Arema manual for railway engineering pdf


 

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/nbafinals.info Arema Manual For Railway Engineering Pdf. AREMA RAILWAY MANUAL. Edition, Complete Document. MANUAL FOR RAILWAY ENGINEERING. View Abstract. Product Details.

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Arema Manual For Railway Engineering Pdf

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.

It publishes recommended practices for the design, construction and maintenance of railway infrastructure, which are requirements in the United States and Canada. As stated in their mission statement, AREMA promotes "The development and advancement of both technical and practical knowledge and recommended practices pertaining to the design, construction and maintenance of railway infrastructure. Brian A. Formed in in St. The group provided a forum to exchange information and create solutions to problems that confront the railway industry. The oldest of the groups was organized in by 61 roadmasters representing 24 railways. The Association provided a means for maintenance officers to meet and discuss their mutual problems. Rail joints , switches , frogs and ties were among the subjects studied, leading to the standardization of maintenance practices. In , the Association of Telegraph and Telephone Superintendents was formed by the telegraph superintendents of the major railroads. In , the Railway Signaling Club was organized at a meeting in Chicago , Illinois and created a code of rules governing the operation of interlockings. AREMA has 29 technical committees, organized in six functional groups. The committees, whose volunteer members come from the railroad industry, meet on a regular basis and use their expertise to come up with the best methods to maintain a railroad. AREMA publishes recommended practices in nine separate documents.

AREMA publishes recommended practices in nine separate documents. The AREMA Manual for Railway Engineering contains principles, data, specifications, plans and economics pertaining to the engineering, design and construction of the fixed plant of railways except signals and communications , and allied services and facilities. Consultants use the manual's recommendations as a basis for design.

Many railroads use the manual as a basis for their track standards and may add to it to describe their specific needs. From Wikipedia, the free encyclopedia. Archived from the original on 8 December Retrieved October ". Railway Track and Structures. Retrieved from " https: A railroad consists of two steel rails which are held a fixed distance apart on a roadbed.

Vehicles, guided and supported by flanged steel wheels and connected into trains, are propelled as a means of transportation. Websters Dictionary defines a railroad as 1. A complete system of such roads, including land, rolling stock, stations, etc. The persons or corporation owning and managing such a system.

The terms railway and railroad are sometimes used interchangeably. However, for this book, railway will generally refer to the track and other closely associated items, i. Railroad will be used where the usage connotes the bigger system. In commencing a railway engineering career, you are joining many fellow workers in a complex and increasingly coordinated activity that is an integral part of any civilized society.

About one-seventh of the workers in advanced economies are involved in some phase of transportation. Transportation, the movement of persons and goods, of which railroading is a large and vital part, is tied in with the location and magnitude of all kinds of human activity which depend on the timely availability of quality goods and services.

This ranges from the necessities of food and fuel and work to leisure pursuits. Many of you will be considered as transportation engineers specializing in railway engineering not operating trains. We can define railway engineering as that branch of civil engineering involved in the planning, design, construction, operation and maintenance of railway land facilities used for the movement of people and goods serving the social and economic needs of contemporary society and its successors.

The complete railway engineer is active in all aspects of civil engineering practice, surveying, geotechnics, hydrology, hydraulics, environmental and sanitary and structural design as well as construction technology. You will frequently encounter the word mode in your railway practice. A mode of transportation is no more than a particular type of transportation defined in enough detail for the purpose at hand.

It can be as general as the medium through or on which transportation takes place; for example, air, sea and land modes. The walking or pedestrian mode involves the moving human. The public transportation mode includes those systems such as rail commuter lines and public bus and taxi service. Often, far more detailed descriptions are needed for effective analysis, communication and understanding.

The railway mode is a type of a land transportation mode as defined above. The light rail transit mode is a further more specifically defined type of rail service, typically today an urban, electrically powered system operating on its own right of way with intersections with intersecting public streets. Other terms used in railway engineering are listed and defined in the Glossary found at the end of this Manual.

As additional lines were built, they facilitated the establishment and growth of towns in the West. Except for the trip from farm to railhead in town, the poor roads and limited canals became irrelevant. The Federal government and states encouraged and provided financial support through land grants and loans, which were paid back with reduced rates for half a century. Since the first railways, there have been many improvements in all aspects of railroading. For example, the development of the iron flanged T rail was achieved by See Figure for an early track section Until mass steel making was developed, there was a continuing controversy between the use of malleable iron vs.

By wooden ties kept in place by ballast stone had replaced simple stone surface support. The information in this chapter is of a general nature and may be considered as typical of the industry.

However, each railway company is unique and as such it must be understood what is included in this chapter may not be correct for a particular company. Passenger railways, on the other hand, are generally owned by governments.

