Track Design Handbook for Light Rail Transit, Second Edition (2012)

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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] 7-22 Figure 7.5.3 Typical structural analysis model 7-24

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] 7-28 

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-on- grade for supporting embedded and direct fixation trackforms. Railway aerial structures come in many forms, and each has a different level of interaction with the tracks carried. At one extreme are heavy ballasted track bridges that have very little structural interaction between the rails and the structure. At the other end of the spectrum are concrete deck structures with continuous welded rail (CWR) directly affixed to the deck. This design is very typical of modern rail transit aerial structures. These structures can have significant interaction between the rail, which does not move, and the structure, which must expand and contract with changes in temperature. Somewhere in the middle of the spectrum are open deck bridges, which are very commonly used for long railroad structures and are often found on older urban rapid transit railways. These lighter structures generally use jointed rail to limit the interaction between the rail and the structure, but many have been successfully upgraded to use CWR. Finally, very nearly in a class by themselves, are bridges that have rails embedded into a concrete pavement, thereby allowing the trackway to be used by both rail and rubber-tired vehicles. The design of aerial structures for light rail transit systems involves choosing a design code, determining light rail vehicle (LRV) forces, confirming track configuration requirements, and applying rail/structure interaction forces. This interaction is affected by such factors as the bearing arrangement at the substructure units, trackwork terminating on the aerial structure, type of deck construction, and type of rail fasteners. The details of the trackwork design significantly affect the magnitude of the forces that must be resisted by the aerial structure. In order to efficiently design an aerial structure for an LRT project, it is critical for the structural engineer to coordinate early and continuously with the trackwork engineer to fully understand the trackwork- related issues that affect the design of an aerial structure. Structural engineers should be involved in the project as early as the planning phase to provide support to the trackwork engineer. Although they are not an aerial structure component, slabs-on-grade present challenges in structural design and detailing on current-day LRT projects. This chapter discusses some of the issues that warrant consideration during the design phase of slabs-on-grade. 7.2 DESIGN CODES As of 2010, there is no nationally accepted design code that has been developed specifically for light rail transit aerial structures. In addition to owner-specific and local design codes, designers must choose between the Standard Specifications for Highway Bridges, published by the American Association of State Highway and Transportation Officials (AASHTO), the Manual for Railway Engineering (MRE) issued by the American Railway Engineering and Maintenance of Way Association (AREMA), and, more recently, the AASHTO Load and Resistance Factor

Track Design Handbook for Light Rail Transit, Second Edition 7-2 Design (LRFD) Bridge Design Specifications. Unfortunately, neither set of AASHTO specifications nor the AREMA manual accurately defines the requirements of an aerial structure to resist light rail transit loads, although the AASHTO specifications are generally more applicable. It has been common for transit agencies to base the aerial structure design criteria on their state department of transportation’s (DOT’s) highway bridge design criteria. Prior to 2007, those criteria were typically derived from the AASHTO Standard Specifications for Highway Bridges. In October of 2007, the Federal Highway Administration (FHWA) mandated that new and replacement highway bridges that are part of federal-aid funded projects are to be designed according to the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications. As such, if transit agencies continue electing to follow local DOT highway bridge design criteria, the AASHTO LRFD criteria will apply. Research shows that several recent LRT projects’ aerial structure design criteria have been established as LRFD based. This is evidence of LRT aerial structures moving toward being designed according to the LRFD methodology, and this apparent trend will likely continue into the future. Most light rail transit loads are greater than the HS20 truck load used by the AASHTO specifications, but light rail transit loads are much less than the Cooper E80 railroad loading cited in the AREMA manual. Figure 7.2.1 plots bending moment versus span length for the Cooper E80 railroad load, the HS20 truck load, and the LRV load from the Dallas and St. Louis transit systems. As shown in Figure 7.2.1, for a 100-foot [30.5-meter] span, the LRV produces a bending moment approximately 50% higher than that produced by the HS20 truck load, but less than 20% of the bending moment caused by the Cooper E80 railroad load. Figure 7.2.1 Vehicle bending moments on simple spans

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. Also, the HS20 live load moments cannot be directly compared to the HL-93 moments since a unique set of load factors applies to each according to the AASHTO Standard Specifications (for HS20) and the AASHTO LRFD Bridge Design Specifications (for HL-93), respectively. The AREMA Manual for Railway Engineering, although applicable to railroad structures, is too restrictive for light rail transit structures due to the great difference in loadings. Wheel spacings for AREMA loading do not correspond to those found on LRVs, and the AREMA impact criterion is not consistent with the suspension and drive systems used on LRVs. The service conditions, frequencies, and types of loading applicable to freight railroad bridges are not consistent with those items on dedicated light rail transit systems.[1], [2] A strong similarity exists between light rail transit design requirements and the AASHTO specifications. For light rail transit aerial structures, the ratio of live load to dead load more closely approximates that of highway loadings than that of freight railroad loadings. In addition, since the magnitude of the transit live load can be more accurately predicted, the conservatism inherent in the AREMA Manual for Railway Engineering is not required in light rail transit structures. It is interesting to note that the older “legacy” rail transit systems (e.g., in Chicago, Philadelphia, and New York) often refer to the AREMA Manual for Railway Engineering for the design of bridges, but the newer systems (e.g., in Atlanta, Washington, D.C., and Baltimore) base their designs on the AASHTO specifications. This is partly due to an increased understanding of an aerial structure’s behavior and the designer’s confidence in the ability to accurately predict transit loads. Although there is no current bridge design code that is completely applicable to light rail transit bridges, the use of the AASHTO specifications will result in a conservative design that is not overly restrictive or uneconomical.[1], [2], [3] 7.3 VEHICLE FORCES The vehicle forces applied to an aerial structure are often set by the transit agency’s design criteria for site-specific circumstances. In addition to the loadings imposed by trains of rail vehicles, many rail transit properties include other, heavier vehicles in their design criteria for aerial structures. These vehicles might include a crane car, an overhead lines maintenance car, a work train consisting of ballast cars and flat cars pulled by a locomotive, and even highway vehicles (during construction). On the other hand, some transit properties establish the LRV as the sole basis of design for the aerial structures. In addition to the LRV and alternative vehicle live loads applied to the aerial structure, the following vehicle-related forces are also typically considered in structural design: • Vertical impact. This represents a dynamic load allowance that is applied to the static wheel load to account for wheel load impact from moving vehicles. • Horizontal impact. Similar in nature to vertical impact, this vehicle force is associated with dynamic effects but is applied horizontally to the aerial structure.

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. • Rolling force (vertical force applied at each rail, one up and one down). Also known as the rocking effect, this vehicle force accounts for the inherent rocking of the vehicle back and forth during its travel. • Longitudinal force. This results from braking and acceleration/tractive effort. • Derailment forces. These can be considered analogous to collision-type loadings or other extreme event loadings associated with highway bridge design. Derailment forces are attributed to LRV(s) coming off of the running rails, which results in a lateral excursion of the vehicle from the centerline of track. Typically, derailment loads are applied both vertically and horizontally to the structure and affect the superstructure and substructure designs. Derailment forces must be developed in conjunction with the trackwork configuration used on the aerial structure. Given the common use of plinths, guard rails, and/or restraining rails, the physical possibility of any significant lateral excursion of the vehicle should be considered early in the design phase. Guard rails and restraining rails limit lateral excursion and, as a result, minimize some force effects due to vehicle derailment. Combinations of vehicle forces, in conjunction with dead loads, wind loads, thermal loads, and seismic loads, are developed to generate the load cases that govern the design of an aerial structure. 7.4 STRUCTURE AND TRACK CONFIGURATIONS 7.4.1 Structure Type Considerations During the early stages of design, the designer must determine the type of superstructure and substructure to be used for a specific aerial transit structure. Whether the superstructure is composed of steel or concrete girders, as well as the configuration of the girders, must be evaluated with respect to the project and site constraints. Commonly considered superstructure types include the following: • Cast-in-place concrete • Precast concrete girders with cast-in-place concrete deck slab • Segmental precast concrete • Steel girders with cast-in-place or precast concrete deck slab • Steel box section with either cast-in-place or precast concrete deck The following factors are used to comparatively study superstructure types:[1], [4], [15] • Effectiveness of structural function (span lengths, vertical clearances, span-to-depth ratio, etc.) • Constructibility issues, such as erection and construction convenience, including transportation of the structural elements to the site • Production schedule constraints

