9 Nytram plot -- rotated to first point of contact 4-19 Figure 4.2.10 Nytram plot -- rotated to second point of contact 4-20 Figure 4.2.11 Static Nytram plot with restraining rail 4-21 Figure 4.2.12 Nytram plot with restraining rail -- rotated to first point of contact 4-22 Figure 4.2.13 Nytram plot with restraining rail -- rotated to second point of contact 4-22 Figure 4.2.14 Rail cant design and wheel contact 4-29 Figure 4.4.1 Track transition slab 4-50 Figure 4.5.1 Ballasted single track, tangent track (concrete cross ties)
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From page 144... ...
Track Structure Design v-4 Figure 4.5.2 Ballasted single guarded curve track (concrete cross ties) 4-57 Figure 4.5.3 Ballasted double tangent track (concrete cross ties)
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From page 145... ...
4-1 CHAPTER 4 -- TRACK STRUCTURE DESIGN 4.1 INTRODUCTION The design standards for contemporary light rail transit (LRT) track structures, whether in an atgrade, aerial, or tunnel environment, differ considerably from the principles for either "heavy" rail transit or railroad service.
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From page 146... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-2 important that the track designer take steps to ensure that the vehicle designer does not select wheel parameters independent of track design. If, as is common, there are several series of vehicles in use on a rail transit system, each with a different combination of truck characteristics, the track designers must consider the worst-case requirements of each car series and optimize the track gauge parameters accordingly.
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From page 147... ...
Track Structure Design 4-3 For trucks with conventional solid axles and not independently rotating wheels, the freeplay assists in the steering or curving of the axle by the differential in wheel diameters, provided the wheel treads are tapered. See Article 4.2.4.1 for additional discussion on this point.
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From page 148... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-4 Railway Engineering Association (later renamed the American Transit Engineering Association or ATEA)
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From page 149... ...
Track Structure Design 4-5 and a larger B2B dimension is what permits LRT operations to successfully use groove rails with narrow flangeways. See Article 4.2.2.5 for additional discussion related to freeplay and the use of narrow flangeway groove rails.
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From page 150... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-6 B rail sections, had smaller gauge corner radii and thus were more conducive to gauge measurement closer to top of rail. Such rail is no longer commonly rolled in North America.
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From page 151... ...
Track Structure Design 4-7 Regardless of whether or not joint operation with a freight railroad is contemplated, there are several key issues to consider. These include the setting of the back-to-back wheel dimension, guard check gauge, and guard face gauge criteria that result from a particular wheel setting.
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From page 152... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-8 the transit system's gauge standards differ from AAR and AREMA standards so that construction and maintenance equipment do not damage the track. Refer to Chapters 13 and 14 for more on this subject.
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From page 153... ...
Track Structure Design 4-9 More latitude for joint operations in embedded track can be achieved using tee rails rather than groove rails; however, a separate flangeway must be constructed and maintained in the pavement surface. Refer to Chapter 5 of this Handbook for additional discussion concerning the application of tee rails to embedded track.
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From page 154... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-10 [1,435-millimeter] track gauge in both tangent track and virtually all radius curves without regard to whether railroad or transit design standards are used for wheel gauge.
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From page 155... ...
Track Structure Design 4-11 tracking performance when passing through reduced radius curves using groove rail. It is thought that reduction of track gauge could also reduce wheel squeal by limiting lateral wheel slip, which is believed to be a main source of such noise.
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From page 156... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-12 is usually the case that when the guard is worn the running rail is also worn to such an extent that it will soon have to come out also.[1] This excerpt provides still timely guidance in determining flangeway requirements, particularly for design of restraining rail systems, and evaluating the possible use of presently available groove rails.
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From page 157... ...
Track Structure Design 4-13 Figure 4.2.4 Filkins-Wharton diagram for determining flangeway widths
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From page 158... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-14 Figure 4.2.5 illustrates the flangeway requirements using outline K-L-M considering both flangeways using 59R2 groove rail and standard track gauge and AAR wheel gauge. Note how the "fattened" wheel flanges just barely fit in the flangeway.
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From page 159... ...
Track Structure Design 4-15 To illustrate the methods involved, a series of figures have been developed that illustrate the fundamentals of adapting track gauge to wheel gauge, wheel contour, and positioning of a truck on a segment of curved track. The figures consider the following parameters: Wheel Profile Modified AAR-1B 5 ¼ inches [133 millimeters]
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From page 160... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-16 Figure 4.2.7 Wheel sections for Nytram plot -- modified AAR-1B transit wheel
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From page 161... ...
