Introduction: The authors of the "Practical Guide To Railway Engineering" wish to provide the reader a general overview of the specific disciplines common to. Written by a group of over 50 railroad professionals, representing over years of experience, the Practical Guide to Railway Engineering may be the most . Introduction to Railroad . The American Railway Engineering and Maintenance of Way Association the railroad industry track engineering standards.
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As we approach a new millenium, transport of people and materials takes on an even co-ordinating the optional subject 'Railway Engineering Concepts' for. Practical guide to railway engineering. [American Railway Engineering and Maintenance-of-Way Association.;] CD-ROM contains PDF version of book. AMERICAN RAILWAY ENGINEERING AND MAINTENANCE OF WAY ASSOCIATION Practical Guide to Railway Engineering Railway Structures. Saida Navarro.
An understanding of track switches, components and their interconnection to the signal system is provided. Crossing warning device theory of operation and differences between conventional and solid state devices is highlighted. The basic principles of CTC, sequence of operation and safety checks are explained along with concepts associated with microprocessor based coded track circuits and solid-state interlockings.
Chapter 8 — Railway Structures Chapter 8 was prepared to accomplish two primary objectives. For the novice engineer, the authors wished to provide an overview of the types of railway bridge structures and their appropriate usage as well as define the primary bridge components and their functions. For the experienced highway design engineer, the common design approach differences between highway and railway bridges are reviewed.
Other critical structure criteria are highlighted such as fatigue, fracture critical members, structure serviceability, bearings and volumetric changes and composite design. Chapter 9 — Railway Electrification Chapter 9 compares the various alternatives available when considering and designing an electrified railway.
Finally, the impact that implementation of electrification will have on existing railroad infrastructure, staff and community is discussed. Chapter 10 — Passenger, Transit and High-Speed Rail Chapter10 presents an overview of typical design principles, construction practices and maintenance considerations applied to passenger rail lines.
It describes how basic railroad engineering principles are applied in specialized ways to accommodate passenger rail requirements. The chapter notes the key distinctions between railroad and transit operations and introduces six major types of passenger rail modes.
The text then discusses the service, infrastructure, regulatory U. It concludes with discussion of the special topics of line capacity and cant deficiency. Chapter 11 - Environmental Regulations And Permitting Chapter 11 is a general overview of environmental regulations and permitting in the U. Information is given on wetland issues along with other topics, such as endangered species, cultural resources, Phase I environmental assessments, hazardous waste, brownfields, asbestos and air quality.
Environmental information includes: the U. Army Corps of Engineers wetland definition, Nationwide and General permits for proposed construction activities, U.
Each topic concludes on where to locate additional information. The potential for incorporating European practices in high-speed North American transit initiatives is clearly obvious. Chapter 13 — Case Studies Chapter 13 presents four case studies drawn from actual railway design projects using formatted templates to identify critical stakeholders, identify controlling criteria, recognize potential problems, and learn from past mistakes. It is intended that this will be part of an accessible library of case study solutions yet to be developed.
Glossary The glossary contains short definitions of the majority of the terms utilized within the text. Sometimes it is impossible for this to occur. By studying the upstream and downstream effects, the designer may be able to apply a certain amount of change that does not harm or cause damage to adjacent property owners.
For example, a 0. However, this is dependent upon the conditions and regulations unique to that project location. Typically, the year base flood elevation is the most commonly regulated stormwater elevation associated with rivers, streams and concentrated flow areas.
Any change to the flood plain will generally result in extensive studies and computer modeling to be submitted for approval. Below is a summary of possible floodplain permitting reviews. Excavation below normal water elevation State Water Resource Department: Floodway Area within a floodplain that demonstrates conveyance County Some counties may not be involved in the review process: An alignment is defined in two fashions.
First, the horizontal alignment defines physically where the route or track goes mathematically the XY plane. The second component is a vertical alignment, which defines the elevation, rise and fall the Z component. Alignment considerations weigh more heavily on railway design versus highway design for several reasons. First, unlike most other transportation modes, the operator of a train has no control over horizontal movements i.
The guidance mechanism for railway vehicles is defined almost exclusively by track location and thus the track alignment. Secondly, the relative power available for locomotion relative to the mass to be moved is significantly less than for other forms of transportation, such as air or highway vehicles. See Table Finally, the physical dimension of the vehicular unit the train is extremely long and thin, sometimes approaching two miles in length. This compares, for example, with a barge tow, which may encompass full trains, but may only be feet in length.
These factors result in much more limited constraints to the designer when considering alignments of small terminal and yard facilities as well as new routes between distant locations. The designer MUST take into account the type of train traffic freight, passenger, light rail, length, etc.
The design criteria for a new coal route across the prairie handling 15, ton coal trains a mile and a half long ten times per day will be significantly different than the extension of a light rail trolley line in downtown San Francisco. However, there does not seem to be any widespread incorporation of this practice. When working with light rail or in metric units, current practice employs curves defined by radius. As a vehicle traverses a curve, the vehicle transmits a centrifugal force to the rail at the point of wheel contact.