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. Substructure type comparisons must consider many of the same factors outlined above for making superstructure type selections. In addition to the parameters listed in regard to choosing the optimum structure type, there are other design and performance issues that warrant consideration during the decision-making process. These include concerns with differential deflections related to station platforms adjacent to track structures, vibration issues, stray current isolation (corrosion control), and miscellaneous structural details. • Differential deflections. In some cases, aerial station platforms positioned adjacent to the track structure may be required. This requirement depends on site constraints, geometric constraints, right-of-way limitations, and other project constraints. The Americans with Disabilities Act Accessibility Guidelines (ADAAG) provide limitations related to horizontal gaps between platforms and LRVs and limitations on vertical deflection between the top of the platform (walking surface) and the floor of the LRV. Detailed structural analyses are required to design the track structure and platform structure to satisfy the differential deflection criteria. Locating stations on-grade rather than on an aerial structure, if at all possible, is recommended.

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. The span length, structure depth, and structural stiffness are variables that affect the structure’s natural frequency. For instance, a long span steel structure with limited structure depth available may not be as stiff as a short span concrete girder bridge without structure depth limitations. Refer to Chapter 9 for further discussion associated with the vibration of the structure-track-vehicle system. • Stray current isolation. The ability of the structural details to provide stray current isolation is an important consideration, and various details offer differing degrees of isolation. For example, steel bearings provide less stray current isolation than elastomeric bearings. In addition, locations of epoxy-coated reinforcement versus uncoated reinforcement can play a role in the overall stray current isolation plan. The weldability of the reinforcing steel is also a consideration as it affects the ability to achieve electrically continuous reinforcement in a concrete deck slab. Refer to Chapter 8 for further discussion related to stray current control issues. • Miscellaneous structural details. Aerial structures not only need to support and accommodate the LRVs, but they also typically support utilities, safety handrailings, maintenance walkways, overhead contact system (OCS, also known as “catenary”) poles, and other miscellaneous items. These items and their locations need to be carefully coordinated to ensure that required vehicle clearance envelopes are satisfied and that the functionality and maintenance needs of the various systems are accommodated. As an example, an OCS pole may need to be located between the two tracks on a double-track structure with direct fixation deck. The pole may be mounted to the substructure and would need to pass up through the deck. The structural details associated with an opening in the deck would need to be coordinated among the track, OCS, and structural engineers to ensure that the details accommodate the needs of each discipline. Structure type selection will also consider the construction costs for each alternative. Historical cost data can be obtained from previously constructed LRT projects and extrapolated to current costs. In addition, most state DOTs archive cost history data related to highway bridge construction. These cost data are relatively easily obtained and can serve as an approximation of costs associated with the construction of LRT aerial structures. It is likely that these highway bridge cost items would need to be adjusted to apply to the specific aerial structure under design. As part of the preliminary design effort for an aerial structure, a study should be performed to determine the most desirable structure configuration based on economic, social, environmental, and technical needs. Various span arrangements should be considered in the design. The final span length selection should include consideration of aesthetics and community factors in addition to structural efficiency. Many times in an urban setting, span lengths are specified that provide the required horizontal and vertical clearances to existing facilities along the rail system’s alignment. The location of existing railroad tracks, roadways, highway bridges, waterways, and major utilities can restrict substructure locations, thereby limiting the choices for span lengths.

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) featured open deck construction, in which timber cross ties were attached directly to the steel superstructure. This type of construction is lightweight, especially compared to the ballasted deck construction described below, and is very common for railroad structures. However, radiated noise from open deck structures was a significant problem, particularly with relatively lightweight superstructures. Vibration of deep web girders on open deck elevated structures—a phenomenon sometimes called “oil canning”—amplified all other noises. • Embedded deck construction became necessary very early for streetcar lines that crossed bridges that were shared with wagons, carriages, and other users of the public streets. In this kind of construction, the only portion of the track structure that is visible is the running surface on the tops of the rails. The remainder of the track structure is concealed by the roadway pavement. • Ballasted deck construction, also common in railroad work, became common for urban rail transit around the beginning of the 20th century, partially in response to the public’s complaints about the noise and vibration generated by open deck structures. However, ballasted deck elevated lines were very expensive due to the much heavier structures required. Track maintenance on ballasted deck aerial structures is also difficult, particularly the spot replacement of defective cross ties. • Direct fixation deck was developed to resolve the shortcomings of the ballasted and open deck designs. The earliest direct fixation track sections (used in both subways and on aerial structures) consisted of timber ties and half-length ties embedded into a poured concrete deck. This type of construction was commonly employed for rail transit structures as late as the 1960s. In contemporary direct fixation track designs the embedded ties have been eliminated and, instead, rail fasteners are used that can be anchored into a reinforced concrete substrate and that incorporate rail restraint, acoustic attenuation, and electrical isolation features. The articles below address each of these major structural trackforms. They are discussed in the sequence in which they most commonly appear on light rail transit projects. 7.4.2.1 Direct Fixation Deck Construction Direct fixation deck construction is the standard practice for most transit properties for bridges that are 300 feet [90 meters] or longer. The current concepts, as developed in the 1960s for then-new heavy rail transit projects such as BART (San Francisco), WMATA (Washington, DC) and MARTA (Atlanta), have the rails attached to the concrete deck by resilient fasteners. The various possible methods of this attachment are detailed in Chapter 4, Article 4.6.

Track Design Handbook for Light Rail Transit, Second Edition 7-8 The advantages of the direct fixation trackform include the following:[2],[10] • Absorbs noise and vibration and provides vertical flexibility with resilient direct fixation fasteners • Improves aesthetics by using shallower, less massive structures • Results in a relatively low dead load compared to other trackforms • Provides both electrical isolation and a means to efficiently adjust the line and grade of the track with rail fasteners • Requires less maintenance and is easier to maintain than alternatives • Retains track geometry much longer than ballasted track • Provides relatively good ride quality • Offers relatively good live load distribution While direct fixation trackwork is appreciably more expensive than ballasted track, the reduced dead load of the track means that the bridge superstructure and substructure can be less robust and hence less expensive, resulting in a net savings per unit length of route. The use of direct fixation track construction was credited with saving millions of dollars on one 1960s heavy rail transit project by eliminating the weight of cross ties and ballast.[14] As detailed in Chapter 4, Article 4.6, direct fixation track can be configured in several different ways. That information will not be repeated here. The important thing to note for purposes of this chapter is that the practical construction tolerances for bridges, tunnel inverts, and slabs-on-grade are appreciably looser than those required for the top-of-rail profile of railway tracks. Installation of direct fixation trackwork therefore requires either unusually tight construction tolerances for the underlying support structure or a relatively simple way of compensating for deviations. In the case of bridges, the construction tolerances for the construction of the deck, including residual deck camber, are usually far less stringent than are necessary to provide a satisfactory top-of-rail profile. Because of this, most rail transit systems use a concrete pad, or plinth, approximately 6 inches [150 mm] tall to support the direct fixation fasteners and anchor them to the superstructure. Intermittent gaps are provided along the length of the plinths to accommodate deck drainage and to provide openings for electrical (systems) conduits placed on the deck. In addition, the deck slab sometimes incorporates recesses to accept the second-pour plinths. Each plinth recess forms a shear key to help resist the lateral loads from the rail and vehicles and also slightly reduces the effective height of the plinth. The second-pour concrete plinths are carefully constructed to meet the alignment and profile requirements of the CWR and fasteners. Reinforcing steel dowels (typically in the form of inverted U-bars and often called “stirrups”) project from the bridge deck, anchoring the second-pour concrete plinths to the deck. (Per CRSI, “dowels” is the correct terminology for these bars, but “stirrups” is a common vernacular among transit trackwork designers and constructors.) The reinforcing in the concrete plinths should be designed to assist in resisting the loads imparted by the rail fastener anchors to the plinths. In addition to the compression forces on the plinths, the fastener anchors must resist horizontal