Track Structure Design 4-17 Figure 4.2.7 illustrates the details of the process. Identification points are established on the surface of the wheel flange to define points of horizontal flange sections and assigned numbers from zero up to 10.
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From page 162... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-18 vary depending on whether the measurement is done at gauge line elevation or at some other elevation at or below the top-of-rail plane. When considering modern rail sections with compound radius gauge corners paired with a conformal wheel profile, a precise evaluation will typically reveal that the first point of contact between wheel and rail occurs at a point about 3/8-inch [9.5mm]
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From page 163... ...
Track Structure Design 4-19 Figure 4.2.9 illustrates the same vehicle truck as shown in Figure 4.2.8. The truck has been rotated about the center of the truck (Point "A")
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From page 164... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-20 • The absolute minimum flangeway widths necessary to permit free passage of the flanges. The last bullet point above becomes an important issue in embedded track using tee rail because if the flangeways are too narrow, unintentional contact could occur between the backs of the wheels and the paving material that defines the edge of the flangeway.
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From page 165... ...
Track Structure Design 4-21 is selected on a system due to restricted sharp radius curves, then a similar scenario should be undertaken using the parameters of the vehicle truck and track system to establish the flangeway. Figure 4.2.11 illustrates the truck shown in Figures 4.2.8 through 4.2.10 statically mounted on a curve with a restraining rail mounted along the inside rail of the curve.
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From page 166... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-22 Figure 4.2.12 Nytram plot with restraining rail -- rotated to first point of contact Figure 4.2.13 Nytram plot with restraining rail -- rotated to second point of contact
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From page 167... ...
Track Structure Design 4-23 All of the illustrations above use AAR wheel profiles and wheel gauge. If the same analysis is performed using a transit wheel profile and/or wheel gauge, different values will ensue.
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From page 168... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-24 self-steering of wheel sets through curves. The cant also moves the vertical wheel loading away from the gauge corner of the rail and toward the center of the ball of the rail head.
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From page 169... ...
Track Structure Design 4-25 result, the axle assembly steers itself around the curve just as a cone rolls in a circle on a table top. Note that rolling radius differential is maximized when the wheel and axle set is free to shift laterally an appreciable amount.
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From page 170... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-26 4.2.5.2 Rail Grinding Rail grinding is essential in transit track maintenance and is discussed in Chapters 9 and 14 of this Handbook. However rail grinding can also play a role in new track design.
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From page 171... ...
Track Structure Design 4-27 general quality of the finished grinding. For example, mill scale is completely a non-issue for freight railroads.
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From page 172... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-28 contacts the rail. Installing rails with no cant creates a contact zone or wear strip that is close to the gauge corner of the rail.
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From page 173... ...
Track Structure Design 4-29 Figure 4.2.14 Rail cant design and wheel contact
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From page 174... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-30 The benefits of differential cant, like those of asymmetrical rail grinding, decline as the wheels and rail wear. As wheel treads wear toward a flat or hollow profile and rails wear to conform with the wheel profile, self-steering capabilities decline.
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From page 175... ...
Track Structure Design 4-31 will be unsatisfactory, wear will be excessive, and the cost of restoration to a satisfactory state will be high. Either immediate corrective repairs, reduced speeds, or both are required once track has deteriorated to this condition.
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From page 176... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-32 Table 4.2.1 Track construction tolerances Construction Tolerances Location Tolerances Type of Track Track Gauge (5) Guard Rail Gauge (5)
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From page 177... ...
Track Structure Design 4-33 this Handbook so as to avoid any confusion with either the guard rails positioned opposite a frog or the "emergency guard rails" that are often positioned between the running rails on bridges. In addition to the discussion that follows here, readers are encouraged to consult two other documents on the topic of restraining rail that were produced by TCRP Project D-07 -- TCRP Research Results Digest 82: Use of Guard/Girder/Restraining Rails [7]
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From page 178... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-34 restraining rail. (Note that a small amount of loading would still be carried by surface friction between the tops of the rails and the treads of the wheels, but that is so small that it is usually neglected.)
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From page 179... ...
Track Structure Design 4-35 contact and that contact on only the restraining rail results in a higher lateral force than curves without restraining rail. This difference is unexplained but may be due to slight differences in the angles of attack between the restraining rail and the outer running rail.