This force is a function of the severity of the curve, speed of the vehicle and the mass weight of the vehicle. This force acts at the center of gravity of the rail vehicle. This force is resisted by the track. If the vehicle is traveling fast enough, it may derail due to rail rollover, the car rolling over or simply derailing from the combined transverse force exceeding the limit allowed by rail-flange contact.
This centrifugal force can be counteracted by the application of superelevation or banking , which effectively raises the outside rail in the curve by rotating the track structure about the inside rail. See Figure The point, at which this elevation of the outer rail relative to the inner rail is such that the weight is again equally distributed on both rails, is considered the equilibrium elevation.
Track is rarely superelevated to the equilibrium elevation. The difference between the equilibrium elevation and the actual superelevation is termed underbalance. Though trains rarely overturn strictly from centrifugal force from speed they usually derail first. This same logic can be used to derive the overturning speed. Conventional wisdom dictates that the rail vehicle is generally considered stable if the resultant of forces falls within the middle third of the track.
This equates to the middle 20 inches for standard gauge track assuming that the wheel load upon the rail head is approximately inches apart. As this resultant force begins to fall outside the two rails, the vehicle will begin to tip and eventually overturn. It should be noted that this overturning speed would vary depending upon where the center of gravity of the vehicle is assumed to be.
There are several factors, which are considered in establishing the elevation for a curve. The limit established by many railways is between five and six-inches for freight operation and most passenger tracks. There is also a limit imposed by the Federal Railroad Administration FRA in the amount of underbalance employed, which is generally three inches for freight equipment and most passenger equipment.
Track is rarely elevated to equilibrium elevation because not all trains will be moving at equilibrium speed through the curve. Furthermore, to reduce both the maximum allowable superelevation along with a reduction of underbalance provides a margin for maintenance. Each railway will have its own standards for superelevation and underbalance, which should be used unless directed otherwise.
The transition from level track on tangents to curves can be accomplished in two ways. For low speed tracks with minimum superelevation, which is commonly found in yards and industry tracks, the superelevation is run-out before and after the curve, or through the beginning of the curve if space prevents the latter. A commonly used value for this run-out is feet per half inch of superelevation. On main tracks, it is preferred to establish the transition from tangent level track and curved superelevated track by the use of a spiral or easement curve.
A spiral is a curve whose degree of curve varies exponentially from infinity tangent to the degree of the body curve. The spiral completes two functions, including the gradual introduction of superelevation as well as guiding the railway vehicle from tangent track to curved track. Without it, there would be very high lateral dynamic load acting on the first portion of the curve and the first portion of tangent past the curve due to the sudden introduction and removal of centrifugal forces associated with the body curve.
There are several different types of mathematical spirals available for use, including the clothoid, the cubic parabola and the lemniscate. From the macro perspective, there has been for over years, the classic railway location problem where a route between two points must be constructed. One option is to construct a shorter route with steep grades. The second option is to build a longer route with greater curvature along gentle sloping topography.
The challenge is for the designer to choose the better route based upon overall construction, operational and maintenance criteria.
Such an example is shown below. Figure Heavy Curvature on the Santa Fe - Railway Technical Manual Courtesy of BNSF Suffice it to say that in todays environment, the designer must also add to the decision model environmental concerns, politics, land use issues, economics, long-term traffic levels and other economic criteria far beyond what has traditionally been considered.
These added considerations are well beyond what is normally the designers task of alignment design, but they all affect it. The designer will have to work with these issues occasionally, dependent upon the size and scope of the project.
There have been a number of guidelines, which have been developed over the past years, which take the foregoing into account.
For the remaining situations, the designer must take into account how the track is going to be used train type, speed, frequency, length, etc. The decision must be in concurrence with that of the eventual owner or operator of the track as to how to produce the alignment with the release of at least one of the restraining guidelines. Sometimes, a less restrictive guideline from another entity can be employed to solve the design problem.
Other times, a specific project constraint can be changed to allow for the exception. Other times, its more complicated, and the designer must understand how a train is going to perform to be able to make an educated decision. The following are brief discussions of some of the concepts which must be considered when evaluating how the most common guidelines were established. A freight train is most commonly comprised of power and cars. The power may be one or several locomotives located at the front of a train.
The cars are then located in a line behind the power. Occasionally, additional power is placed at the rear, or even in the center of the train and may be operated remotely from the head-end.
The train can be effectively visualized for this discussion as a chain lying on a table. We will assume for the sake of simplicity that the power is all at one end of the chain. Trains, and in this example the chain, will always have longitudinal forces acting along their length as the train speeds up or down, as well as reacting to changes in grade and curvature. These forces are often termed buff negative and draft positive forces. Trains are most often connected together with couplers Figure The mechanical connections of most couplers in North America have several inches up to six or eight in some cases of play between pulling and pushing.
This is termed slack. If one considers that a long train of cars may be ' long, and that each car might account for six inches of slack, it becomes mathematically possible for a locomotive and the front end of a train to move fifty feet before the rear end moves at all.