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. These dowels attach the plinth and fastener anchorage system to the supporting slab. Depending on the loads being resisted, the rebar dowels have been installed longitudinally along the length of the plinths or transverse to the plinths. In either orientation, the height that the dowels project above the slab depends on the details of the reinforcing in the plinths. Figures 4.6.1 through 4.6.5 in Chapter 4 of this Handbook illustrate common reinforcing steel details for concrete plinths. See Chapter 13, Article 13.3.2 for additional discussion on the relative merits of installing the dowels transverse to the track versus parallel to the rails. Some transit projects have installed the dowels in the slab while the concrete was still wet, and other projects have drilled and grouted the dowels into the completed supporting slab. Should the drilling and grouting method be considered, the designer needs to evaluate the chance that the reinforcing in the slab will be damaged by the drilling, and the capacity of the grout is dependent on the proper mix being installed. With either installation method, it is critical that the contractor install the dowels at the proper height projecting above the supporting slab to properly engage the plinth reinforcing. The typical light rail transit project includes a multitude of contractors and subcontractors working in the same corridor. As such, no matter which orientation of rebar dowels is chosen, the dowels need to be protected from damage by the follow-on construction activity. This is particularly important if the dowels are epoxy-coated rebars. One method to protect the dowels is to require the contractor to install temporary timber blocking that is the same height as the dowels and adjacent to them. Figures 7.4.1 and 7.4.2 illustrate two stages of the construction of a bridge deck to support direct fixation track. In Figure 7.4.1, the deck is ready to be poured. The formwork has been positioned along the path of each rail so as to provide a recessed key in the deck for the direct fixation track plinths. The holes in the forms are for the placement of dowels when the deck is poured. In Figure 7.4.2, the deck has been poured and the formwork stripped, and the installation is ready for the construction of the direct fixation track system. Although direct fixation is the current standard practice for deck construction, there are some disadvantages to consider. Disadvantages of direct fixation deck include the following: • Rail/structure interaction must address thermal forces • Relatively high initial cost • Tight construction control required • Specialized rail fasteners required

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.) Figure 7.4.2 Completed deck with plinth recesses and dowels

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] or less. Advantages of the ballasted deck include the following:[2],[4],[10] • Provides an intermediate cushion between the rails and the structure to enhance ride quality. • Limits the transfer of thermal forces from the track to the superstructure. • Uses standard ballasted track rail fastening hardware. • Reduces noise and vibration compared to open deck construction. • Permits standard track maintenance methods to adjust alignment and profile. • Provides good live load distribution and good track support. Disadvantages of the ballasted deck include the following: • The cost of deck waterproofing beneath the ballast layer. • The relatively heavier deck load. • The greater depth of deck required. • Rail breaks can result in horizontal, vertical, and angular displacements. • The cost of maintenance of the ballast layer, including track resurfacing (tamping) and periodic ballast cleaning. Without the latter, the ballast on a bridge will tend to lose much of its original resiliency. 7.4.2.3. Embedded Deck Construction Where lanes on a bridge deck are shared by LRVs and rubber-tired traffic, the rails are embedded in the concrete deck so that the tops of the rails are at the same elevation as the top of the pavement. Various items are considered when preparing the designs for the embedded rails, and the bridge engineer must coordinate extensively with the trackwork engineer at the early stages of the design process to understand the requirements of the track system. Some items for consideration are the following: • Determine the dimensions and details of the trough or recess in the deck to accommodate the rails, fasteners, fastener anchorage items, waterproofing system, and stray current protection system. • Check the strength of the concrete deck to support the LRV loads, especially in the thinner deck section beneath the rails. One method to avoid thickening the deck for the entire width of the bridge is to position beams beneath the deck directly under the rail locations. • Carefully detail the reinforcing steel in the deck under and around the rail troughs in order to maintain the required deck strength.

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. The fill material should be chosen to minimize the long-term maintenance of this area. • Develop the transverse expansion joints in the deck that accommodate the bridge’s thermal movement to efficiently interface with the rails passing through them. Special rail expansion joints may need to be detailed adjacent to these bridge expansion joints so that the thermal movement of the bridge is not restrained. • Consider the rail/structure interaction forces that develop due to the restraint offered by the continuously welded rail in the rail troughs. Determine the gap in the rail that could develop should there be a rail break. • Review the details of the deck drainage system in relationship to the embedded rails. The runoff crossing the deck will be interrupted by the rails and end up traveling longitudinally along the deck in the rail trough. A drainage system needs to be developed to periodically capture this runoff along the rails. • With the loads from the LRVs being concentrated below the rail trough and not dispersed to a wider deck section by the plinths, the structural engineer has to carefully examine the superstructure to confirm that there are not any adverse effects. For steel structures, this includes an examination of the fatigue stresses and fatigue-sensitive connection details on the bridge. • Review the surroundings of the aerial structure location to determine whether there are any noise-sensitive facilities beneath or adjacent to the aerial structure. The embedded track construction is likely to generate more noise from the LRVs running along the embedded track than the direct fixation alternative that uses plinths and fasteners on top of the deck. Without appropriate detailing, the entire bridge deck could resonate, amplifying the rolling noise that originates at the wheel/rail interface. It is highly unlikely that any rail fastening design detail would permit the bridge superstructure to expand and contract independent of a rail that is embedded within it and do so reliably over the long term with minimal maintenance. Therefore, a rail expansion joint is virtually certain to be necessary at any expansion joint in the bridge deck. The rail expansion joint will necessarily provide an opening in the deck surface so that the rail ends can move relative to each other. That opening will inevitably become clogged with dirt and debris if the joint is not configured so that storm water will flush it to scuppers below the deck. Figure 7.4.3 illustrates a rail expansion joint at the abutment of an embedded track bridge. Note that the rail expansion joint does not straddle the deck expansion joint but is instead fully supported by the abutment. See Chapter 5 for additional discussion of embedded rail expansion joints. See Chapter 8 for additional discussion of stray current issues.