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From page 180... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-36 • Frequently used tracks tend to develop a polish on the wheel/rail contact surfaces, which obviously reduces friction. Informal field observations suggest that tracks with these shiny rails that are both sharply curved and frequently used can be successfully operated without restraining rail and also with relatively little noise.
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From page 181... ...
Track Structure Design 4-37 standards recommended guard face angles that varied by the curve radius and most likely mimicked the angles to which restraining rails naturally wore in service. It should be noted that the contact point between an inside restraining rail and the back of the wheel usually occurs appreciably ahead of a vertical projection from the centerline of the axle.
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From page 182... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-38 nominal solution. More recent evidence suggests that no benefit is obtained by extending restraining rail more than about 10 feet [3 meters]
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From page 183... ...
Track Structure Design 4-39 Figure 5.2.5 of this Handbook, particularly for track embedded in pavement. For open track design, such as ballasted or direct fixation track, a separate restraining rail mounted alongside the running rail is more commonly used.
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From page 184... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-40 more than the usual ¼ inch [6 mm] above the running rail, particularly if one looks objectively at the typical construction and maintenance tolerances for the pavement on urban streets.
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From page 185... ...
Track Structure Design 4-41 pavement can be abraded and damaged by the backs of the wheels as they briefly act as a de facto restraining rail. Arguably, in open trackforms (i.e., ballasted and direct fixation track)
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From page 186... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-42 on a life cycle cost basis, including vehicle-related procurement and operation and maintenance costs. 4.4 TRACK SUPPORT MODULUS Railway track acts as a structural element that undergoes stress and strain as a vehicle passes over it.
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From page 187... ...
Track Structure Design 4-43 In direct fixation track, the track modulus is typically much higher, because the rail fasteners are made of elastomer with relatively high stiffness. In direct fixation track, the track designer is more frequently challenged to engineer a lower modulus into the track where possible, while still retaining required levels of gauge restraint and corrugation control.
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From page 188... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-44 The above analysis assumes that either the desired rail deflection is known or that maximum rail deflection is the primary criterion for the track design. Increasing the track modulus will dramatically reduce the bending moments in the rail.
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From page 189... ...
Track Structure Design 4-45 • 3500–5000 psi [24–34 N/mm2] : track - 24 inches [609.6 mm]
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From page 190... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-46 It is worthwhile to note that the spring rate of a direct fixation rail fastener is virtually never linear from a condition of zero load up to maximum service load. Instead, due to the elastic behavior of elastomers under loading, a plot of load versus deflection would be a curve.
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From page 191... ...
Track Structure Design 4-47 has not fully recovered when the next wheel load is applied. Low ratios of dynamic to static stiffness are achieved with natural rubber fasteners with low shape factor or in shear.
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From page 192... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-48 Track modulus values have very little meaning for designs where the bare rail is completely encased in concrete without rail boots, such as occurs in some "bathtub" embedded track designs. Rail deflections, if any, are extremely small -- possibly as low as 0.001 inches [0.025 millimeters]
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From page 193... ...
Track Structure Design 4-49 stability, as do non-ballasted "open" deck bridge structures where the rail is supported on rigid structural abutments and spans. 4.4.3.1 Interface between Track Types The transition interface points between embedded and ballasted track segments and between direct fixation and ballasted track are typically locations of sudden major changes in track modulus.
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From page 194... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-50 Figure 4.4.1 Track transition slab Center-to-center distances between cross ties are generally reduced in the transition slab section to provide additional stability and increase the track modulus. Cross tie lengths are also often increased incrementally for the same reason, and such arrangements have been standard details for most freight and passenger railroads and many transit agencies for a century or longer.
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From page 195... ...
Track Structure Design 4-51 measures had little benefit in terms of either reducing rail deflection or increasing track stiffness.[6] Additional research might be warranted into this topic.
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From page 196... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-52 track in advance of the interface is suggested. In the case of embedded track using rail boot, this might require placing additional elastomeric pads beneath the boot or transitioning to a trough type of embedded track with additional polyurethane grout beneath the base of rail.
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From page 197... ...
Track Structure Design 4-53 Additional suggested design features include the following: • Diverting surface runoff from the direct fixation track or embedded track sections so that it doesn't enter the transition area. In direct fixation track, provide an end barrier wall and drain surface runoff to the side of the track beyond the embankment.