As a result, the dynamic portion of the buff and draft forces can become quite large if the operation of the train, or more importantly to the designer, the geometry of the alignment contribute significantly to the longitudinal forces.
As the train moves or accelerates, the chain is pulled from one end. The force at any point in the chain Figure is simply the force being applied to the front end of the chain minus the frictional resistance of the chain sliding on the table from the head end to the point under consideration.
As the chain is pulled in a straight line, the remainder of the chain follows an identical path. However, as the chain is pulled around a corner, the middle portion of the chain wants to deviate from the initial path of the front-end.
On a train, there are three things preventing this from occurring. First, the centrifugal force, as the rail car moves about the curve, tends to push the car away from the inside of the curve. When this fails, the wheel treads are both canted inward to encourage the vehicle to maintain the course of the track. The last resort is the action of the wheel flange striking the rail and guiding the wheel back on course.
Attempting to push the chain causes a different situation. A gentle nudge on a short chain will generally allow for some movement along a line. However, as more force is applied and the chain becomes longer, the chain wants to buckle in much the same way an overloaded, un-braced column would buckle See Figure The same theories that Euler applied to column buckling theory can be conceptually applied to a train under heavy buff forces.
Readers of this chapter are invited to read the AREMA Communications and Signals Manual of Recommended Practices for a comprehensive study of the various elements of signaling, including recommended practices. As traffic increased, it became necessary to operate trains in both directions over single track.
To permit faster and superior trains to pass and provide for opposing trains to meet, it was necessary to construct sidings. It was then necessary to devise methods to affect opposing and passing movements without disaster and with a minimum of confusion and delay. This was achieved by introducing time schedules so that the meeting and passing of trains could be prearranged.
Thus, the "timetable" was born. Bond wires are applied to ensure a path of low and uniform resistance between adjoining rails. Insulated joints define track circuit limits. Track circuits vary in length as required. AREMA definitions of terms commonly applied to track circuit operation are: Ballast Leakage: The leakage of current from one rail to the other rail through the ballast, ties, etc. Ballast Resistance: The resistance offered by the ballast, ties, etc. Floating Charge: Maintaining a storage battery in operating condition by a continuous charge at a low rate.
Rail Resistance: The total resistance offered to the current by the rail, bonds and rail connections. Shunt Circuit: A low resistance connection across the source of supply, between it and the operating units. Short Circuit: A shunt circuit abnormally applied. Shunting Sensitivity: The maximum resistance in ohms, which will cause the relay contacts to open when the resistance is placed across the rails at the most adverse, shunting locations.
The relay is connected at the other end of the track circuit with one lead of the relay coils going to rail S and the other to rail N. With the battery and relay connected, current has a complete path in which to flow, as indicated by the arrows.
When an alternate path for current flow exists from one rail to the other via the ballast, the track circuit becomes a parallel circuit. The current through each ballast resistance and the current through the relay coils adds up to the total current drain from the battery during normal conditions.
When a train enters a track section, the wheels and axles place a shunt short on the track circuit. This creates a low resistance current path from one rail to the other and in parallel with the existing ballast resistance and relay coil. When maximum current from the battery is reached because of current flow through the relay coils, ballast resistance and low resistance path created by the train shunt, the relay armature drops.
Most of the current flows through the low resistance shunt path. This reduces the current in the relay sufficiently to cause the armature to drop, thereby opening the front contacts. In Figure , the heavy dark arrows indicate the high current path through the shunt.
When a train is present on that section of track, the relay de-energises and the heel contact makes with the back contact lighting the red signal. When the last pair of wheels moves off the track circuit, the current will again flow in the un-shunted track circuit, through the coils of the relay, causing the front contacts to close and light the green signal.
An appreciation of the effect of ballast resistance is necessary to understand track circuit operation. When good ties are supported in good crushed stone and the complete section is dry, the resistance to current flow from one rail to the other rail is very high.
This condition is known as maximum ballast resistance and is ideal for good track circuit operation. When the ballast is wet or contains substances such as salt or minerals that conduct electricity easily, current can flow from one rail to the other rail.
This condition is minimum ballast resistance. With minimum ballast resistance, ballast leakage current is high. When the ballast resistance decreases significantly, the relay can be robbed of its current and become de-energized, or fail to pick up after it has been de-energized by a train and the train has left the track circuit.
Because the ballast resistance varies between a wet day minimum ballast resistance and a dry day maximum ballast resistance the flow of current from the battery will vary. When a train occupies a track circuit, it places a short circuit on the battery. In order to limit the amount of current drawn from the battery during this time, a resistor is placed in series with the battery output to prevent the battery from becoming exhausted.
A variable resistor is used in order to set the desired amount of discharge current during the period the track circuit is occupied. This resistor is called the battery-limiting resistor. When the battery-limiting resistor is adjusted as specified, higher current will flow through the relay coil on a dry day due to maximum ballast resistance. If this current is too high the relay will be hard to shunt. To overcome this condition a variable resistor is inserted in series with the relay coil at the relay end of the track circuit and is used to adjust the amount of current flowing in the relay coils.