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. Open deck construction is typically only employed when rehabilitating an older railroad bridge that, for whatever reason, cannot be changed to another trackform. (However, at least one LRT project has built entirely new open deck bridges.) Figures 7.4.4 and 7.4.5 illustrate such structures. Notable is the non- conventional use of concrete AASHTO beams to support the ties rather than the usual steel girders or timber stringers. Figure 7.4.4 Example of open deck viaduct LRT aerial structure

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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. • Provision of sufficient lateral bracing in the superstructure framing. The lateral bracing system provides stability to the superstructure and resistance to potential superstructure overturning effects that can be associated with centrifugal forces in curved alignments and derailment forces. • Confirmation that cross ties can be sufficiently dapped to accommodate the as-erected girder camber and deflection conditions. • Evaluation of structural vibrations as related to rider comfort and dynamic interaction between the aerial structure and the LRVs. 7.4.2.5 Adding Rails to Existing Structures There are instances when it may be advantageous and cost-effective to utilize an existing bridge for the construction of a light rail transit line. Although LRV loading is higher than the truck loads that highway bridges are designed to resist, an existing bridge may be a candidate for strengthening in order to add tracks for light rail transit. Assuming that embedded track isn’t required to accommodate mixed rail and highway traffic on the existing bridge, and no accommodation for future rails was provided when the bridge was originally built, concrete plinths would typically be added on top of the deck for anchorage of the direct fixation fasteners. There are a number of items that need to be studied in order to confirm the feasibility of adding tracks to an existing bridge: • A structural analysis of the existing bridge needs to be performed to determine the strengthening needed to support LRV loads. Even if the deck itself is structurally sound, it is unlikely that the rails will fall directly above existing stringers. So as to minimize shear stresses in the deck, supplemental stringers may be required below the deck directly beneath where the rails will be installed. The existing bearings need to be analyzed in addition to the superstructure and substructure. Careful consideration of the LRV loads needs to occur, since these loads are different in magnitude and direction than the loads from the highway trucks that the bridge was first designed to carry. For steel structures, the existing connection details need to be reviewed to determine whether any of them are fatigue-sensitive details, requiring an analysis of the fatigue stresses. • The details of adding the concrete plinths to the bridge deck need to be determined. In order to drill holes in the deck to insert rebar dowels and grout to tie the plinths to the deck, it is highly desirable to identify the locations of the existing reinforcing in the deck so the drill holes do not damage the existing reinforcing. There is non-destructive test equipment that can be used to locate the existing reinforcing.

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. • The drainage patterns on the deck will be altered by the installation of the concrete plinths. Horizontal gaps should be included along the length of the plinths to permit runoff to flow to the existing deck drains. Alternatively, new deck drains can be added. • The bridge will need to be retrofitted to provide support for the OCS poles, unless the bridge is short and the OCS poles can be installed at the approaches to the bridge. The barriers along the sides of the deck and the bridge piers are possible locations for the OCS poles. On a through truss bridge, it is typically possible to hang the OCS directly from existing cross members above the deck. • As with embedded deck construction, the surroundings of the bridge location need to be reviewed to determine whether there are any noise-sensitive facilities beneath or adjacent to the bridge. Vibration of the deck and the webs of any tall steel girders can result in a noise phenomenon sometimes known as “oil canning.” Mitigation measures may need to be performed to address the rolling noise from LRVs crossing the deck.   • The details to transition the light rail tracks from the deck to approach areas beyond the abutments must be determined. Abrupt changes in track stiffness should be avoided. 7.5 RAIL/STRUCTURE INTERACTION 7.5.1 General With widespread use of CWR, the designer of an aerial structure must be aware of trackwork design and installation procedures, as well as vehicle performance and ride comfort issues. Trackwork design and installation procedures are especially critical in establishing the magnitude of the interaction forces between the rail and aerial structure. As the temperature changes, the superstructure (including both the deck and the supporting girders) expands or contracts. Depending on the type of trackform, the interaction of the track structure and the bridge structure will exhibit different behavior. On an “open” trackform (ballasted, open deck, or direct fixation), rails are effectively stationary because of both their continuity throughout the length of the bridge and their being anchored off the bridge. In the case of ballasted track, the relative movement between the superstructure and the combined ties and rails is accommodated by slight movements of the cross ties within the ballast. Effectively, the ballast beneath the cross ties is a shear plane between the track and the bridge. In a direct fixation bridge, the movement of the superstructure relative to the rails as the temperature changes imposes deformation on the fastening system that attaches the rails to the bridge deck. Open deck structures are more indeterminate, and some relative movement can be expected between the rails and the rail fastenings and between the bridge ties and the girders.

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. Rather than the classical jointed rail with bolted connections every 39 feet [12 meters], the trackwork is normally constructed with continuous welded rail. With either rail configuration, the rails can be fastened directly to the aerial structure’s deck, installed on ties and ballast, or installed on ties without ballast. The bolted connections used with jointed rail allow sufficient longitudinal expansion and contraction to reduce the accumulation of thermal stresses along the rails. But there are some disadvantages; bolted joints[4] • Generate noise and vibration • Are troublesome to maintain • Contribute to derailments if not maintained • Cause rail fatigue in the proximity of the rail joints • Cause wear of the rolling stock • Reduce ride quality • Increase the dynamic impact forces applied to the aerial structure • Are points of high electrical resistance for traction power return currents CWR has been the most common track configuration for rail transit systems for several decades. This is mainly due to its ability to overcome many of the disadvantages of jointed rail. Specifically, CWR [5],[6] • Minimizes noise and vibration • Reduces track maintenance • Improves track safety • Eliminates the joints that cause rail fatigue • Limits wear of the rolling stock • Provides a smooth, quiet ride • Limits the dynamic impact forces applied to the aerial structure • Provides a consistent path for traction power return currents. The use of CWR, combined with direct fixation of the rails to the supporting structure, is an improvement in the support and geometric stability of the trackwork. As a result, rider comfort and safety are enhanced, and track maintenance requirements are decreased. The use of CWR requires designers of trackwork and aerial structures to consider issues that typically do not arise when using jointed rail, such as the following:[7],[8],[ 9]

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. Rail breaks often occur at structural expansion joints in the aerial structure and must be accommodated without catastrophic effects such as derailment of the vehicle. Depending on the length of the aerial structure, the CWR has to be sufficiently restrained on the aerial structure to limit the length of the gap if the rail does break. CWR is a standard now employed in the transit industry. Therefore, transit system designers must understand how it interacts with aerial structures as the temperature changes in order to provide a safe track and structure. Expansion (sliding) rail joints are used in certain circumstances to reduce the interactive forces between the CWR and the structure. These include locations where special trackwork is installed on the aerial structure and where the aerial structure includes very long spans and or spans of extremely sharp curvature. Rails can be attached to the structure in a variety of ways. The most common mechanism is the use of direct fixation fasteners with elastic spring clips. High-restraint rail clips have also been used in the vicinity of substructure units (piers and abutments) with fixed bearings, as well as adjacent to special trackwork. Also, zero-longitudinal-restraint fasteners can be installed to minimize the interaction forces between CWR and the aerial structure. A common configuration will employ high-restraint rail fastenings in a track zone centered above a “fixed-fixed” bearing location while low- or zero-restraint fastenings are used at intermediate zones straddling “expansion-expansion” bearings. The decision concerning which type of deck construction to use with CWR has profound implications for construction cost. Based on the difference in cost of aerial structures with and without CWR, and the resultant thermal effects considered in the structural design, the most conservative design using CWR could increase structure costs by approximately 20%.[5] However, there are many variables to consider when choosing the type of deck to use on any particular transit structure. 7.5.3 Force Distribution between Rails and Superstructure The thermal action in a direct fixation bridge exerts additional interactive axial forces and deformations on the rails and superstructure. Reaction loads are applied to the substructure

