American Railway Engineering and Maintenance-of-Way Association America's First African American Female Combat Pilot to Speak at Railway Interchange. Most of the recommended practice relating to railway structures is contained within Volume 2 of the AREMA Manual for Railway Engineering. Chapters within . arema manual of railway engineering document arema manual of railway in various formats such as pdf, doc and epub which you can directly download.
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The text hereafter is not intended to supplant the AREMA Manual for Railway Engineering, the AREMA C&S Manual or other comprehensive texts covering. access the full manual, you will need to place an order in our online http://arema. org/publications/mre/ayofoto.info Arema Manual For Railway Engineering Pdf. AREMA RAILWAY MANUAL. Edition, Complete Document. MANUAL FOR RAILWAY ENGINEERING. View Abstract. Product Details.
Lateral loads from equipment are not included in the design of concrete bridges. Insulated fiber bushings and washer plates are used to isolate the bolts from the bars. However, a number of criteria should be entertained before doing so, including: When it is impossible to drive a concrete pile to its full depth, it may be cut off by using a concrete friction saw and by torching the reinforcement with an oxy-acetylene torch. As a vehicle traverses a curve, the vehicle transmits a centrifugal force to the rail at the point of wheel contact. Starting Resistance The resistance caused by friction within a railway vehicles wheel bearings can be significantly higher at starting than when the vehicle is moving.
On long or multiple spans, deck drainage should be provided with adequate outlets for the drains, so that the water will be carried off quickly in order to prevent seepage and consequent deterioration of the slab. Concrete Trestles Trestles of this type usually consist of concrete pile bents spaced from 14 to 20 feet apart. The height should not be greater than the span. The bents may also consist of narrow concrete piers or concrete columns footed on concrete pedestals.
A ballast deck is almost invariably used on a concrete trestle. Concrete trestles are more expensive in first cost than those of timber. The replacement of a timber trestle with one of concrete Figure may be accomplished with minimum delay to railway traffic. The concrete pile bents are driven and caps cast. After curing, the timber deck is removed and the concrete slab placed between trains.
In some cases, precast caps may be utilized over the top of H-pile or even timber pile. Caps are often prestressed units or cast-in-place with a high early strength concrete. The cap or transverse strut at the top of the piles or columns forming the bent must be designed as a reinforced concrete beam to transfer the load from the slab uniformly to the supporting piles or posts.
The floor slab or span may be poured in place after the bents have been constructed, but the use of precast panels with the ballast pan integral is common.
Concrete Girders These are sometimes adopted for the construction of bridges designed to span openings between approximately 25 ft and 60 ft in length. Through, half-through and deck types are used, although the latter is generally preferable. Common beam sections are slabs, tees and voided single and double cell boxes Figure These shapes are well suited for spans up to approximately fifty feet.
In most cases, box sections are the preferred section, since they provide a solid deck suitable for ballasted track with no additional construction.
This type construction can provide ballast deck spans up to feet. However, given the time required to form and cure the cast in place concrete, this type of construction is only suited for new railway line, off-line or shoe-fly construction. Precast, post-tensioned segmental concrete construction has also gained acceptance in new construction.
This type of bridge allows construction of very long spans. However, given the time required to set and anchor each segment, this type of construction is also only suited for new, off-line or shoe fly construction. Given the advances in precast concrete technology and acceptance, cast-in-place, reinforced concrete is seldom used in span construction. The time required to form and cure cast-in-place concrete renders it inappropriate for construction under traffic. The common forms of moveable spans are Bascule, Lift and Swing.
Variations of these structures are also found in shop environments where turntables and transfer tables are use to reposition cars and locomotives between various tracks. Determination of whether a movable bridge to be utilized is dependent largely on the horizontal and vertical clearance requirements posed.
Actual design requires additional considerations, since the structure is a precision machine that must maintain perfect alignment every time it is lowered to maintain track, signal and possibly electrical continuity. Specific design elements that must be entertained beyond the structural characteristics of the bridge include: Selecting the type of movable bridge to be used is dependent on the width of the channel and the type of navigation using the channel.
Appropriate foundations must be selected. Lastly, the duration and required frequency of bridge openings and closings must be considered. The potential impact to rail and other vehicles must be evaluated. Bascule Bridges Bascule bridges are single leaf spans of either plate girder or truss construction.
