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From page 411...
... i-7 Chapter 7 -- Structures and Bridges Table of Contents 1-7 NOITCUDORTNI 1.7 1-7 SEDOC NGISED 2.7 3-7 SECROF ELCIHEV 3.7 4-7 SNOITARUGIFNOC KCART DNA ERUTCURTS 4.7 4-7 snoitaredisnoC epyT erutcurtS 1.4.7 7-7 noitcurtsnoC kceD fo sepyT 2.4.7 7-7 noitcurtsnoC kceD noitaxiF tceriD 1.2.4.7 11-7 noitcurtsnoC kceD detsallaB 2.2.4.7 11-7 noitcurtsnoC kceD deddebmE 3.2.4.7 31-7 noitcurtsnoC kceD nepO 4.2.4.7 51-7 serutcurtS gnitsixE ot sliaR gniddA 5.2.4.7 61-7 NOITCARETNI ERUTCURTS/LIAR 5.7 61-7 lareneG 1.5.7 71-7 liaR dedleW suounitnoC 2.5.7 7.5.3 Force Distribution betwee 81-7 erutcurtsrepuS dna sliaR n 12-7 sreiP eht ta tnemegnarrA gniraeB 4.5.7 7.5.5 Rail/Structure In 12-7 sisylanA noitcaret 32-7 secnerruccO paG liaR/kaerB liaR 6.5.7 82-7 serutcurtS laireA no RWC gnitanimreT 7.5.7 92-7 SRENETSAF NOITAXIF TCERID 6.7 13-7 EDARG-NO-SBALS TROPPUS KCART 7.7 33-7 SECNEREFER 8.7 List of Figures Figure 7.2.1 Vehicle bending moments on simple spans 7-2 Figure 7.4.1 Direct fixation deck formwork for plinth dowels and recessed key for plinths 7-10 Figure 7.4.2 Completed deck with plinth recesses and dowels 7-10 Figure 7.4.3 Rail expansion joint on embedded track bridge 7-13 Figure 7.4.4 Example of open deck viaduct LRT aerial structure 7-13 Figure 7.4.5 Detail of open deck on LRT aerial structure 7-14 Figure 7.5.1 Radial rail/structure interaction forces[17] 7-21 Figure 7.5.2 Bearing configurations for elevated structure girders[17]
From page 412...
... Track Design Handbook for Light Rail Transit, Second Edition 7-ii Figure 7.5.4 Typical structural model components 7-24  Figure 7.5.5 Rail break gap size predicted by finite computer model[5] 7-27  Figure 7.5.6 Tie bar on aerial crossover 7-30  List of Tables Table 7.5.1 Effects of unbroken rail and column longitudinal stiffness on loads transferred to the substructure 7-26  Table 7.5.2 Comparison of rail break gap by different formulas[5]
From page 413...
... 7-1 CHAPTER 7 -- STRUCTURES AND BRIDGES 7.1 INTRODUCTION This chapter principally discusses the interaction between railway tracks and aerial structures, such as bridges and viaducts, that carry them and presents the items to be considered during the design of aerial structures. Additional discussion is provided concerning the design of slabs-ongrade for supporting embedded and direct fixation trackforms.
From page 414...
... Track Design Handbook for Light Rail Transit, Second Edition 7-2 Design (LRFD) Bridge Design Specifications.
From page 415...
... Structures and Bridges 7-3 The LRFD-based live load (designated as HL-93) has not been plotted in Figure 7.2.1 as it is a "notional" load and not intended to specifically represent a group of axles or a specific truck configuration.
From page 416...
... Track Design Handbook for Light Rail Transit, Second Edition 7-4 • Centrifugal force. This accounts for the radial force and overturning effect resulting from a vehicle traveling through a horizontally curved alignment.
From page 417...
... Structures and Bridges 7-5 • Capital cost • Maintenance cost • Availability of materials and finished product • Availability of construction expertise • Site working conditions, including weather, local ordinances, and working restrictions • Aesthetics • Owner's preference • Urban constraints • Durability • Construction schedule Substructure components typically are composed of reinforced concrete structures. Commonly considered substructure types include the following: • Single-column hammerhead-type piers • Multicolumn piers with rectangular caps • Trestle bents • Wall piers • Abutments of various types The substructure type and its supporting foundation play a role in the overall stiffness of the structure as related to the distribution of forces, as described in Article 7.5.4.
From page 418...
... Track Design Handbook for Light Rail Transit, Second Edition 7-6 • Vibration. The natural frequency of an aerial structure is directly related to the type and configuration of the superstructure.