Structures and Bridges 7-19 (piers and abutments) through the fixed bearings and by shear or friction through the expansion bearings. The aerial structure must also resist lateral components of the longitudinal loads on curved track. When the cumulative resistance of the fastening devices (rail clips) along a length of superstructure is overcome, the superstructure slides relative to the rail. Since CWR is not able to expand or contract, temperature increases above the rail installation temperature cause compressive forces that could buckle the rail. Rail fasteners prevent buckling of the rail. Temperature decreases below the rail installation temperature cause tensile forces that increase the probability of a rail break (a “pull-apart”). A rail break not only results in a gap in the rail that could cause a derailment, it creates unbalanced forces and moments in the aerial structure. Rail breaks are discussed in further detail in Article 7.5.5. Based on these thermal effects, there are three problems to address in the design of aerial structures with CWR: • Controlling the stresses in the rail attributed to the different longitudinal motions of the rail and the superstructure because of temperature changes or other causes • Controlling the rail break gap size and resulting loads into the superstructure • Transferring the superstructure loads and moments into the substructure A structural system is formed when CWR is installed on a direct fixation aerial structure. The major components of this system include the following:[6] • Long, elastic CWR, with ends anchored in the track beyond the abutments • Elastic rail fasteners that attach the rails directly to the superstructure • The elastic superstructure • Elastic bearings connecting the girders to the substructure • The elastic substructure anchored to rigid foundations There are a number of principal design factors that affect the magnitude of the interaction movement and forces between the rails and the structure, including the following:[10],[11] • The composition of the girder material (steel or concrete), which will affect the expansion/contraction response to temperature changes • The girder length and type (simple span or continuous), which will affect the magnitude of thermal movement that the rail fasteners must accommodate • The girder’s support pattern of fixed and expansion bearings from adjacent spans on the piers (refer to Article 7.5.3) • The magnitude of the temperature change • The rail fastener layout and longitudinal restraint characteristics, including these four concepts of fastener and restraint: − Frictional restraint developed in mechanical fasteners

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). The thermal force in the rail is calculated by the following equation:[4],[7],[8] Fr = Ar Er α (Ti – To) (Equation 1) where Fr = thermal rail force Ar = cross-sectional area of the rail Er = modulus of elasticity of steel α = coefficient of thermal expansion Ti = final rail temperature To = effective construction temperature of the rail On horizontal curves, the axial forces in the rail and superstructure result in radial forces. These radial forces are transferred to the substructure by the bearings. The magnitude of the radial force is a function of rail temperature, rail size, curve radius, and longitudinal fastener restraint. Refer to Figure 7.5.1 as well as other pertinent publications for the equation to calculate the radial rail/structure interaction force. Various solutions have been implemented in an attempt to minimize the interaction forces caused by placing CWR on aerial structures, including use of the following: • Ballasted track instead of direct fixation track (refer to Article 7.4.2) • Zero-longitudinal-restraint fasteners (refer to Article 7.6) • High-restraint fasteners near the structure’s point(s) of fixity and low-restraint fasteners on the remainder of the structure • A series of rail expansion joints and low-restraint fasteners to allow the rail to move independently of the structure; this requires highly restrained zones to transfer traction and braking forces to the structure

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. On bearing at th al interactive gh the inter g the piers, t these forces Rail/Structu ns differ wit n aerial struc nd supportin Figure 7.5. angement at rail/structure angement us ents. Conf sion bearing d on moder g arrangeme rail transit s this arrange out at the pie the contrary e end of the forces would active forces he structural . re Interactio hin the trans tures subject g structure in 1 Radial rail the Piers interaction fo ed. As show iguration A s) from adjac n transit sys nt typically us ystems with ment, the th rs. This is tru , if an expans adjacent s have a cumu at symmetri engineer mu n Analysis it design prof ed to therma volves the co 7-21 /structure in rces transfer n in Figure is a symme ent spans at tems that ut ed on railroa CWR, the s ermal interac e as long as ion bearing pan on the s lative effect. cal bearing a st still desig ession regar l interaction f ntrol of rail c S teraction fo red to the su 7.5.2, there trical bearing the same pie ilize CWR. d and highw ymmetrical b tive forces in the adjacent at the end of hared pier ( rrangements n the bearing ding the leve orces from C reep, broken tructures a rces[17] bstructure de are three co arrangeme r. Configura Configuratio ay bridges. earing arran duced into t spans are o one span is Configuration tend to can s and their a l of complex WR. The int rail gaps, str nd Bridge pends heavi mmonly use nt, with fixe tions A and n C is a non gement is th he rail tend t f similar lengt coupled with C), then th cel out befor nchor bolts t ity required t eraction of th esses induce s ly d d B - e o h a e e o o e d

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] tions for ele dequate and teraction. O s and structu al engineer h this comple ibuted to the rstructure and the resu t failures ds from the e rail/structu eer. Depen Second Ed ucture, and vated struc provide a thers have f ral behavior as the adva x interaction. rmally induc lting loads tr superstructur re interactio ding on the le ition longitudinal a ture girders[ good unders ound that sim critical to sig ntage of bein The design ed differenti ansferred int e through the n forces is ngth of the a nd transvers 17] tanding of th pler analys nificant CWR g able to us elements tha al movemen o the structur bearings an clearly at th erial structur e e is - e t ts e d e e

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. At times, the transit agency’s design criteria will include the required analysis methodology. Aerial structure alignments and geometrics are becoming more complex in order to fit into today’s urban and suburban communities, and computer modeling and analysis techniques are becoming commonplace in the structure design environment. As a result, it is now typical for structural modeling to be part of the aerial structure design effort. Structural analysis models can vary widely depending on the structure’s geometry and structural characteristics. In general, these analysis models consist of a network of beam-type elements and nodes that join or connect the different beam elements together. This skeleton or framework defines the critical structural elements and how they interact with each other and the track. The computer model allows the experienced structural engineer to mimic foundation conditions, model variations in structural stiffness, and apply forces to the structure in order to study the overall behavior and response of the structure. Figures 7.5.3 and 7.5.4 depict the configuration of generic structural models used to analyze LRT aerial structures. For additional information related to structural modeling techniques used for LRT aerial structures, see Design Guideline for the Thermal Interactive Forces Between CWR and the New Jersey Transit LRT Aerial Structures, Hudson-Bergen Light Rail Transit System. [35] 7.5.6 Rail Break/Rail Gap Occurrences A rail break occurs when a thermally induced tensile force resulting from a significant decrease in temperature exceeds the ultimate tensile strength of the rail. The rail break is likely to occur at or near an expansion joint in the superstructure or at a poor quality weld, a rail flaw, or other weak spot in the rail. The structure’s expansion joint is a likely area where a rail break can occur because the girder’s end rotations increase flexural stresses in the rail and the tensile stress already in the rail is likely to be at its maximum value at this location.[4],[7],[12] A cold-weather broken rail on a rail transit bridge is an important consideration because of the potential to transfer a large eccentric force to the bridge and because a derailment might occur because of the resulting rail gap. For these reasons, aerial structure designers must consider the rail break condition. Limits on the size of the rail gap have to be established, usually based on the rail vehicle’s wheel diameter. It is commonly assumed that only one rail of a single- or double-track alignment will break at any one time. When the rail breaks, the direct fixation rail fasteners situated between the break and the thermal neutral point experience a sudden loading as the rail retreats from the point of the break. Then, the rail slips through the fasteners whose pads have deformed beyond their elastic limit, engaging enough fasteners to resist the remaining thermal force. Once a sufficient number of fasteners are engaged to balance the thermal force in the rail, the rail ceases to move.