The relay current before the shunt is applied The effectiveness of the shunt When a train occupies and shunts a track circuit, the relay will not drop immediately. Counterweights are used in conjunction with various lengths of gate arms for the purpose of off-setting the weight of the gate arm itself, in order that the motor without excessive current draw can raise the gate. The counterweights are adjustable in two ways to provide a sufficient number of foot- pounds of torque in both the vertical and horizontal positions.
Counterweights are to be installed as per manufacturer's instructions. Gate arms are to be torqued in the vertical and horizontal position according to the manufacturer's handbook, which is included with each mechanism. Settings may vary depending on which type of gate model is used. Gate Lighting: The light nearest the tip of the gate arm is at the prescribed distance from the tip and burns steadily as per the railways standards.
The other lights are located to suit local conditions and flash alternately in unison with the lights on the gate mast. When positioning the lights on the gate arm, the rightmost light must be in line with the edge of the roadway and the center light should be placed between the two outer lights.
If the train stops or backs up, the crossing warning device will stop operating. The industry has taken it one step further by converting the motion sensor into a device that can predict the speed of an oncoming train to activate the crossing at a pre-determined time. The automatic warning device is hardware and software driven. The above example illustrates a bi- directional configuration.
A key function of the transmitter section is to maintain a constant AC current on the track. The transmitter wires TX send an AC signal: Down one rail in both directions bi-directional Through the termination shunt at the ends of the circuit Through the other rail, returning to the AC source The receiver wires RX define the limits of the island circuit and monitor the transmitter signal. Track impedance, in the form of inductive reactance resistance to AC , depends on the length of the track circuit, which is defined by the termination shunt and the applied frequency.
For this reason, the longer approach circuits should use a low frequency, while the shorter island tracks should use a higher frequency. With no train on either approach, the electronic box at the crossing creates a volt DC signal distant voltage. When a train comes onto the crossing approach, the following occurs: Lead axle shunts the track. Lead axle becomes a moving termination shunt, which shortens the track circuit as it approaches the crossing.
Track impedance resistance decreases as the track circuit shortens. As the track impedance decreases, the distant voltage 10 VDC decreases towards 0 volts at the crossing. The rate of voltage drop is dependent on the speed of a train. From this, you can probably see that with a little creative programming, the box can predict the speed of a train and activate the crossing at the appropriate time or stop the crossing operation if the train stops or backs up.
For this configuration bi-directional , no insulated joints are required. However, if there are insulated joints because of the presence of a DC track circuit, bypass couplers can be used to allow the AC signal around the joints while blocking DC.
Output terminals from the crossing predictor provide 12 volts DC to the crossing control circuits. The crossing control circuits are either relay logic control circuits or solid-state control circuits. Crossing control circuits operate the bell, flashing lights and gate arms. CTC allows for more than one train to be in a block, travelling in the same direction at the same time and eliminates the need for train orders and timetable superiority.
Control point circuitry, controlled block signals, dual control power operated switch machines, electric locks in conjunction with switch circuit controllers and advanced communications systems are all integral parts of a CTC system. Signal indications authorize train movement in CTC. Once a train is allowed into a block by the dispatcher control signal often referred to as home signal , the train is controlled by automatic block signals intermediate signals. Important Note: The sequence of operations described below is a typical model only.
For compliance to FRA requirements and regulations refer to Parts and Modern installations are microprocessor based with solid-state support circuits and advanced communication links.
For this discussion, we will consider a relay-based system. A later section of this chapter will introduce solid-state systems. Control and indication codes rely on step-by-step operation of relays and mechanisms at the field location, working in synchronism with step-by-step operation of relays at the control office.
Control Codes: To transmit a control, the dispatcher positions the necessary levers and buttons on the control machine. Next, he pushes the appropriate start button that causes a code to be transmitted. All field locations connected to the code line see the control code, but only the one called is selected. At the selected location, the control portion of the code is delivered through field application relays to cause the function relays to operate switches, signals, etc. Indication Codes: When a field change occurs in the position of a switch, the aspect of a signal, or the condition of a track circuit, an indication code is set up at the field location, which in turn automatically transmits the indication back to the control office.
When the indication code is received at the control machine, the appropriate indications light up on the dispatchers panel to show the conditions existing at the field locations. Control Point: Control Points may consist of a single switch or a cross-over between tracks, or various combinations of switches and crossovers with associated signals.
From the control machine, the dispatcher remotely controls the power switch machines. A network of signals is associated with each power switch to ensure that train movements are made safely. CTC is basically a series of controlled switches and signals at wayside locations, connected with automatic signalling. Control Office: A dispatcher's duties require that he set up routes and signals for traffic, arrange meets of trains and provide protection for roadway workers.
Railways have implemented computers to assist with train control systems. The computers are equipped with mass storage devices on which train and signal activity are archived for future reference. This information is accessed for purposes ranging from accident investigation to train delay reports. The dispatching computers are located in a special room.