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From page 198... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-54 could consist of either timber cross ties with conventional tie plates, cut spikes, and rail anchors or concrete cross ties with elastic rail fastenings that incorporate conventional insulating components (so as to retain traction power currents within the rail)
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From page 199... ...
Track Structure Design 4-55 with standard rail insulation. The yard maintenance facility tracks were generally built with timber cross ties either with or without insulated fasteners.
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From page 200... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-56 Resilient rail base pads are placed on concrete cross ties to protect the concrete tie seat and to impede the impact and vibration associated with wheel passage from migrating from the rail to the cross tie. Resilient rail base pads are a determining parameter of track modulus.
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From page 201... ...
Track Structure Design 4-57 Figure 4.5.2 Ballasted single guarded curve track (concrete cross ties) Although wood is an insulating material, timber cross ties provide only a limited barrier against stray current and become less effective in that regard over time.
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From page 202... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-58 portions of North America, Douglas fir is readily available and considered equivalent to eastern hardwoods. For additional information on timber cross ties, refer to Chapter 5.
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From page 203... ...
Track Structure Design 4-59 Figure 4.5.4 Ballasted double curved track (concrete cross ties) The concrete cross tie design includes the specific type of elastic fastening system (e.g., spring clip)
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From page 204... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-60 Langer[11]
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From page 205... ...
Track Structure Design 4-61 Subgrade Load at Tie Centerline is similar to subballast load calculation except depth includes ballast and subballast heights. Using the above formulas, Table 4.5.1 presents the values according to the parameters.
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From page 206... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-62 Lateral track stability is provided by ballast friction contact along the sides and bottom of the tie and by the end area of the tie. The end area of the tie provides a calculated degree of lateral stability; however, increasing the ballast shoulder width beyond an 18-inch [450-millimeter]
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From page 207... ...
Track Structure Design 4-63 Tropical hardwood ties, manufactured from species such as Bonzai, Ekki, and Azobe have been used in North American railway and transit trackage with mixed success. The reader is cautioned about using these tropical woods.
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From page 208... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-64 Similar to timber switch tie installations, insulated special trackwork plates may be required to control stray current on concrete switch ties. Insulated switch, frog, and guard rail fastening plates may be similar to conventional timber cross tie installations.
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From page 209... ...
Track Structure Design 4-65 provide additional lateral stability. The subballast and subgrade sections must be increased to provide sufficient support width if the ballast shoulders are increased.
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From page 210... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-66 4.5.6.4 Subgrade The subgrade is the finished embankment surface of the roadbed below the subballast that supports the loads transmitted through the rails, ties, ballast, and subballast. The designer should review the geotechnical engineer's analysis of the subgrade materials/soils along the entire route to determine whether all locations have both uniform stability and the strength to carry the expected track loadings.
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From page 211... ...
Track Structure Design 4-67 4.5.8 Retained Ballasted Guideway Right-of-way constraints and other situations often make it impossible to construct a ballasted track with open drainage ditches alongside of the roadbed. In such cases, a ballasted guideway can be constructed between curbs or walls and drainage provided by an underdrain system.
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From page 212... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-68 provides the barrier at the base of the fastening plate on timber cross ties. Concrete cross ties provide the isolation at the base of the rail using a pad on the rail seat and insulating pads between the base of rail and the rail clips.
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From page 213... ...
Track Structure Design 4-69 interlockings, at each end of station platforms, at-grade crossings, within individual turnouts and crossovers, and at other locations to be determined by the train control requirements. For additional information on transit signal work, refer to Chapter 10.
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From page 214... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-70 be cleaned out by a continuous maintenance program; otherwise, both stray current leakage and signal system malfunctions will develop. Due to the superior subgrade at most at-grade road crossings, the transition from ballasted track to the ballasted roadway track becomes a factor.
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From page 215... ...
Track Structure Design 4-71 4.6.2 Direct Fixation Track Criteria To develop direct fixation track design, the following track components and standards must be specified: • Rail section. • Track gauge.
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From page 216... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-72 are still in service. The design was simple: timber cross tie track was constructed in skeleton form, blocked up to grade and alignment, and then the lower portions of the cross ties were encased in concrete, locking the track structure in place.
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From page 217... ...
Track Structure Design 4-73 4.6.3.1 Reinforced Concrete Plinths The most common direct fixation track design is the raised reinforced concrete plinth system. The authors of this Handbook strongly recommend the use of plinths for most direct fixation track installations.