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

Structures and Bridges 52-7 The unbalanced force from the broken rail is resisted by the other unbroken rail(s) and the aerial structure. The portion of the rail break force that is resisted by the unbroken rail(s) versus the aerial structure is significantly affected by the substructure’s longitudinal stiffness (the force required to induce a unit deformation in a component), the bearing configuration, and the rail fastener’s restraint characteristics.[5] Refer to Table 7.5.1 for a comparison of the rail gap size for different column stiffness and levels of fastener restraint. Note that progressively lower loads are transferred to the columns as column stiffness decreases. As a result, higher loads are transferred to the unbroken rails. This increases the thermally induced stress in this rail and raises the possibility of a second rail break. With higher restraint fasteners, more load is transferred to the unbroken rail and less to the column than is the case with medium-restraint fasteners. Researchers have found that the superstructure’s bearing arrangement, as discussed in Article 7.5.3, has little effect on rail gap size. Nonetheless, decreasing the fastener’s longitudinal stiffness or slip force limit, or both, will result in an increased rail gap size. The redistribution of the rail break force to the substructure causes a longitudinal deflection in the substructure. The resulting substructure deflection and the thermal slip of the broken rail combine to create the total gap in the broken rail. Rail gap size is generally estimated using the following equation:[5] G = 2 (XC1 + XC2 – XC3) (Equation 2) where G = rail gap, inches [cm] XC1 = Pfns/Kf, the maximum longitudinal deflection of the non-slip fastener XC2 = α∆ TLs, the nominal rail contraction XC3 = (nsPfs + nnsPfns) Ls/2ArEr, the reduction in rail contraction caused by fastener constraint and where the factors in the formulae above are as follows α = coefficient of expansion, 6.5 x 10-6 in/in/°F [1.17x10-5 cm/cm/°C ] for steel ∆T = temperature change, °F [°C] Ls = length of span (fixed to expansion point), inches [cm] Pfs = minimum longitudinal restraint force in controlled-slip fastener, lb [kg] Pfns = minimum longitudinal restraint force in non-slip fastener, lb [kg] Kf = fastener longitudinal stiffness lb/in [kg/cm] nns = number of non-slip fasteners in span ns = number of controlled-slip fasteners in span Ar = cross-sectional area of rail (11.25 in 2 [72.58 cm2] for 115 RE rail) Er = rail steel modulus of elasticity, 30 X 10 6 lb/in2 [2.1 X 106 kg/cm2 ]

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) Load (lb) Gap Size* (in) Load (lb) Gap Size* (in) Rigid 131,000 0.67 134,000 0.79 500,000 50,600 0.89 35,800 0.89 100,000 17,700 1.17 11,600 0.96 40,000 9,300 1.27 5,800 1.15 * Assuming a symmetrical girder bearing configuration of E—F/F—E/E—F and a 60o F temperature drop. A simplified form of Equation 2 has been used to estimate rail gap size, based on a length, L, on either side of the break over which full rail anchorage is provided, so that G = (α∆T)2 ArEr / Rf (Equation 3) where Rf is the longitudinal restraint per inch of rail in pounds per inch (kilograms per centimeter). Equation 2 provides a reasonable estimate of rail gap size for medium- and high-restraint fasteners, but significantly underestimates the rail gap size for low-restraint fasteners. Low- restraint fasteners generally do not adequately control the size of the rail gap. Although Equation 3 provides relatively accurate estimates in many cases, it does not provide accurate estimates where high-restraint fasteners are used. Improved accuracy can be obtained with Equation 2 if the term XC2 is modified to use the estimated total number of fasteners over which the locked-in load is distributed. Therefore: G = 2(XC1 + XC2 − XC3) (Equation 4) which appears identical to Equation 2 except that the XC2 factor is modified as follows: XC2 = 0.5 α∆T nxLs nx = PT/Pfmax = PfmaxKr/2PTKf PT = α∆T ArEr, the thermal load, lb [kg] Pfmax = (nnsPfns + nsPfs)/(nns + ns), the average fastener restraint limit, lb [kg] Kr = ArEr/Lf, the rail spring, lb/inch [kg/cm] Kf = fastener longitudinal stiffness lb/inch [kg/cm] Lf = fastener longitudinal spacing, inches [cm]

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. estimate ra models show n provided in an be used Figure 7.5.5 7.5.5 Rail b arizes estima een estimate nd stiffness) il vehicle der he diameter 50 millimeter om seen in to 2 inches. note that eff h of the rail g d, the forces ed to achiev igh longitudin relatively low rs must coor il gap size a the fastene Article 7.5.4 to determin for the rail g reak gap siz ted rail gap s d, the variab should be ad ailment caus of the vehic s] for a 16-inc rail transit ve orts to contro ap should be and mome e an econom al restraint longitudina dinate the o 7-27 ssuming line r load distrib applies to th e rail gap a ap sizes pred e predicted ize using diff les affecting justed to limi ed by a rail le’s wheel. h [400-millim hicles, so th l rail gap siz minimized t nts transferre ical structur should be us l restraint s pposing des S ar load dist utions to be e rail break/g nd loads res icted using a by finite com erent equatio the magnitu t the size of gap. The siz Typically acc eter] diamete ere is a cons e offer oppo o reduce the d to the str e. To resolv ed. To add hould be u ign requirem tructures a ributions. T nonlinear. ap analysis s ulting from finite-eleme puter mode ns and softw de of the ga the gap. Thi e of the rail epted rail ga r wheel.[4] N iderable fact sing solution possibility o ucture due t e safety iss ress the stru sed. The t ents to balan nd Bridge ypically, finite The compute ince the sam the rail brea nt model. l[5] are. Once th p (such as ra s will minimiz gap is usual ps are in th otably, whee or of safety s. For safet f a derailmen o a rail brea ues, fastener ctural issue rackwork an ce the need s - r e k e il e ly e ls in y t. k s s, d s

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) Case Equation 2 Equation 3 Equation 4 TBTRACKb ∆Tg = 0 TRKTHRMb ∆Tg = 0 TRKTHRMb ∆Tg = 60 1 0.69 0.62 0.83 0.55 0.74 0.67 2 0.72 1.23 1.35 0.85 1.29 1.31 3 0.89 1.23 1.55 0.97 1.38 1.47 4 0.51 0.15 0.99 0.40 0.50 0.79 5 0.57 0.62 0.68 0.66 0.63 6 0.85 2.29a 2.47 1.77 2.39 2.68 7 0.82 1.43a 1.62 — 1.54 1.61 8 1.21 0.68 1.44 — 1.20 1.14 Notes. ∆Tg = Temperature change in the girder; the girder bearing configuration = E-F/F-E/E-F; the length of the span = 80 ft; the length of the fastener = 30 in; and the temperature change in the rail = 60° F (temperature drop). a Using average of Rf = nsPfs + nnsPfns)/(ns + nns) where ns = the number of slip fasteners, and nns = the number of non-slip fasteners. b TBTRACK and TRKTHRM are programs developed to calculate rail break gap size. — Designates cases for which no data were published. 7.5.7 Terminating CWR on Aerial Structures As much as possible, CWR should not be terminated on an aerial structure due to the large termination force transferred to the structure. Problems arise when specialwork must be located on an aerial structure due to the length of the structure, the needs of the transit operations, or other occurrences. Unbalanced thermal forces exist in specialwork locations due to discontinuities in the rail. Standard turnout units, by design, transfer high forces through the units on an aerial structure, which causes misalignment and wear.[12] For these reasons, designers should avoid placing specialwork on aerial structures. When this cannot be avoided, there are ways to accommodate the specialwork without causing it to malfunction. To accommodate the large forces occurring at locations of specialwork, either rail anchorages or rail expansion joints can be used. Rail anchorages (not to be confused with rail anchors—the snap-on device used to control rail movement in timber tie ballasted track) are large units that clamp onto the rail and anchor it to the plinth concrete below with no possibility of slippage. See Chapter 5, Figure 5.10.2, for a typical rail anchorage assembly. One of these units is placed in each rail on each approach to the special trackwork unit. Rail anchorages create a zero force condition through the specialwork, but pass the rail termination force to the underlying structure. However, the massiveness of the resulting substructure may be both aesthetically and economically undesirable. Sliding rail expansion joints have been used both with and without the rail anchorages noted above. The following should be considered when using sliding rail expansion joints: • The construction length of the sliding rail joints • The length of structure required to accommodate the specialwork and sliding rail joint • The design, location, and installation details of the rail anchors