These passenger railway companies normally do not own the trackage infrastructures. Except for certain connecting routes and dedicated high-speed corridors, they merely operate the passenger equipment on existing tracks owned by freight railways. Local rapid transit systems are usually operated as public utilities by the individual municipalities or transit authorities on their own trackage. Commuter services may be operated by government agencies or private sector on either their own or other railway owned trackage.

Tractive Effort lbs. For example, a hp locomotive will have approximately 74, lbs. The maximum tractive force that can be developed at the rail is equal to the weight on drivers multiplied by the adhesion coefficient of friction of the wheels on the rail. The primary factors, among others, affecting adhesion are rail condition and speed. Adhesion decreases as speed increases.

As all the wheels on most diesel locomotives are driving wheels, the weight of the locomotives must be about four times the tractive force developed. The HHP high horsepower units for main line service weigh about tons each on 6 axles.

The maximum tractive force is therefore approximately 97, lb. As the train speed increases, the tractive effort from the locomotives decreases and the drawbar pull available to move the train also decreases.

Due to the limited strength of drawbars and coupler knuckles, the number of locomotives or motorized axles that can be used in the head end of a train is restricted.

Although rated with a minimum strength of , lb. Grade E knuckles used on some captive services may have an ultimate strength of , lb. To avoid the risk of drawbar failure enroute, it is recommended to limit the number of motorized axles in a locomotive consist to 18 three 6-axle units. Other resistances due to wind velocity, tunnels or different train marshalling will not be discussed here.

Rolling Resistance Rolling Resistance is the sum of the forces that must be overcome by the tractive effort of the locomotive to move a railway vehicle on level tangent track in still air at a constant speed. These resistive forces include: Rolling friction between wheels and rail that depends mainly on the quality of track. Bearing resistance, which varies with the weight on each axle and, at low speed, the type, design and lubrication of the bearing.

Train dynamic forces that include the effects of friction and impact between the wheel flanges against the gauge side of the rail and those due to sway, concussion, buff and slack-action. The rail-flange forces vary with speed and quality of the wheel tread and rail, as well as the tracking effect of the trucks. Air resistance that varies directly with the cross-sectional area, length and shape of the vehicle and the square of its speed. In general, rolling resistance of a train, R in lb.

Davis Formula The first empirical formula to compute rolling resistance was developed by W. Davis in The original Davis formula provided satisfactory results for older equipment with journal bearings within the speed range between 5 and 40 mph. Roller bearings, increased dimensions, heavier loadings, higher train speeds and changes to track structure have made it necessary to modify the coefficients proposed by Davis. Interested readers may refer to Section 2. 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.

The ambient temperature and the duration of the stop as shown below affect temperature of the bearing. If necessary, the locomotive engineer can bunch up the train first, then start the train one car at a time.

The cars already moving will help start the ones to the rear. This is called taking slack to start. Grade Resistance Grade Resistance is the force required to overcome gradient and is equal to 20 lb.

This force is derived from the resolution of force vectors and is independent of train speed. An up grade produces a resistive force while a down grade produces an accelerating negative resistive force.

Curve Resistance Curve Resistance is an estimate of the added resistance a locomotive or car must overcome when operating through a horizontal curve. The exact details of the mechanics contributing to curve resistance are not easy to define.

It is generally accepted in the railway industry that curve resistance is approximately the same as a 0. At very slow speeds, say 1 or 2 mph, the curve resistance is closer to 1. However, a basic understanding of elementary track componentry, geometry and maintenance operations is necessary if intelligent decisions are to be made within the options that are typically available.

The purpose of the rail is to: Transfer a train's weight to cross ties. Provide a smooth running surface. Guide wheel flanges. Nominal cross-section dimensions are 7" x 9", although larger ties are specified by some railways. The primary use for switch ties is relegated to turnouts thus their name.

However, they are also used in bridge approaches, crossovers, at hot box detectors and as transition ties. Some railways use switch ties in heavily traveled road crossings and at insulated rail joints. Switch ties ranging in length from 9'-0" to 12'-0" can also be used as "swamp" ties. The extra length provides additional support for the track in swampy or poor-drained areas. Some railways have utilized Azobe switch ties an extremely dense African wood for high-speed turnouts.

The benefits associated with reduced plate cutting and fastener retention may be offset by the high import costs of this timber. Softwood Ties Softwood timber Figure is more rot resistant than hardwoods, but does not offer the resistance of a hardwood tie to tie plate cutting, gauge spreading and spike hole enlargement spike killing.

Softwood ties also are not as effective in transmitting the loads to the ballast section as the hardwood tie. Softwood and hardwood ties must not be mixed on the main track except when changing from one category to another. Softwood ties are typically used in open deck bridges. They can be supplied as crossties i. They are made of pre-stressed concrete containing reinforcing steel wires.