They open vertically by pivoting at one end of the span to provide the navigable opening. They are suitable for small to medium span lengths and consist of one of three basic types: In the trunnion bascule bridge Figure 8- 29 , the leaf rotates about a horizontal axis with the trunnion supporting the entire structure when raised.
The curved tracks, which are segmental girders with a tread plate, are attached to the tail end of the structure. Lugs or teeth See Figure are attached to the bottom of the curved track and they engage a matched track plate girder to prevent slippage. The entire weight of the bridge is supported by the curved track when the structure is opening.
At the top pivot end of the structure is a counterweight, which offsets the weight of the span. A powered pinion gear See Figure engages a fixed horizontal rack gear attached to a frame on the adjoining span.
As the pinion gear moves forward or backward on the rack, the curved track enables the horizontal motion of the rack to translate into vertical motion of the structure. Obviously, the opening angle secured is a function of the horizontal roll distance available for the curved track to move.
The bascule span rotates about the main trunnion. The counterweight is attached to a rotating framework. The operating strut is composed of a rack gear. The outer end of the operating strut is pinned to the top chord of the bascule truss and the counterweight link. The opposite end of the counterweight link is also pinned.
A fixed pinion gear moves the operating strut towards the pivoting end of the bridge. Because of the pinned connections, the counterweight frame rotates downward, thus raising the bascule span.
The counterweight offsets the weight of the bascule span. Typically, these spans are truss constructed to accommodate negative bending over the center pier while in the open position. This type of structure is suitable for short to medium span lengths on each side of the pivot pier, with the spans usually symmetric in length.
There are two types of swing span structures in common use. The center bearing swing span supports the weight of the structure by a center thrust bearing Figure The center of gravity of the structure is immediately over the bearing to ensure that the bridge is balanced when in the open position. Balance wheels stabilize the structure as it opens and closes. The span rides on tapered rollers, which carry the weight while opening and closing as well as providing stability during movement.
Cables raise vertical lift spans vertically. The span remains in a horizontal position when raised or lowered. Unlike the previous examples, the navigable opening provided by the vertical lift span remains limited by the height of the lift towers. These spans may be a rolled beam, plate girder or of truss construction. The weight of the span is offset by two counterweights located at the top of each tower. The lift machinery is mounted above the deck.
The towers can be of either braced or unbraced construction. Access to the tower is required to grease sheaves and service lifting machinery. A system of span guides and bridge locks ensures that the bridge is properly aligned when the span is lowered to operate trains.
Any misalignment will not permit the bridge locks to engage and a proceed signal will not be displayed. Vertical lift bridges are suitable for medium to long spans where height clearance is required. Vertical lift bridges are categorized by the location of the drive machinery.
Counterweight ropes attach to the span and to the counterweight. The machinery turns the sheave, thus raising the span and lowering the counterweight. Again, the counterweight ropes attach to the span and to the counterweight, but the lifting force is provided by operating ropes cables and drums, one at each corner. A drive shaft from the motor, located in the control house extends out to the drum located at Figure Span Driven Vertical Lift - Courtesy of each end of the span.
Christian Brown, HNTB Although the weight of the span lifted is massive, the load imposed on the operating cables is relatively small, due to the offsetting weight of the counterweight. In each case, a movable bridge requires balancing the structure. Although gravity plays only a small part in opening and closing bascule and vertical lift bridges, it plays no part at all in swing bridges.
The critical balancing component is the counterweight. Typically, the counterweights are of concrete or steel-encased concrete construction. Balance pockets are provided for not less than 3. The configuration of the counterweight is important too, especially for bascule bridges. The lowered counterweight must maintain clearance between other structural members of the bridge.
For vertical lift bridges, the designer performing calculations dealing with balance must consider on the counterweight side of the tower: Other design considerations include electrical control equipment, including, but not limited to: Maintenance issues include: These spans are usually supported on a center pivot pier as well as a circular track at the end of the span. Conversely, a transfer table carries locomotives or cars in a lateral direction.
These spans are usually supported on a track at each end of the span. However, they tend to be significantly sturdier due to the higher live loads, which must be supported.