This room contains an air conditioning system to keep the environment at a constant temperature and humidity, and a fire protection system to safeguard against fires in and around the computer room. As well, the system is equipped with an un-interruptible power supply UPS to keep it up and running in the event of a commercial power failure. The uninterruptible power supply is made up primarily of storage batteries and a diesel generator.
The generator is used to keep the batteries fully charged if the power failure persists. The computer duplicates all of the interlocking checks performed by the field circuitry, safeguarding against any potentially unsafe requests by any of the system users. The purpose is to inform engineers of design considerations for railway structures that are different from their non-railway counterparts.
Due to variations in design standards between the different railways, consult the controlling railway for their governing standard before starting design. Common examples of track carrying structures are bridges, trestles, viaducts, culverts, scales, inspection pits, unloading pits and similar construction. Examples of common ancillary structures are drainage structures, retaining walls, tunnels, snow sheds, repair shops, loading docks, passenger stations and platforms, fueling facilities, towers, catenary frames and the like.
While the design of ancillary structures for the railway environment may introduce considerations not found in their non-railway counterparts, these considerations are usually well defined in the governing railways standards. Accordingly, this chapter will focus primarily on track carrying structures. When designing railway structures, the various sources of their loads must be considered, as they would be with any other similar, non- railway structure.
In addition to the dead load of the structure itself, there are the usual live loads from the carried traffic. To these are added the dynamic components of the traffic such as impact, centrifugal, lateral and longitudinal forces. Then there are the environmental considerations such as wind, snow and ice, thermal, seismic and stream flow loads.
Once the designer has established the first pass at the load environment for the subject structure, the primary difference between a highway structure and a railway structure should become obvious.
In the typical railway structure, the live load dominates all of the other design considerations. For the engineer accustomed to highway bridge design, where the dead load of the structure itself tends to drive the design considerations, this marks a substantial divergence from the norm. Specifically, the unacceptability of high deflections in railway structures, maintenance concerns and fatigue considerations render many aspects of bridge design common to the highway industry unacceptable in the railway environment.
Chief among these are welded connections and continuous spans. In addition to the types of construction, the engineer must also choose from among the available material alternatives. Generally, these are limited to timber, concrete and steel, or a combination of the three. Exotic materials can also be considered, but they are beyond the scope of this book. Each material has its specific advantages. Timber is economical, but has strength and life limitations. Structural timber of the size and grade traditionally used for railway structures is getting more difficult to obtain at a price competitive with concrete or steel.
Concrete is also economical, but its strength to weight ratio is poor. Steel has a good strength to weight ratio, but is expensive. The material chosen for the spans will generally determine the designation of the bridge. For instance, steel beam spans on timber piles will be considered a steel bridge. The point where one form of construction with a certain type of material becomes advantageous over another is a matter of site conditions, span length, tonnage carried and railway preference.
While initial cost of construction is a major point in the decision process, the engineer must keep in mind such additional factors as construction under traffic and the long-term maintainability of the final design. For short height structures, trestle construction is favored due to the economies of pile bents. Conversely, taller structures over good footing are likely to be viaducts with longer spans supported by towers.
Where there is insufficient clearance over navigable waterways, moveable spans may be necessary. The addition of longer or moveable spans to clear main channels does not significantly affect the design of the balance of the structure. However, as the structure becomes taller, the economies of pile bents are diminished due to the need to strengthen the relatively slender components. The alternative to conventional trestle construction is trestle on towers, otherwise known as viaducts.
Trestle on towers can offer a significant reduction in footprint for only a moderate increase in span requirements. It is customary for the spans to be of alternating lengths, with the short span over the tower equal to the leg spacing at the top of the tower. This ensures that each span remains a simple span with full bearing at the ends of the span. Of course, trestle construction represents the typical site conditions.
More demanding site conditions may require exotic solutions. For example, very tall, very short length conditions may lend themselves to arch construction, whereas for transit operations, very long main span requirements may lend themselves to suspension type construction and some trestles on towers may be better constructed as a series of arches. In comparison to the rest of the superstructure design, bridge deck decisions are relatively simple.
The choices are open deck and ballast deck.
On open deck bridges Figure 8- 5 , the rails are anchored directly to timber bridge ties supported directly on the floor system of the superstructure. On ballasted bridge decks, the rails are anchored directly to timber track ties supported in the ballast section. The ballasted bridge decks require a floor to support the ballast section and such floors are designated by their types, such as timber floors, structural plate floors, buckle plate floors or concrete slab floors, all of which transfer loads directly to the superstructure.
The latter types of structures have many examples still in service today, but are not generally cost-effective for new construction. Some might consider the notion of bridge railings to be an odd bridge design consideration.
Railway bridges traditionally have not been designed for the conveyance of anything other than railway traffic, which does not in and of itself, require any sort of railing whatsoever. Recently, however, a greater focus upon railway worker safety has resulted in railings being widely incorporated.
Open Bridge Decks Many different considerations enter into the choice of open or ballast decks, and the selection usually is governed by the requirements of each individual structure. Open decks are less costly and are free draining Figure , but their use over streets and highways requires additional measures such as canopies, plates or wooden flooring to protect highway traffic from falling objects, water or other materials during the movement of trains.