The concrete crosstie weighs about lbs. The concrete tie utilizes a specialized pad between the base of the rail and the plate to cushion and absorb the load, as well as to better fasten the rail to the tie. Failure to use this pad will cause the impact load to be transmitted directly to the ballast section, which may cause rail and track surface defects to develop quickly.

An insulator is installed between the edge of the rail base and the shoulder of the plate to isolate the tie electrically. An insulator clip is also placed between the contact point of the elastic fastener used to secure the rail to the tie and the contact point on the base of the rail. Steel Ties Steel ties Figure are often relegated to specialized plant locations or areas not favorable to the use of either timber or concrete, such as tunnels with limited headway clearance.

They have also been utilized in heavy curvature prone to gage widening. However, they have not gained wide acceptance due to problems associated with shunting of signal current flow to ground. Some lighter models have also experienced problems with fatigue cracking. The joint is considered to be the weakest part of the track structure and should be eliminated wherever possible.

Joint bars are matched to the appropriate rail section. Each rail section has a designated drilling pattern spacing of holes from the end of the rail as well as dimension above the base that must be matched by the joint bars.

Although many sections utilize the same hole spacing and are even close with regard to web height, it is essential that the right bars are used so that fishing angles and radii are matched. Failure to do so will result in an inadequately supported joint and will promote rail defects such as head and web separations and bolt hole breaks.

There are three basic types of rail joints Figure See Figure They are typically 24" in length with 4-bolt holes for the smaller rail sections or 36" in length with 6-bolt holes for the larger rail sections.

Alternate holes are elliptical in punching to accommodate the oval necked track bolt. Temporary joints in CWR require the use of the 36 bars in order to permit drilling of only the two outside holes and to comply with the FRA Track Safety Standards requirement of maintaining a minimum of two bolts in each end of any joint in CWR.

Compromise Joints Compromise bars connect two rails of different weights or sections together. See Figure They are constructed such that the bars align the running surface and gage sides of different rails sections.

There are two kinds of compromise joints: Directional Right or Left hand compromise bars are used where a difference in the width of the head between two sections requires the offsetting of the rail to align the gage side of the rail. To determine a left or right hand compromise joint: Stand between the rails at the taller rail section. Face the lower rail section. The joint on your left is a "left hand". Insulated Joints Insulated joints are used in tracks having track circuits.

They prevent the electrical current from flowing between the ends of two adjoining rails, thereby creating a track circuit section. Insulated joints use an insulated end post between rail ends to prevent the rail ends from shorting out.

There are three types of insulated joints: Continuous Non-continuous Bonded Continuous insulated joints Figure 3- 11 are called continuous because they continuously support the rail base.

No metal contact exists between the joint bars and the rails. Insulated fiber bushings and washer plates are used to isolate the bolts from the bars. The joint bars are shaped to fit over the base of the rail. This type of insulated joint requires a special tie plate called an "abrasion plates" to properly support the joint. Non-continuous insulated rail joints are called non-continuous because these joints don't continuously support the rail base.

A special insulating tie plate is required on the center tie of a supported, non-continuous insulated joint. Metal washer plates are placed on the outside of the joint bar to prevent the bolts from damaging the bar. There are two common kinds of non-continuous insulated joints: Glass fiber. Polyurethane encapsulated bar. The roadway is considered to be any construction within the right-of-way except the track, bridge structures, signals and crossings.

The right-of-way is often thought of as the strip of land on which the railway and its supporting features are built. The right-of-way typically includes ditches running along the track and related drainage structures required to divert water past and away from the railway. The issue of drainage is covered in Chapter 5. It also includes any embankments and cuts on which, or through which, the railway is built, their side slopes and the vegetation covering the slopes.

It may also include any retaining walls or other earth-supporting structures required to hold railway embankment and cut side slopes in place. It includes fences, signs, utilities and outlying structures.

The bulk of this chapter deals with what the railways are built upon, the soil. Just as concrete and steel are the materials used by the structural engineer, soil is the main building material for the railway.

In the same way as there are various types of steel, or diverse mixtures of concrete, there are many classifications of soil. Some soils are suitable for use as ballast and sub-ballast sand and gravel , some as subgrade materials sand, gravel, clay, etc.

A major difference between soils and most other construction materials is that soil is a natural material and is subjected to little or no processing before use. It is therefore essential to identify the various soils and avoid using those that may give problems, since it is seldom that soil can be processed to improve its properties.

For instance, it is not unusual for track that functioned very well for more than 50 years to suddenly develop severe geotechnical problems. In solving problems today, the experiences and effects of the last to years of railway practice must be considered. Not only are the railways dealing with ever- increasing loads and ever-increasing traffic, but also a maintenance effort focused on rails and ties.