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. See Figure 7.5.6 for a picture of a tie bar installation at an aerial structure crossover. With a tie bar system, the CWR is interrupted at the crossover and the rail ends are attached as rigidly as possible to special “AXO” girders adjacent to the outer ends of the specialwork. The AXO girders are similar to standard girders except for the addition of an embedded steel plate to which the tie bar is attached by welding. The tie bar, a structural steel member with a cross section equal to two rails, is located on the centerline of each track and is welded to the embedded plates on the centerline of the two AXO girders. The tie bar rests on Teflon bearing pads placed directly on the concrete deck for the length of the crossover. When the temperature changes, the thermal force built up at the end of the CWR is transferred to an AXO girder through a group of rail fasteners equally spaced along the girder. An equal and opposite thermal force is developed in the tie bar and transferred to the AXO girder through a welded connection. Therefore, the net longitudinal thermal force is directed through the tie bar instead of the through the piers or the specialwork, where the trackwork could be damaged. One problem with all of the methods discussed above is making certain that the rails remain electrically isolated from both the superstructure and each other. If electrical isolation is compromised, signal system failures and stray currents could result. However, the mechanical/structural properties of the dielectric materials used to isolate the rails may not accommodate the stresses associated with the absolute termination of the CWR. Avoidance of special trackwork on aerial structures is quite likely the only way to fully mitigate this concern. 7.6 DIRECT FIXATION FASTENERS Since the majority of transit properties now use CWR with direct fixation deck construction, the aerial structure designer should understand the types of rail fasteners presently available. Direct fixation rail fasteners secure the CWR to the deck of the aerial structure. The bottom portion of the fastener is bolted to the deck (either directly or through plinths), and the top portion is bolted or clipped to the bottom flange of the rail. Low-restraint, moderate-restraint, and high-restraint fastener rail clips are available. In addition, some transit properties have utilized zero-longitudinal-restraint (ZLR) fasteners in certain circumstances. Although ZLR fasteners allow the superstructure to move longitudinally without generating thermal interaction forces, the rail gap size at a rail break has to be carefully considered when they are used. With a conventional direct fixation fastener, the elastomer provides isolation of the high wheel/rail impact forces from the deck, electrical isolation, vertical elasticity to dampen noise and vibration, longitudinal elasticity to accommodate rail/structure interaction movements, and distribution of the wheel loads longitudinally along the rail. The fastener also provides full restraint in the lateral direction, maintains the desired rail tolerances, and prevents rail buckling under high temperature.

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],[8] (Photos courtesy of Bay Area Rapid Transit)[6] Figure 7.5.6 Tie bar on aerial crossover

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) Longitudinal fastener stiffness 3,400 to 18,000 lb/in (600 to 3,200 N/mm) Longitudinal restraint 2,000 to 3,500 lb (9,000 to 15,750 N) Direct fixation fasteners are commonly spaced at 30 inches [762 millimeters] on center. Other spacings may be required if determined by analysis of rail bending stresses, interaction forces of the rail and rail fasteners, and the rail gap size at a rail break location. Trackwork and structural engineers need to carefully coordinate fastener spacing on sharply skewed bridges to ensure that the fasteners are adequately supported on each side of the joints in the deck. Another key consideration to be coordinated between the trackwork engineer and the structural engineer is structure vibration. The structural engineer typically determines the unloaded (i.e., without live load) natural frequency of the aerial structure. Then the spacing and characteristics of the rail fasteners can be chosen in an effort to control resonance. It is common for transit agencies to specify structure vibration limitations in the aerial structure design criteria. These limitations are established in an effort to minimize the potential dynamic interaction between the structure and the LRVs. Further discussion of structure vibration and the role of direct fixation rail fasteners in mitigating vibration can be found in Chapter 5 and Chapter 9 of this Handbook. 7.7 TRACK SUPPORT SLABS-ON-GRADE It is common to have slabs-on-grade along many portions of a LRT project corridor. Typically this slab is also functioning as concrete pavement for use by rubber-tired vehicles operating in the same lanes as the LRV, but it can also include slab track in exclusive or semi-exclusive guideways such as direct fixation track and some forms of grass track. The design, analysis, and detailing of slabs-on-grade are important to the durability and performance of the slab. An investigation of the approaches used to design and detail slabs-on-grade used by various transit agencies shows how methodologies and detailing practices vary considerably. The basic analysis of the slab-on-grade should typically involve performing a classical beam-on- elastic foundation analysis. This analysis will consider the effects of subsurface conditions, geotechnical design parameters (subgrade modulus), and the anticipated LRV loads. For instance, the stiffness or “softness” of the subgrade will in most cases dictate the relationship between the slab thickness required and the amount of flexural reinforcement needed. If a very stiff subgrade is present, minimal bending would be expected to occur in the slab, thus resulting in minimal reinforcement. Conversely, if loose and/or compressible soil exists, the tendency for the slab to differentially deflect and flex will be high, thus warranting a thicker and/or more heavily reinforced slab.

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. Additionally, consideration needs to be given as to whether the reinforcing in these slabs should be epoxy coated or uncoated and electrically bonded or not. These issues need to be coordinated during the design process in order to provide durable, long- lasting slabs. Chapter 8 provides additional information related to corrosion control and stray current mitigation. Four different configurations of slabs-on-grade are common to LRT projects. These include the following: • Single-pour embedded slabs, in which the rails are embedded in the slab. • Two-pour embedded slabs, in which the rails sit on top of the first pour and, after being checked for proper alignment, are embedded with a second pour. A horizontal construction joint is located at the base-of-rail elevation. • Slabs supporting an open, direct fixation trackform. • Slabs supporting a grass track detail. In general, the slab-on-grade designer will need to consider the following items during the design phase, regardless of which type of slab is used: • The structural engineer may be the lead designer of the slab-on-grade, but this engineer will seek input from the trackwork engineer, geotechnical engineer, roadway/pavement engineer, and corrosion engineer in order to confirm an efficient design for the slab-on-grade. • The trackwork engineer will be consulted to confirm the details of the rail troughs or rail embedments to be formed in the slab-on-grade. The details of the waterproofing system, stray current isolation system and rail fastener and anchorage systems are critical in the design of the slab-on-grade. • Subsurface data, including the engineering properties of the subgrade material, are required for the structural analysis to determine the thickness and reinforcing requirements for the slab. The subgrade modulus of the soil beneath the slab is critical to the elastic foundation analysis that is typically performed. Some transit properties include minimal, or even no reinforcing steel in the support slabs while other systems, projects, or specific locations on a project require that a slab be heavily reinforced. The stiffness and uniformity of the subgrade materials are key items in determining the thickness of the support slab and whether or not it needs to be reinforced. • The presence of utilities beneath the slab-on-grade needs to be confirmed at the early stages of design. A decision needs to be made regarding the methods by which the utilities will be serviced, maintained, and replaced if necessary as they pass below the track slab. On some projects, this issue has led to a decision that the track slab needed to be able to span across an open utility trench without disrupting LRT revenue service, resulting in an extremely heavily reinforced slab. On one such project, a utility repair was undertaken several years later; however, it is questionable whether the utility contractor’s task was actually made easier by the bridging slab. More conventional methods, such as pipe jacking or directional boring, which do not require a bridging slab, may have been just as efficient and could have saved the rail project a substantial percentage of the original embedded track construction