From page 419...
... Structures and Bridges 7-7 7.4.2 Types of Deck Construction There are four different types of bridge deck construction used in rail transit construction. Below, these four types of bridge deck construction are listed in roughly chronological order, based on when they were first used for rail transit guideways: • The earliest elevated transit trackway (beginning around 1870)
From page 420...
... Track Design Handbook for Light Rail Transit, Second Edition 7-8 The advantages of the direct fixation trackform include the following:[2]
From page 421...
... Structures and Bridges 7-9 shear and tension forces as a result of the braking, accelerating, and lateral forces from the LRVs. The reinforcing in the plinths needs to be designed in conjunction with the rebar dowels that project up from the concrete slab beneath the plinths.
From page 422...
... Track Design Handbook for Light Rail Transit, Second Edition 7-10 (Photo courtesy of Bryant Contracting, Inc.) Figure 7.4.1 Direct fixation deck formwork for plinth dowels and recessed key for plinths (Photo courtesy of Bryant Contracting, Inc.)
From page 423...
... Structures and Bridges 7-11 7.4.2.2 Ballasted Deck Construction Ballasted deck construction is still considered a valid choice by most transit agencies. It is usually used on short to moderate length bridges, generally 300 feet [91 meters]
From page 424...
... Track Design Handbook for Light Rail Transit, Second Edition 7-12 • Determine the material that is to be placed adjacent to the rails to fill the trough up to the top of the deck. This material needs to be able to seal the trough so rain and snowmelt do not penetrate into the trough, compromising the service life of the deck surface.
From page 425...
... Structures and Bridges 7-13 (Photo courtesy of Trammco) Figure 7.4.3 Rail expansion joint on embedded track bridge 7.4.2.4 Open Deck Construction Although it is not used as commonly as either direct fixation or ballasted deck construction, open deck construction is a valid alternative for track support on bridges.
From page 426...
... Tra c As w coord suppo const k Design H ith the other inate closely rt structure. ruction includ The longitu The specia other types needs to b maintain a The detai accommod The relativ to the fixed The mater roadways surfaces o Details of t open deck Methods to structure.
From page 427...
... Structures and Bridges 7-15 • The maintenance and emergency walkway requirements and details. • Coordination with emergency response teams so they are aware of the open deck configuration and provision of the means to evacuate people from the LRVs, if necessary, in a safe manner.
From page 428...
... Track Design Handbook for Light Rail Transit, Second Edition 7-16 • In addition to the LRV loads, the existing bridge needs to be analyzed to address stray current corrosion, vibrations from the LRVs, and rail/structure interaction caused by the continuous welded rails restraining the thermal movement of the superstructure. • The expansion joints in the deck may need to be retrofitted to accommodate the installation of the concrete plinths.
From page 429...
... Structures and Bridges 7-17 7.5.2 Continuous Welded Rail The majority of the early elevated rail transit systems used trackwork composed of jointed rail supported on simple-span guideway structures. Alternatives have been developed for modern rail transit trackwork on aerial structures.
From page 430...
... Track Design Handbook for Light Rail Transit, Second Edition 7-18 • Providing sufficient rail restraint to prevent horizontal or vertical buckling of the rails • Providing anchorage of the CWR to prevent excessive rail gaps from forming if the rail breaks at low temperature • Determining the effect a rail break could have on an aerial structure • Calculating the thermal forces applied to the aerial structure, the rail, and the fasteners as the aerial structure expands and contracts and the CWR remains in a fixed position • Providing a connection between the CWR and aerial structure (direct fixation fasteners) that is sufficiently elastic to permit the structure to expand and contract without overstressing the fasteners An important element in the design of trackwork using CWR is the consideration of rail breaks.
From page 431...
... Structures and Bridges 7-19 (piers and abutments) through the fixed bearings and by shear or friction through the expansion bearings.
From page 432...
... Track Design Handbook for Light Rail Transit, Second Edition 7-20 − Elastic restraint developed in elastic fasteners − Elastic restraint developed in elastic fasteners with controlled rail slip − Elastic and slip fasteners installed in accordance with the expected relative movements between girder and rail: to control rail creep, install sufficient elastic fasteners near the fixed bearing; to provide full lateral restraint and minimal longitudinal restraint, install slip fasteners over the balance of the girder length Depending on the method used to attach the rails to the structure, the structural engineer must design the structure for longitudinal restraint loads induced by the fasteners, horizontal forces due to a rail break, and radial forces caused by thermal changes in rails on curved alignments. Today's designer can use computer models to simulate the entire structure/trackwork system to account for variations in the stiffness of the substructure and the dissipation of rail/structure interaction forces due to the substructure's deflection (see Article 7.5.4)
From page 433...