Each railroad has different preferences relating to the types of materials installed. Many prefer metal pipes to concrete, as they tend to be less susceptible to failure due to settlement. Newer materials such as plastic have not generally gained wide acceptance for use under track. Box culverts Figure are almost exclusively concrete. The preference of cast- in-place versus pre-cast differs between railways.
Box culverts may be one cell, two cell or three cell, depending on the size of drainage stream. French drains are constructed adjacent to and parallel to foundation structures to drain away ground water. They are typically corrugated metal pipe with perforations along the bottom invert to allow drainage of the surrounding soil.
This material may be an embankment for supporting track loads or natural earth along the edge of a cut and separated from the wall by a wedge of filled-in material.
Normally, retaining walls usually do not carry vertical loads. However, bridge abutment walls are types of retaining walls that are required to carry bridge superstructure vertical loads in addition to large net overturning moments. Ordinarily, gravity retaining walls are built of reinforced concrete, mass concrete and formerly of stone masonry. Overturning forces are resisted by the "gravity" weight alone of the masonry or concrete. Failure of a retaining wall can occur by sliding along a horizontal plane, by overturning or rotating and by crushing of the masonry.
The design of the wall, and especially the footing, should include such special features as indicated by the character of the supporting earth at each location. Crib Walls Crib walls, also known as bin walls Figure , are composed of interlocking prefabricated members arranged to form a series of cells or "bins," that are then filled with compacted backfill.
Crib walls are frequently used as an alternative to stone or concrete retaining walls. Although a carefully constructed foundation forms the base of a solid retaining wall, crib walls are ordinarily supported directly on the particular material encountered at each location. Consequently, the use of crib walls should be confined to locations where the supporting material is reasonably firm and stable and is free of impounded water. Tensile forces within each cell resist overturning forces.
The cells are anchored by "deadmen" in the back of the fill. Many old railway crib walls were often constructed using old railroad ties. Since the width of a crib wall increases as the height of the wall increases, space limitations may impose restrictions upon crib wall use.
Three different types of ready-made cribbing are available: Creosoted timber, steel and reinforced concrete. Cribbing of these types are used for: Creosoted timber cribbing is made up of two different types of units: Each header and stretcher is dapped and bored prior to treatment. Drift bolts are driven during erection to give additional stability to the interlocking timbers.
Metal cribbing consists of box-like headers and stretchers. Each stretcher usually has lugs at both ends, which fit into corresponding slots in the header units. Every header is locked at opposite ends to the stretchers directly underneath by bolts. The stretchers usually are of a plain square section while the headers are of rectangular section with T-shaped heads for tying the stretchers together.
As excavation proceeds on one side of the wall, horizontal sections known as walers are welded or fixed to the piling to provide additional support. Sheet pile walls are fairly expensive and require extensive information on buried utilities prior to driving. Wooden sheet piling includes a variety of proprietary designs intended to provide a tongue-and-groove effect at the mating surfaces. A variety known as Wakefield is formed of three planks fastened together to create a tongue and groove.
Steel sheet pile retaining walls consist of individual sheet piles driven into the ground that are interlocked to each other to form a vertical steel wall. Submerged timber piling has long life at points where it is left as a part of the permanent construction and may sometimes be salvaged, if desired.
However, the relatively thin planks are readily split or broomed by contact with stones or other hard materials encountered in passage through the soil. Metallic piles have great penetrating power under the impact of the hammer and are less susceptible to damage in driving than timber piles.
These piles may be left as a permanent part of the under water construction, or they can be withdrawn and re-used.
Concrete sheet piling, when properly and thoroughly cured before use, and under normal conditions, is permanent in water and air, and is particularly applicable where the sheet piling is to remain as a part of the permanent structure. Due to the relatively brittle nature of the material, the salvaging of such piling is difficult. For temporary use, timber or metallic sheet piling is preferable.
Sheet piling may be driven to form either single or double partitions. Steel sheet piling is frequently left in place as a part of the completed structure. Soldier pile and lagging retaining walls are cheaper than sheet pile walls and are more appropriate in areas where buried utilities are expected.
The soldier piles are usually steel rolled sections driven vertically into the ground at 5-foot to foot center-to- center distances. As excavation proceeds, concrete or timber lagging is placed horizontally between the soldier piles. Horizontal steel walers are added as bracing is needed. MSE retaining walls represent a relatively new method of resolving earth retainage problems.