Open-deck construction establishes a permanent elevation for the rails. Normal surfacing and lining operations, particularly in curves, eventually result in line swings leading into the fixed bridge. The grade frequently is raised to the extent that the bridge eventually becomes low. The bridge dumps are of a different modulus than the rigid deck. Thus, it becomes difficult to maintain surface off of the bridge as well. This equates to extensive maintenance costs that shortly will surpass the first cost savings gained by installing an open deck bridge over a ballast deck bridge.
In welded rail, tight rail conditions can occur at the fixed ends of an open deck bridge, thus requiring an increased level of surveillance in hot weather. Requirements for Ties For ballast deck structures, bridge ties are no different than those found in traditional track construction. However, in track constructed with concrete ties, the track is often times transitioned to timber ties before crossing the structure.
Some railway companies and agencies have had difficulty with fouled ballast, track alignment and deck surface damage resulting from the use of concrete ties on bridges. Ballasted Decks A ballasted deck Figure provides a better riding track. The track modulus is consistent on the dumps of the bridge as well as across the bridge. Thus, one is unlikely to have surface runoff problems on the bridge dumps. Surfacing and lining operations can continue across the bridge unimpeded.
However, care must be exercised to maintain a permanent grade line in the vicinity of and over a ballasted deck bridge to be certain that excessive quantities of ballast are not accumulated on the bridge structure through track raises during successive reballasting operations. Ballasted decks Figure , irrespective of the type of bridge floor, afford a considerable measure of protection to the steel floor system against damage from derailed car wheels traveling across the bridge.
Over roadways, vehicles and the public are protected from dropping ballast and material off of the cars. Ballast The depth of ballast contributes to the satisfactory functioning of ballasted decks on railway bridges.
It is generally agreed that 6 inches to 12 inches of ballast under the ties is adequate and that more than 12 inches is undesirable because of the potential of overload involved, except when provision is made in the design for a greater load. Many designers calculate the dead load on the basis of 18 inches to 24 inches of ballast to accommodate future raises. Figure Ballast Decked Bridge. Though this is commonly a result of increases in traffic or higher safety standards, the ability to perform major repairs or upgrades of highway structures by temporary removal of the bridge from service is generally not a significant concern.
Railway bridges, on the contrary, are designed to have a significantly longer life, and indeed, a considerable number of railway structures in service today are in the neighborhood of years old.
Though the design criteria within AREMA reflect this consideration, the operating impact and expense must be called to mind when considering the replacement of an existing structure. Often times a designer will have a proposed design solution rebuffed by a railway for this reason.
Though the solution offered may be widely accepted in highway design, the permanence required by the railway environment may not have been yet proven to the railway. Railway structures require a much greater consideration of longitudinal loading than a typical highway bridge. This is the result of two environmental variables. Vehicle and individual wheel loads of railway vehicles are many times greater than roadway vehicles.
Likewise, unlike roadways, the vehicle running surface the rail is continuous between the bridge structure and the adjacent roadbed.
The track structure by its very nature is moderately flexible, distributing loads in all directions over a length of track. The introduction of a fixed object e. When comparing railway bridges to roadway, pedestrian, and other sorts of bridges, the live loading relative to the dead load is much greater and more consistent. This consistent loading and unloading over a greater stress range results in fatigue considerations more prevalent in railway bridge design than other types.
Many of these guidelines are consistent in character, if not identical to other codes. However, there are many distinctions, which are the result of the different service demands of railway structures as well as railway practice or preference developed over the past years. The designer must be cognizant of the fact that each chapter is effectively independent of the others, and not all handle similar design considerations in the same fashion.
Where a single structure may incorporate several different types of materials e. The reader is also cautioned that the Manual for Railway Engineering is always under revision.
The following material is current as of the date this text was published and is provided herein only for general informational understanding. Referencing the latest issue of the Manual for Railway Engineering is essential before undertaking any design activity.
Dead Load The dead load consists of the estimated weight of the structural members, plus that of the tracks, ballast and any other railway appendages signal, electrical, etc. The weight of track material running rails, guard rails, tie plates, spikes and rail clips is taken as pounds per lineal track foot. Ballast is assumed to be lbs per cubic foot. Treated timber is assumed to be 60 lbs per cubic foot.
Waterproofing weight is the actual weight. The designer should allow for additional ballast depth for future grade or surfacing raises generally 8 On ballasted deck bridges, the roadbed section is assumed to be full of ballast to the top of tie with no reduction made for the volume that the tie would include. This type of superstructure comprises a steel beam or girder and a concrete deck slab.
The connection between the two materials is designed and constructed to transfer adequate shear force, such that the two materials behave as a single, integral unit under load. Some of the important issues include: Selection of the effective flange width of the concrete as a function of slab thickness, steel beam spacing or span length; Proportioning of the cross-section by the moment-of-inertia method; Application of the dead load forces to the non-composite or composite section, depending on construction sequencing and methods; Considering the effect of creep due to long term dead loads acting on the composite section.