Structures and Bridges 7-33 cost. At least two legacy streetcar systems use no reinforcing steel at all in their embedded tracks. A major water main break directly beneath one such track was repaired without damage to the track as the track itself was sufficiently stiff to bridge over the excavation. Streetcar service was interrupted for a short time while the repair was in process, and shuttle buses were substituted over the affected portion of the route. • The roadway/pavement engineer will review the slab-on-grade design from the perspective of supporting the loads from rubber-tired vehicles. An analysis with the wheel loads placed along the edges of the slab-on-grade may be critical to confirming the adequacy of the slab design in those areas. Obviously, the LRVs can only travel along the rails embedded in the slab, but the rubber-tired vehicles can travel in any direction. • Based on the details of the stray current isolation system surrounding the rail in the trough, the structural engineer will consult with the corrosion engineer to determine the need for either epoxy-coated rebar or electrically bonded rebar in the slab. In addition, the corrosion engineer will evaluate the depth of the slab and its proximity to underground utilities to assist the structural engineer in determining a cost-effective slab design. The trackwork engineer and the structural engineer must coordinate on which slab type is best suited for a particular application on a site-specific basis. 7.8 REFERENCES [1] Harrington, G., Dunn, P.C., Investigation of Design Standards for Urban Rail Transit Elevated Structures, UMTA, June, 1981. [2] Niemietz, R.D., Neimeyer, A.W., “Light Rail Transit Bridge Design Issues,” Transportation Research Record 1361, Transportation Research Board, National Research Council, Washington, D.C.,1992, pp. 244–253. [3] Nowak, A.S., Grouni, H.N., “Development of Design Criteria for Transit Guideways,” ACI Journal, September-October, 1983. [4] ACI Committee 358, Analysis and Design of Reinforced Concrete Guideway Structures, ACI 358.1R-86. [5] Ahlbeck, D.R., Kish, A., Sluz, A., “An Assessment of Design Criteria for Continuous-Welded Rail on Elevated Transit Structures,” Transportation Research Record 1071, Transportation Research Board, National Research Council, Washington, D.C., 1986, pp. 19–25. [6] Clemons, R.E., “Continuous-Welded Rail on BART Aerial Structures,” Transportation Research Record 1071, Transportation Research Board, National Research Council, Washington, D.C., 1986, pp. 29-34. [7] Grouni, H.N., Sadler, C., Thermal Interaction of Continuously Welded Rail and Elevated Transit Guideways, Ontario Ministry of Transportation and Communications. [8] Guarre, J.S., Gathard, D.R., Implications of Continuously Welded Rail on Aerial Structure Design and Construction, June, 1985.

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. [10] Clemons, R.E., Continuous Welded Rail on Aerial Structure: Examples of Transit Practice, APTA, January, 1985. [11] Fine, D.F., Design and Construction of Aerial Structures of the Washington Metropolitan Area Rapid Transit System, Concrete International, July, 1980. [12] Lee, R.J., Designing Precast Aerial Structures to Meet Track and Vehicle Geometry Needs, 1994 Rail Transit Conference. [13] [Deleted from 2nd Edition] [14] Meyers, B.L., Tso, S.H., “Bay Area Rapid Transit: Concrete in the 1960s,” Concrete International, February, 1993. [15] Desai, D.B., Sharma, M., Chang, B., Design of Aerial Structure for the Baltimore Metro, APTA Rapid Transit Conference, June 1986. [16] Naaman, A.E., Silver, M.L., “Minimum Cost Design of Elevated Transit Structures,” Journal of Construction Division, March, 1976. [Deleted from 2nd Edition] [17] Fassmann, S., Merali, A.S., “Light Rail Transit Direct Fixation Track Rehabilitation: The Calgary Experience,” Transportation Research Record 1361, Transportation Research Board, National Research Council, Washington, D.C.,1992, pp. 235–243. [18] AREA Manual for Railway Engineering, Section 8.3, “Anchorage of Decks and Rails on Steel Bridges,” 1995. [Deleted from 2nd Edition] [19] Beaver, J.F., Southern Railway System’s Use of Sliding Joints, AREA Bulletin 584, February, 1964. [Deleted from 2nd Edition] [20] Billing, J.R., Grouni, H.N., “Design of Elevated Guideway Structures for Light Rail Transit,” Transportation Research Record 627, Transportation Research Board, National Research Council, Washington, D.C.,1977, pp. 17–21. [Deleted from 2nd Edition] [21] Casey, J., “Green Light,” Civil Engineering, May, 1996. [Deleted from 2nd Edition] [22] Deenik, J.F., Eisses, J.A., Fastening Rails to Concrete Deck, The Railway Gazette, March 18, 1966. [Deleted from 2nd Edition] [23] Dorton, R.A., Grouni, H.N., Review of Guideway Design Criteria in Existing Transit System Codes, ACI Journal, April 1978. [Deleted from 2nd Edition] [24] Fox, G.F., Design of Steel Bridges for Rapid Transit Systems, Canadian Structural Engineering Conference, 1982. [Deleted from 2nd Edition] [25] International Civil Engineering Consultants, Inc., Task Report on a Study to Determine the Dynamic Rail Rupture Gaps Resulting from a Temperature Drop for BART Extension Program, July 26, 1991. [Deleted from 2nd Edition] [26] Jackson, B., “Ballastless Track, A Rapid Transit Wave of the Future?” Railway Track and Structures, April, 1984. [Deleted from 2nd Edition]

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. [Deleted from 2nd Edition] [28] Magee, G.M., Welded Rail on Bridges, Railway Track and Structures, November, 1965. [Deleted from 2nd Edition] [29] Mansfield, D.J., “Segmental Aerial Structures for Atlanta’s Rail Transit System,” Transportation Research Record 1071, Transportation Research Board, National Research Council, Washington, D.C., pp. 26–28,1986. [Deleted from 2nd Edition] [30] “Philadelphia’s El Gets Major Facelift,” Mass Transit, May/June, 1995. [Deleted from 2nd Edition] [31] McLachlan, L.J., “University Boosts Light Rail Traffic,” Developing Metros, 1994. [Deleted from 2nd Edition] [32] Middleton, W.D., “Engineering the Renaissance of Transit in Southern California,” Railway Track and Structures, March, 1993. [Deleted from 2nd Edition] [33] Middleton, W.D., “DART: Innovative Engineering, Innovative Construction,” Railway Track and Structures, December, 1994. [Deleted from 2nd Edition] [34] Patel, N.P., Brach, J.R., “Atlanta Transit Structures,” Concrete International, February, 1993. [Deleted from 2nd Edition] [35] PBQD, Design Guideline for the Thermal Interactive Forces Between CWR and the New Jersey Transit LRT Aerial Structures, Hudson-Bergen Light Rail Transit System, July 1995. [36] PBQD, Rail/Structure Interaction Analysis - Retrofit of Direct Fixation Fasteners with Spring Clips, WMATA - Rhode Island Avenue, February, 1995. [Deleted from 2nd Edition] [37] PBQD, Thermal Study of Bridge-Continuous Rail Interaction, Metro Pasadena Project, Los Angeles River Bridge, August, 1994. [Deleted from 2nd Edition] [38] Swindlehurst, J., “Frankford Elevated Reconstruction Project,” International Bridge Conference, June, 1984. [Deleted from 2nd Edition] [39] Thorpe, R.D., “San Diego LRT System: Ten Years of Design Lessons,” Transportation Research Record 1361, Transportation Research Board, National Research Council, Washington, D.C., pp. 171–175, 1992. [Deleted from 2nd Edition] [40] Varga, O.H., The Thermal Elongation of Rails on Elastic Mountings, AREA Bulletin 626, February, 1970. [Deleted from 2nd Edition] [41] Yu, S., “Closing the Gaps in Track Design,” Railway Gazette International, January, 1981. [42] Zellner, W., Saul, R., “Long Span Bridges of the New Railroad Lines in Germany, Bridges: Interaction Between Construction Technology and Design.” [Deleted from 2nd Edition]