Instead of regarding soil as a mass to be contained by force, the earth itself is reinforced to become an integral part of the structure. MSE Figure MSE Wall - Courtesy of Sam Dragonetti, UMA walls rely on increasing the strength and stability of earth embankments by placing corrosion resistant reinforcing straps, welded wire mesh, or geotechnical fabric within the earth embankment as it is constructed.
The walls then behave as gravity structures in an integral unit and provide structural flexibility. Native soils at the site or from excavation are usually acceptable for backfill. The resulting structure is strong, yet resilient. MSE walls generally include a fascia panel typically precast concrete, but can also be welded wire mesh, cast-in-place concrete, or other materials.
Precast panels or cast-in-place fascia allow for a wide variety of architectural treatments and finishes. An MSE constructed with a face of welded wire Figure can be covered with air- blown mortar, seeded with grass or plants, or filled with rock. MSE walls perform extremely well in a multitude of conditions. It performs particularly well in seismic zones, due to the built-in flexibility of the system, which allows for some Figure MSE Green Wall - Courtesy of Charley Chambers movement without distressing the structure or causing cracks.
It can also tolerate a certain amount of settlement, making it a desirable solution even in relatively poor subsoil conditions. The primary reason for the use of MSE walls is its inherent low cost. Installation is fast and efficient, using a simple, repetitive construction procedure.
After placing the initial course of panels, the first lift of backfill is spread and compacted. The reinforcement steel straps, welded wire or geotechnical fabric is laid on the compacted lift and connected to the panels if used.
Next, a lift of backfill is spread and compacted over the reinforcing material. This procedure repeats until the design height is reached. Regardless of height or length, the structure is stable during construction. Equipment may operate on any layer of backfill. MSE walls are well suited for restricted sites or close property lines since construction is performed behind the wall face without any forms or scaffolding. MSE structures should be considered for projects that have problems that may include costly right-of-way acquisition, lack of suitable borrow sources, topographic restraints or difficult subsurface conditions.
Although the use of MSE technology has been proven and accepted in standard practice in highway applications, the use of MSE walls in the railway industry is limited and should be approved by the impacted railway before design starts. Drainage of Retaining Walls Water under the foundation and behind the wall is the most frequent cause of failure of a retaining wall. Effective weep-holes through the footings and the body of the wall ordinarily will prevent the impounding of water behind the wall.
However, additional measures may be necessary, such as the installation of drainage pipes to collect and deliver the water to the weep-holes or other suitably located outlets, and also sub-drainage adjacent to the footings, to lower the water level in the cut. Tunnels have also been constructed to carry rail lines underground, beneath cities, rivers and canals.
The engineering associated with tunnel design and construction is not specific to railway engineering. However, there are aspects of tunnel design that railway engineers need to pay particular attention to including: Pressure relief through proper ventilation is required to release this pressure build up.
In tunnels, consideration should also be given to alternate track support structures, as the ballast may tend to break down faster due to the lack of flexibility in the sub-grade. Consider direct rail fixation or alternate methods. Once the ground has been investigated, an excavation and support procedure must be chosen that can handle any unpredictable ground conditions without unnecessary interruptions or risk.
There are two major classifications of tunnels: Rock cut tunnels and soft ground tunnels. Both types of tunnels have had significant improvements in their construction over the last century, including faster construction rate, increased usable cross-sectional area per excavated volume, reduced support volume and improved construction safety. Some major lessons that have been learned from tunneling in poor ground are: The shield consisted of 12 sections that could be advanced independently by pushing against the brick liner.
The first circular shield, propelled by screw jacks, came out fifty years later in In , the Price Rotary Digger Shield was developed. The Price Rotary Digger Shield was the first soil excavation machine.
The first successful mechanized Tunnel Boring Machine, for rock drilling, took another sixty years to be developed. It was used in for the Oahe Dam project in South Dakota. In , the first immersed railway tunnel was built under the Detroit River. Since then there has been an increasing use of immersed tunnels for crossing water bodies.