... 7.5.4 The m on the bearin bearin are c symm As a most cance and g fixed therm Althou loadin resist 7.5.5 Opinio desig rails a Bearing Arr agnitude of bearing arr g arrangem gs (or expan ommonly use etrical bearin guideline for desirable. In l each other eometry.
From page 434...
... Trac in the forces Some impor metho relate comp requir The c discre k Design H CWR, axial developed i Figure suggest tha tant consider ds are unrel d design ele uter software e investigatio The contro between th The contro during low The transf into the su hoice of the tion of the ex andbook f stresses ind n the support 7.5.2 Beari t hand calcu ations of rai iable in pred ments.[5] Tod to more "exa n include the l of stresses e rail and su l of the rail b -temperature er of thermall bstructure method us perienced st or Light Ra uced in the ing substruct ng configura lations are a l/structure in icting stresse ay's structur ctly" analyze following: in rails attr pporting supe reak gap size rail pull-apar y induced loa ed to analyz ructural engin il Transit, 7-22 guideway str ure.[8]
From page 435...
... Structures and Bridges 7-23 and other considerations, simple formulas may be used to determine the structural requirements. Alternately, complexities such as curved alignments, varying span lengths, and the type of structural elements may require that a rigorous three-dimensional structural analysis be performed.
From page 436...
... Track Design Handbook for Light Rail Transit, Second Edition 7-24 Figure 7.5.3 Typical structural analysis model Figure 7.5.4 Typical structural model components
From page 437...
... Structures and Bridges 52-7 The unbalanced force from the broken rail is resisted by the other unbroken rail(s) and the aerial structure.
From page 438...
... Track Design Handbook for Light Rail Transit, Second Edition 7-26 Table 7.5.1 Effects of unbroken rail and column longitudinal stiffness on loads transferred to the substructure[5] Medium Restraint High Restraint Column Stiffness (lb/in)
From page 439...
... Equat eleme mode struct condit Table rail ga fasten the ch limited range that s limitin It is in reaso On th shoul with a fasten struct for ea ions 2 and 3 nt computer ling discussio ural model c ion. Refer to Figure 7.5.2 summ p size has b er spacing a ance of a ra based on t of 2 inches [ mall are seld g the rail gap teresting to ns, the lengt e other han d be minimiz relatively h ers with a ural enginee ch structure.
From page 440...
... Track Design Handbook for Light Rail Transit, Second Edition 7-28 Table 7.5.2 Comparison of rail break gap by different formulas[5] Rail Break Gap Size Estimates (in)
From page 441...
... Structures and Bridges 7-29 As an alternative to rail anchors, some transit systems have used a tie bar device to accommodate specialwork on their aerial structures. Tie bars carry the CWR stresses around the special trackwork unit at deck elevation rather than transferring those loads down to the substructure.
From page 442...
... Track Design Handbook for Light Rail Transit, Second Edition 7-30 The level of longitudinal restraint chosen for the fastener is a compromise between the restraint required to limit the rail gap size and the desire to minimize rail/structure interaction forces.[6]
From page 443...
... Structures and Bridges 13-7 The following are typical ranges of direct fixation fastener properties: Vertical fastener stiffness: 75,000 to 150,000 lb/in (13,300 to 26,600 N/mm) Lateral fastener stiffness: 22,000 to 64,000 lb/in (3,900 to 11,400 N/mm)
From page 444...
... Track Design Handbook for Light Rail Transit, Second Edition 7-32 The presence of steel reinforcing in slabs-on-grade used to support LRV traffic raises concerns related to stray current and its mitigation. Questions arise related to whether the slab reinforcing attracts or deters stray current.
From page 445...
... Structures and Bridges 7-33 cost. At least two legacy streetcar systems use no reinforcing steel at all in their embedded tracks.
From page 446...
... Track Design Handbook for Light Rail Transit, Second Edition 7-34 [9] New York City Transit, Metropolitan Transit Authority, Continuous Welded Rail on Elevated Structures, August 1991.
From page 447...
... Structures and Bridges 53-7 [27] Kaess, G., Schultheiss, H., "Germany's New High-Speed Railways, DB Chooses Tried and Tested Track Design," International Railway Journal, September, 1985.

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