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From page 218... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-74 direct fixation track designs affect the lengths and shapes of the plinths and the reinforcing bar configurations as follows. 4.6.3.1.1 Concrete Plinth in Tangent Track Concrete plinths in tangent track generally follow one of two designs, both shown in Figure 4.6.1: • Concrete plinths of sufficient width and height mounted directly on the top of the concrete deck, slab, or invert.
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From page 219... ...
Track Structure Design 4-75 4.6.3.1.1.2 Concrete Plinth in Concrete Recess. The concrete plinth design has a variant wherein the second-pour concrete can be recessed into a shallow trough in the base concrete slab.
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From page 220... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-76 Figure 4.6.2 Concrete plinth design -- graduated J-bars to match superelevated plinth heights
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From page 221... ...
Track Structure Design 4-77 Figure 4.6.3 Concrete plinths -- superelevated track with restraining rail 4.6.3.1.4 Concrete Plinth Lengths Concrete plinths can be formed in various lengths. Typical plinths of intermediate lengths will accommodate three to six direct fixation fasteners between drainage chases, as shown in Figure 4.6.4.
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From page 222... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-78 Concrete plinth lengths are dependent on several track design factors: whether the track is tangent or curved, whether formwork in curved track is curved or chorded, and the locations of construction joints and expansion joints in the deck, slab, or invert. Concrete plinths in curved track are generally constructed in short tangent segments for ease of formwork.
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From page 223... ...
Track Structure Design 4-79 4.6.3.1.6 Plinths on Decks Twisted for Superelevation Several rail transit projects since 2000 have used segmental concrete girders where track superelevation has been achieved by twisting the deck. This allows all plinths to be the same height, as they are in tangent track.
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From page 224... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-80 The plinth reinforcement that is installed by the trackwork constructor consists of a series of "J- hook" bars and longitudinal bars. A transverse collector bar is sometimes placed at the ends of each concrete plinth for stray current control as shown in Figures 4.6.5A and 4.6.5B.
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From page 225... ...
Track Structure Design 4-81 Figure 4.6.5B Concrete plinth reinforcing bar details (continued) The design size of the concrete plinth will determine the lengths and bend radii of the "J" hooks and the length of the longitudinal bars.
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From page 226... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-82 The concrete plinth reinforcing bar system can be made electrically continuous by following these steps: • The deck or invert stirrups installed during the initial construction must be connected (welded) to the deck or invert reinforcing bar network.
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From page 227... ...
Track Structure Design 4-83 Figure 4.6.6 Cementitious grout pad design -- direct fixation track Grout pads typically support only a single fastener, although current practice is to build longer pads to support at least four fasteners. The longer design provides improved integrity of the pads and ease of maintenance if a fastener is replaced or needs to be repositioned.
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From page 228... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-84 Alternatively, the assembled rail and rail fasteners can be suspended at proper grade and alignment above the concrete invert and the grout either pumped, injected, or "dry packed" under the rail fastener. If this approach, known as top-down installation, is taken, it is essential to ensure that the grout does not enter the recesses on the bottom surface of the direct fixation rail fastener because this could compromise the rail fastener spring rate.
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From page 229... ...
Track Structure Design 4-85 climates that are subject to freeze-thaw cycles. Abbreviated height grout pads make installation of signal cables, conduits, and traction power bonding cables more difficult.
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From page 230... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-86 The electrical barrier for encased tie, direct fixation track systems is provided at the rail base. Similar to concrete tie fastenings, the electrical barrier is established by an insulated resilient rail seat pad and spring clip insulators.
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From page 231... ...
Track Structure Design 4-87 to begin and end at virtually the same point along the structure so as to match the superelevated bridge deck. • Extremely precise placement of the anchor inserts in the bridge segments at the casting yard, keeping in mind that each segment could be up to 30 feet [9.1 meters]
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From page 232... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-88 that storm water will drain off the surface of the plinth, particularly in track that is longitudinally level or only on a slight gradient. Lateral adjustment capability and fastener anchor bolt locations are important elements in the design and configuration of direct fixation rail fasteners.
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From page 233... ...
Track Structure Design 4-89 the invert. Close coordination as to interfacing design between track and system designers of disciplines is required to be certain that conduits, cabling, and the racks that support them do not block surface deck runoff and are not positioned so close to the deck that they will trap windblown debris, thereby creating dams and standing water.