NATM is a tunneling philosophy based on scientifically established principles and proven ideas and is not a construction excavation and supporting method. The following excerpts are taken from Professor E. The inherent strength of the soil or rock surrounding the tunnel should be conserved and mobilized to the maximum extent possible. Controlled deformation of the ground is required to develop its full strength safely. However, excessive deformation, which will result in loss of strength or in unacceptably high surface settlements, should be avoided.
These conditions may be achieved in a variety of ways, but generally a primary support system consisting of systematic rockbolting or anchoring and a thin semi-flexible shotcrete lining are used. Whatever support system is used, it is essential that it is placed and remains in intimate contact with the ground and deforms with it. The dimensioning of the secondary support is based on an assessment of the results of systematic measurements of stresses in the primary support elements and deformations of the tunnel surface and the ground surrounding the tunnel.
Where possible, the tunnel should be driven full face in minimum time with minimum disturbance of the ground by blasting. While in general, the above is a good guideline to follow in the construction of tunnels, special consideration should be given to the following difficult rock conditions: Brown November 8.
The function of this type of shed is to deflect falling rock or debris from above the track, which might otherwise come in contact with the track or operating equipment.
The sheds are generally constructed Figure Avalanche Shed on Canadian Pacific Railway - from large timbers or cast in place Courtesy of Canadian Pacific Railway concrete and incorporate a sloped roof over the track with sufficient clearance to allow trains and equipment to pass through the shed unimpeded.
The roof of the shed is sloped, falling from a higher elevation on the uphill side to a lower elevation downhill, providing a barrier to deflect falling rock or debris over and away from the track, allowing it to accumulate or continue down the slope of the mountain. Sheds of this type are often constructed at the portal or entrance to mountain tunnels. Snow sheds follow a similar principle to deflect debris away from the track, in this case, specifically to deflect or prevent the accumulation of drifting snow that might otherwise make the track impassable.
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. 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. Vehicle loading in railway design can be comprised of several parts, including the static load of the vehicle and the dynamic effect of the moving vehicle.
Prior to this time, the live loads used in bridge design were subject to the judgment of the engineer and tended to vary to the extent that it was difficult to relate the relative strength of one structure to the next.
This was a time when many prominent structures were being proposed and constructed on a contractual basis by many railways. It was difficult to objectively compare the proposals of different competing engineers for specific projects.
It would seem that the adoption of his loadings in the first edition of the AREA Manual for Railway Engineering in seemed to settle the debate. Though widely applied prior to and subsequent of this event, it would be nearly 20 years before all major North American railways incorporated E-series loadings in railway structure design.
Cooper E-series loading consists of two 4-driving axle steam locomotive and tenders followed by a uniform load. Figure Coopers E Loading The E-series loading is scaleable with the number representing the driving axle load in kips. An E loading is eight times heavier than an E load. The continued application of these loadings is in part due to the legacy of the structures, which remain i. In load rating situations, loads are converted to E-series ratings for comparison.
For example, a modern-day coal train on any specific bridge may equate to an E load on a specific bridge, while a passenger train may equate to an E That same bridge may have a service rating of E and an ultimate rating of E This practice has lead to a wide amount of confusion over the serviceability of existing bridges, which may have been constructed nearly years ago when the prevailing standard was E The first key to understanding is that the rating of the coal and passenger train of E and E, respectively, is specific only to the bridge in the example.
Because the actual loading was converted, the same trains will likely rate as something different on another bridge. An intermodal train will rate as something completely different than the other two trains considered. Secondly, the bridge ratings provided represent the limiting structural member.
During the rating process, each structural member is evaluated for strength and fatigue, where required, and assigned an E-series rating. This rating represents the bridge loading which would produce the maximum allowable load on the member under consideration. At the end of the process, each component is compared with the component rating the lowest representing the load rating of the bridge. A summarized example of bridge rating results for an open deck plate girder bridge is shown in Figure The first given is the service rating.
This is the maximum loading to which the bridge can be subject without limiting the life of the structure.
In other words, the theoretical life of the structure handling this load would be infinite. The ultimate rating is normally used for occasional traffic or in special conditions where the structure replacement may occur in the near future. It is unlikely that the bridge was actually designed as an E structure.
A number of things have occurred since Cooper Loadings were first recommended and used for design. AREMA rating guidelines specifically require that structures under consideration be rated with the current design guidance. As design methods and material behavior have been developed and better understood over the past years since the bridge was erected, design practice may have changed in such a fashion that the original structure was over-designed.