Shear connectors may be either steel channels or headed studs welded to the top flange of the steel and embedded in the concrete deck. Additional consideration is warranted for railway bridges in other aspects of design, however. One issue to address in composite design is the magnitude of live load to be resisted.
Although not specifically addressed in the AREMA Manual for Railway Engineering, railway companies generally require that the steel beams or girders be proportioned to carry without contribution from the concrete deck slab, a Coopers live load of only a slightly reduced magnitude than that of the entire structure. For example, a bridge with a composite design load of E is often required to have the steel section alone provide support for an E or higher, and maybe as much as E as well, depending upon the railroad and the type of structure considered.
This ensures that if the concrete deck is damaged during a derailment, the steel section will be sufficient to carry rail traffic, even if the concrete must be torn out and an open deck installed.
If the steel alone is sized for the design load, the cost savings through efficient use of materials is somewhat less for railway structures than it is for highway and building structures that make full use of the composite section to resist live load and impact. The full composite section should be designed as being sufficiently stiff to meet the deflection limitations.
Even if the steel section is adequate to carry the final design loading without contribution from the concrete slab, composite action still must be investigated. The neutral axis of the composite section will be higher on the cross-section than that for the non-composite section. This will increase the stress range in the material below the neutral axis, and fatigue details should be checked for this increased range.
While composite steel and concrete spans provide a stiff design with the benefits of a ballasted-deck bridge, they are unlikely to be used to replace existing structures on existing alignments. Compared to precast concrete deck panels, the additional time required to form, place and cure the cast-in-place concrete deck of a composite span requires off-line construction to minimize impact to rail operations.
Parallel construction of a composite span with a lateral roll-in during a train free window is one way to work around this problem. Additionally, since the deck concrete is not under compression from prestressing or post-tensioning, the use of a waterproofing system to protect the deck may be warranted.
Where structure depth is limited by vertical clearances below the structure, a steel plate may be used instead of a concrete deck. The steel plate may or may not be included in the beam design, depending on the connection to the beam. Bridge Design Assumptions and Constructibility Issues When planning railway structures, it is imperative to be mindful of the factors that frequently control design and construction. Many in the railway industry would agree, that the driving factor of design and construction is track time.
Operations are key, and with greater traffic demands on an ever-aging infrastructure, track time is at a premium.
The designer is challenged with producing plans and specifications that will yield the best structure in the shortest amount of time. Many times the design efficiency is sacrificed for a shorter construction period. Lets briefly examine one simple scenario: In its nearly year life span, the steel superstructure had been raised while being converted from a ballasted deck to an open deck. This conversion included the use of what is known as a grillage.
A grillage also known as cribbing is a temporary steel support, usually in the form of short sections of steel H-piles, welded together side-by-side to form a shallow foot bearing seat. The steel is subsequently encased in concrete. This technique is most common to rehabilitation projects. This chapter presents an introduction to electrification of rail systems. It is intended to provide a historical perspective and an overview of typical design principles, construction practice, and maintenance considerations.
Those interested in learning more are invited to review AREMAs Manual for Railway Engineering, Chapter 33, Electrical Energy Utilization, and Chapter 17, High Speed Rail Systems, which contain sections devoted to electrification power supplies, traction power systems studies and guidelines for the design of overhead contact systems.
When the cost of diesel fuel was 9 cents a gallon and the supply seemed unlimited, United States railways were not interested in alternative methods of propulsion. Railway electrification interest peaks during times of uncertainty in the energy industry. When fuel rose to 34 cents per gallon and the oil embargos occurred, much effort was expended studying alternatives to hydrocarbon fuels.
Studies made in the s also showed that approximately 6 years after electrifying a route, the operating cost would break even when compared to the operating cost of diesel service.
At 30 years, the annual operating cost of an electrified system would be one-third that of diesel service. In other words, over the effective life of a railway, the cost to operate a diesel-electric system far exceeds that of an electric system. These increased costs mainly come from the price of fuel and maintenance. Diesel locomotives average 3 to 10 gallons or more of fuel per mile and three times the amount of maintenance of straight electric locomotives.
The most significant aspect arising from these studies is that in order to realize the long-term savings, a huge capital investment is needed. Even when engineering economic studies show that an electrified system would be beneficial, raising enough money to perform the capital upgrade is a daunting challenge.
Private railways would most likely require government assistance or financing from the utilities. The more significant issues are noted below: Tracks may need to be upgraded, including new track work or re-alignment. Sites must be found and real estate acquired for substations. In rights-of-way with restrictive width, the location of the system-wide ductbank requires coordination with track drainage, the foundations for OCS poles and emergency walkways.
In all cases, maintenance access must be provided. If DC traction is used, the effects of electrolytic corrosion due to leakage stray currents must be mitigated. Existing civil structures may have insufficient clearance to accommodate the proposed electrification system. It may be necessary to lower tracks through overhead crossing bridges.
New bridges resulting from grade-crossing elimination will need to be built with adequate electrical clearance. Future widening of existing overhead bridges must be considered.