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From page 234... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-90 situation get out of control have found it necessary to completely replace both direct fixation rail fasteners and rails that were damaged beyond salvage by corrosion. The most effective corrosion protection measure in such cases is likely a concerted housekeeping program that includes periodic power washing of the entire track structure to remove the contaminants that act as catalysts of corrosion in damp environments.
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From page 235... ...
Track Structure Design 4-91 greatly impact direct fixation track design, it can affect specific parts of the design. The prime example of this interrelationship is the need for insulated joints in the running rails to accommodate train control requirements.
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From page 236... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-92 • The LRT track is located in an exclusive separated guideway or lane with curbs, but, for reasons of aesthetics or housekeeping, it is deemed inappropriate to use an open trackform such as ballasted or direct fixation. In addition to typical structural design issues that affect any track, embedded track design must also address difficult questions with respect to electrical isolation, acoustic attenuation, and urban design, all in an environment that does not facilitate easy maintenance.
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From page 237... ...
Track Structure Design 4-93 • Embedded Track is founded on a concrete slab, similar to non-ballasted track [e.g., direct fixation track] … the paving infill is usually concrete or asphalt, but can also be pavers, paving stones, grass, etc.
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From page 238... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-94 4.7.2.1 Embedded Rail Details at the Rail Head The rail section and wheel profile used on a transit system must be compatible. Further, the rail installation method must be carefully detailed if the track system is to be functional, have minimal long-term maintenance requirements, and realize the expected rail life.
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From page 239... ...
Track Structure Design 4-95 False flanges should not be allowed to progress, especially to the -inch [3-millimeter] height, and the track designer should stress that the vehicle system maintenance policies must include a regular wheel truing program.
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From page 240... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-96 • Narrow wheels result in limited tread support at open flangeways and increase the possibility of wide gauge derailments. This typically forces the adoption of either flange-bearing special trackwork or the use of movable point frogs.
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From page 241... ...
Track Structure Design 4-97 The initial impact absorber on the track is the rail, specifically the rail head, followed by the fastening or supporting system at the rail base, and then the remaining track structure. The track structure's degree of resiliency dictates the amount of load distributed to the rail and track structure and the magnitude of force returned to the wheels and vehicle.
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From page 242... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-98 hardens, it tends to fracture and break down. The resulting water intrusion will accelerate deterioration of the entire track structure, especially in freeze/thaw climates.
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From page 243... ...
Track Structure Design 4-99 4.7.3.3 Floating Slab Embedded Track Ground-borne noise and vibration are a concern for embedded track sections adjacent to or near facilities that are sensitive to noise and vibration. These include hospitals, auditoriums, recording studios, symphony halls, schools, laboratories, and historic (potentially fragile)
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From page 244... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-100 4.7.3.4 Proprietary Resilient Embedded Rail Designs The design of a noiseless and vibration-proof track system has been and will likely continue to be an elusive task. Experimental, usually proprietary, design concepts to curtail noise and dampen vibrations are continuously emerging from many sources.
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From page 245... ...
Track Structure Design 4-101 Figure 4.7.2 Embedded track on leveling beams
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From page 246... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-102 Figure 4.7.3 Concrete slab with individual rail troughs designed jointly by track designers, structural engineers, noise and vibration experts, and the project's vehicle engineers. Similar joint efforts are required to adapt generic concepts for floating slabs supporting special trackwork to each specific project.
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From page 247... ...
Track Structure Design 4-103 within the tolerances required for track gauge. It must be possible to adjust the track gauge both in and out at any point along the rail in both tangent track and curves.
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From page 248... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-104 4.7.4.1.1.3 Rail Fastenings for Leveling Beams. The booted rail must be firmly held to the ties or leveling beams.
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From page 249... ...
Track Structure Design 4-105 might be required was at passenger stops, where braking and tractive efforts (including the use of magnetic track brakes) would wear the top of rail much faster than on plain running track.
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From page 250... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-106 alignment during the initial embedment material pour. Once set, the rail position cannot be adjusted to meet construction tolerances or future maintenance needs.
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From page 251... ...
Track Structure Design 4-107 • Rigid control of rail position during two-pour initial installations. • Anchor plates can be reused during future rail changeout to control rail position.