Likewise, the structure in the example was erected during a time when the load produced by a locomotive was considerably more significant than the loading produced by the cars that it was moving. Particular components of the live loading, such as the impact factor, are generally more severe for steam locomotives than diesel-electric power utilized today. The AREMA bridge rating guidelines allow for an impact load reduction based upon speed, but no such allowance is made for the design of new structures.
AREMA design guidelines specify miles per hour as a practical limit for the recommended design practices. Fortunately, freight train speeds approaching 80 mph are limited to a few select corridors, thus allowing for some additional strength allowance for bridges located on other routes. Yet, the designer must verify the specific loading to be applied from the railway.
Chapter 15, Steel Structures, only, incorporates an alternate loading, which is more representative of the heavy axle loading of modern intermodal equipment and unit trains both in magnitude of loads and frequency of occurrence , particularly on short spans. These cars produce nearly the equivalent of E on shorter spans. The alternate 4-axle load, introduced in , addresses the fatigue problems associated with short span steel members.
Heavy unit trains, similar to the ones mentioned above, increase the rate of fatigue life consumption in short members by inducing very high stress levels and increasing the number of stress cycles experienced by the member. See Figure The alternate live load induces higher moments and shears than an E load on shorter spans.
The resulting higher design stresses lead to bigger sections, which are expected to offer more fatigue life under regular operating conditions. This will control the design of short spans, stringers and floorbeams. For spans greater than 54 ft approximately , Cooper's E loading governs. This makes the design of composite structures complicated, as not only are the material properties between structural components different, but suddenly the loads are different as well.
The loading of structures with multiple tracks also varies slightly between chapters. In general, reduction in live loading is allowed for members receiving live load simultaneously from three or more tracks to model the reduced probability of occurrence.
For open deck structures, the live load is assumed to be distributed equally to beams equally-spaced under the rails and no longitudinal distribution of the live load is assumed.
For ballasted deck structures, the lateral and longitudinal live load distribution are a function of the distance from bottom of tie to top of supporting structure and the length of the tie, with longitudinal distribution not exceeding the axle spacing Impact Impact is an occurrence of dynamic increment and impulsive loads.
Although each of the latter components of impact can be quantified on a one time individual basis, the designer does not have control over their imposition. AREMA has developed empirical relationships based on experimental observations to evaluate design impact values percentage of live load for various bridge types. The impact produced is represented as a vertical load applied at the top of the rail at the same location as the Cooper axle loadings, expressed as a percentage of the live load.
The impact on a ballasted deck structure can sometimes be reduced compared to that for an open deck structure because of the absorbing effect of the ballasted track. Identify watershed areas based upon contour interpretation. Identify existing bridges, culverts and problem areas. Identify sheet and concentrated flow. Identify closed drainage systems. Select outlet points for each watershed area. Select the proper hydrology criteria i. Calculate or run the model and assign flow rates to each of the watersheds.
Add flow rates and hydrographs, as necessary, to determine proper flow through the watershed. Select the proper hydraulic method to determine storm water elevations.
Conduct a plan-in-hand field review. This can take the form of new ditches and culverts or it can take the form of improving existing problem areas. Keep in mind that any improvement to an existing drainage system will more than likely affect surrounding drainage patterns and elevations on adjacent or downstream properties.
For example, increasing the size of an existing cross culvert introduces more storm water flow rate to downstream property owners. The designer should determine whether this situation is going to present a problem. Below is a recommended approach to the design of a proposed drainage system: Complete and review the existing drainage study.
Superimpose the proposed improvements on a copy of the existing drainage study map. Locate new drainage features such as ditches, bridges and culverts. Are there floodplain and wetland impacts? Never relocate an existing outlet point unless it is absolutely necessary. Try to maintain existing watershed limits sometimes these do change. Calculate the new hydrology for the watershed. Calculate the new hydraulics for the watershed. Compare the new data with the existing data at the same points. Initiate Permitting process.
For adjacent properties, it is ideal to obtain the same results between existing and proposed conditions and it may take a few iterations to obtain those results. 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. Anonymous aZrC1EZ. Ndoro Bei.
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