Tunnels may be suitable for electrification, or may require costly remedial work, enlargement or daylighting. Integration of the electrification support structures with existing station canopies must be considered. Station canopies that project over platform edges may need modification. Where OCS poles cannot be installed for lack of clearance, attachments, such as wall brackets, will need to be added to civil structures.
Pictured at the right is an example of an OCS cantilever attachment to an overhead structure. Signals and communication systems will need to be replaced or upgraded. Because electric traction systems use the same running rails for traction return current, it is necessary for the two electrical systems to be electrically isolated.
The signal circuits need to be immunized from the traction power circuits. Grounding and bonding of exposed metals is necessary to protect the public from electrical hazards, as well as insuring that there is no interference with the signals and communications systems. A central location will be needed to supervise the power system. SCADA, pilot wires or a relaying system must send information to a central point to insure power is being supplied to the system when necessary.
Maintenance More details on these and other aspects impacting the railway route are given later. The advent of electrification increases the level of overall maintenance on the right-of- way. To help the structures fit into the urban environment, the structures will often serve double duty by acting as light poles, traffic signal poles, etc. On straight or slightly curved track, either cantilevers or cross-spans support the trolley wire such that it is placed over the center of the track.
When the track requires tight curves, the trolley wire is held in place with cross-spans, pull-offs and back bones. Although trolley poles pivot at the base, the trolley harp does not pivot so that the trolley wire must be placed towards the center of the curve on sharp curves to allow the trolley shoe to track efficiently.
The trolley shoe must be drawn tangentially along the trolley wire, thereby not rubbing against the cheeks of the groove. Only by using rigid harps can the trolley shoe diverge onto the correct trolley wire at turnouts, as the pole operates passively being positioned only by the direction of the streetcar on its tracks.
Catenary Systems Two-wire systems are referred to as simple catenary and utilize a contact wire and above it, a messenger wire. The messenger wire serves two purposes 1 to support the contact wire vertically between structures by use of hangers and 2 to provide more electrical conductivity.
Variations of simple catenary exist, such as low profile simple catenary, which can be considered as a three-quarter- scale version of the most economic simple catenary style. The low profile simple catenary has reduced visual impact by virtue of requiring only one cross-span wire for support between poles compared to the necessary two cross-span wires with full simple catenary as pictured Figure Nevertheless, it is still only about half the cost of a single contact wire system with parallel underground feeders, which would be electrically equivalent.
Twin contact wires are also commonplace on light rail systems in Europe. Other systems using three conductors called compound catenary are operating, but are more costly and are generally not considered necessary for new installations.
Compound catenary utilizes three or more conductors, with a main messenger being the top conductor, the contact wire serving as the bottom conductor, and an auxiliary messenger located between the two.
Other styles, which have been installed in the past, include stitched catenary, triangular catenary and hanging beam catenary, and all continue in use today. This is accomplished by inclining the OCS so that the messenger wire is moved to the outside of the curve while the contact remains close to the track centerline. Sloping hangers support the contact wire at a carefully calculated angle to provide the lateral restraint.
Inclined catenary has fixed terminations, which means that the contact wire moves up and down relative to the track surface as temperatures change. Thus greater clearances are required under structures and over grade crossings. Because of the special techniques needed to align inclined catenaries, the trend today is to replace them with chordal simple catenary, where the messenger is located directly above the contact wire.
Catenary systems are designed to allow the contact wire to operate satisfactorily over the full extent of the carbon-rubbing strip of the pantograph. Careful calculations are performed to determine the extent that the wire can be staggered at the OCS registrations supports and to ensure that the pantograph does not dewire in a combination of adverse operating conditions, including strong winds, maximum vehicle sway and poor quality track.
These calculations are then used to determine how much the contact wire can be allowed to be placed off the centerline of the track and still allow safe operations. On tangent tracks, the wire is intentionally staggered from one side of the track centerline to the other at successive poles to prevent grooves from forming in the middle of the pantograph carbons.
Although small variations to sag and tension do not adversely affect current collection, also called commutation, large variations, say over 6 inches, can be unacceptable. In order to control conductor sag between supports, two options are available: Limit span length length between poles Tension compensation described later Both options apply to Single Contact Wire SCW systems and to multiple conductor catenary systems to be described later.
The more significant impacts involve: Additionally, track renewals and track lowering measures, as described below, should have been finished. Future track improvements may need to be accelerated to avoid the need for later changes.
Old redundant track should be removed before initiating electrification so that cranes are not impeded by the presence of high voltage catenary wires, conductor rails or cables. Substations Typically, 25kV substations require a site area of about an acre in size, with road access suitable for trucks delivering the largest piece of substation equipment.
DC substations are smaller, ranging in size from to square feet, but are generally more numerous than AC substations. Supporting Structures for the Contact System On existing main line routes, particularly those with more than two tracks, there will probably not be enough room between tracks to install OCS pole foundations.
Therefore, the poles will be allocated to the outside of the line. Since third rail is attached to the end of the ties, ROW limits are not as critical for third rail systems as for overhead systems.