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From page 252... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-108 Principal measures to minimize traction current leakage are the following: • The use of a rail section providing electrical resistance not exceeding 0.0092ohms/1000 feet at 20 degrees C • The use of continuous welded rail providing superior traction power return over conventional electrically bonded jointed track.
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From page 253... ...
Track Structure Design 4-109 Encasing the rail in an insulating elastomeric (rubber) boot and thereby totally encapsulating the surface except for the rail head and gauge face.
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From page 254... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-110 4.7.4.3.1 Extruded Elastomeric Rail Boot and Trough Components Rail boot has proven to be a highly satisfactory rail base support material that provides minimal rail deflection. Extruded elastomeric rail boot sections are designed to fit and enclose the entire rail section, exposing only the head and flangeway in groove rail and the head and gauge face surface in tee rail.
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From page 255... ...
Track Structure Design 4-111 Using extruded insulation requires the two-pour method for base slab installation, including installation of the rail prior to placing the surrounding extruded component sections. Finally, the top concrete surface is then placed beyond the gauge and field sides of the extrusion.
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From page 256... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-112 mixing, handling, and application be carefully undertaken only by qualified contractors with product representatives present for initial installations to train the installation crew. Polyurethanes can be very difficult to install in tracks with any significant gradient as they flow to form a level surface when in liquid form.
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From page 257... ...
Track Structure Design 4-113 this accumulated moisture can freeze and damage both the rail embedment materials and the electrical isolation systems. Less severe damage can occur in the absence of freezing temperatures.
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From page 258... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-114 Polyurethane trough fills, on the other hand, provide an excellent bonding to the rail and the surrounding concrete, sealing off water seepage. 4.7.4.4.1 Surface Drainage Embedded track installations complicate pavement surface drainage because the exposed rail head and flangeways intercept and redirect lateral storm water runoff.
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From page 259... ...
Track Structure Design 4-115 The design of the rail through the drainage chase opening should consist of the exposed rail supported on each side of the chase wherein the rail acts as a suspended beam. The bottom of the flangeways must have openings wide enough to ensure that they will not become clogged with leaves or other debris.
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From page 260... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-116 Figure 4.7.9 Depressed pavement without flangeways 4.7.5 Ballasted Track Structure with Embedment Early 20th-century embedded track designs for urban streetcars and trams was typically ordinary ballasted track with timber cross ties that was subsequently embedded to the top of rail with some type of conventional paving material. Blockstone or brick was very common, and concrete and asphalt were also used.
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From page 261... ...
Track Structure Design 4-117 induced movements. The key to this ability was the use of hot tar to seal the joints between the pavers, thereby excluding most moisture.
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From page 262... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-118 Figure 4.7.11 Bituminous pavers with sealed joints Figure 4.7.12 Use of brick or stone pavers with embedded tee rail
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From page 263... ...
Track Structure Design 4-119 4.7.6 Embedded Special Trackwork The embedded special trackwork portion of any transit system will require special treatment and quite possibly a different design concept from the main line embedded track design. In contemporary light rail transit systems, embedded special trackwork generally consists of turnouts for entry onto other track(s)
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From page 264... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-120 other. In addition, additional measures may be necessary to provide acoustic attenuation, both to mitigate possible ground-borne vibrations and to prevent the pavement that encases the rails from resonating and amplifying vibrations within the track structure.
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From page 265... ...
Track Structure Design 4-121 Embedded special trackwork will also require the use of special leveling beams or plates to support the various track elements. These must be designed to develop uniform rail deflections matching the adjacent track system.
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From page 266... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-122 4.7.8 Transit Signal Work Transit signal requirements in embedded track sections differ from the general design standards for ballasted and direct fixation track. Embedded track within city streets may share the right-ofway with automobiles, trucks, and buses both at intersections and along the track.
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From page 267... ...
Track Structure Design 4-123 LRT lines and reconstruction of older tram routes. Landscaped track was developed for various reasons, including • Reducing the visual impact of the track system compared to either ballasted or direct fixation track.
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From page 268... ...
Track Design Handbook for Light Rail Transit, Second Edition 4-124 outside of New Orleans. Kenosha placed turf over fairly conventional concrete cross tie ballasted track.
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From page 269... ...
Track Structure Design 4-125 hold gauge throughout the installation. The base of rail is not connected to the concrete plinth.
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From page 270... ...
Track Design Handbook for Light Rail Transit, Second Edition 621-4 [5]
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Key Terms
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