Ballast, being less visible, receives less attention, and the subgrade, less still except when problems develop. Nonetheless, knowing the history of a section of track is an important component of effective track maintenance.

American Railway Engineering and Maintenance-of-Way Association - Wikipedia

Components and Functions Figure The Track Structure The track structure is made up of subgrade, sub-ballast, ballast, ties and rail as illustrated in Figure Each of these contributes to the primary function of the track structure, which is to conduct the applied loads from train traffic across the subgrade safely.

The magnitudes of typical stresses under a 50, lb axle load are shown in Figure These stresses are applied repeatedly, and each repetition causes a small amount of deformation in the subgrade. In theory, the track structure should be designed and constructed to limit rail deflections to values which do not produce excessive rail wear or rates of rail failure. In reality, cumulative deformation of the subgrade causes distortion of the subgrade, leading to formation of ballast pockets" Figure or outright shear failure.

Every subgrade will undergo some deflection strain as loads stress are applied. The total displacement experienced by the subgrade will be transmitted to other components in the track structure.

The stiffer the subgrade i. It is important that adequate subgrade strength and stiffness be available on a year-round basis, particularly during spring thaw and following heavy precipitation events.

The strength, stiffness and total deflection of the subgrade can be improved by: Limiting access to water to avoid buildup of porewater pressure and subsequent reduction of strength. Improving the soil properties, using techniques such as compaction, in situ densification, grouting and preloading.

Maintain good drainage. Maintain stable subgrade geometry. Sub-ballast The purpose of sub-ballast is to form a transition zone between the ballast and subgrade to avoid migration of soil into the ballast, and to reduce the stresses applied to the subgrade. In theory, the gradation of the sub-ballast should form a filter zone that prevents migration of fine particles from the subgrade into the ballast.

In practice, insufficient attention has been placed to sub-ballast gradation historically, and much of the sub-ballast does not adequately perform that function. This notwithstanding, the number of occurrences of subgrade contamination of ballast are relatively few.

How Track Fails In a nutshell, track fails when differential rail deflections become excessive. This differential deflection may be expressed in differential elevation between tracks, punching of ties, elastic or plastic deformation of the subgrade, or degradation of ballast.

When the bearing capacity of the subgrade is exceeded, the subgrade will deform plastically, resulting in a small amount of permanent deformation under each wheel load. A progressive deterioration of the track begins, as illustrated in Figures to 4- It starts with minor deflections and may progress to a fully visible surface heave, where subgrade material is pushed above the elevation of the rail and ties.

Under those conditions, ballast drainage is impeded, resulting in further softening and degradation of the subgrade to a point where large, saturated pockets of ballast are trapped in the subgrade. Frost heave and further degradation commonly follow, leading eventually to a severe loss of utility of the track structure.

Bearing capacity failures discussed in the previous section are one type of shear failure that occurs when the soil cannot sustain vertical load applied to it and vertically downward movement results. The term landslide is used to define all types of mass movement of soil or rock, where the mass moves down slope under the influence of gravity only. There are many types of landslides, but the distinguishing feature is that a mass of material is moved and gravity is the driving force.

Main Features of Landslides The diagnostic features of most landslides include a scarp that forms at the head of the landslide. This is usually a near vertical wall of soil, usually freshly exposed by movement. The slump blocks are unique, identifiable blocks of soil, usually bounded by scarps that show both vertical and horizontal movement.

The main body of the slide is the mass of soil that is pushed ahead by the slump blocks, and may be marked by numerous tension cracks. Bulging of the soil, and thrusting of the slide debris over the natural surface usually mark the toe of the slide.

The slip plane or shear zone is usually a distinct and identifiable plane that marks the lower limit of movement and the upper limit of undisturbed soil.

It should be noted that the shear zone is not usually planar, but rather may be circular, or a composite curvilinear surface that passes through the weakest zones in the subsurface. Slides that Affect the Track Instability that affects the track can be classified according to the impact that it has on the track.

These are described in various illustrations. Figure illustrates a slide that encompasses a track and will disrupt the track by cutting the alignment. Once the track moves out of line, it is no longer serviceable. Figure Slides Covering Track Figure shows the track being heaved up in response to upward movement of the toe of a landslide.

Figure Slides Heaving Track Figure illustrates an event where a landslide threatens the track, perhaps by encroaching on the down slope shoulder. Figure Slides Threatening Track Figure illustrates how base failure in fills on soft foundations can cause the fill to spread and settle. While this may be mistaken as settlement, it is actually a shear movement involving the foundation soils.

It is common on organic terrain and other soft foundations. Many of the ancient landslides are extremely large, and the limits of the landslides may be difficult to detect. The shear strength of the soils. Porewater pressure within the soils that make up the slope this can be roughly measured by knowing the water table. The geometry of the slope, particularly the slope angle and changes of slope.

Any surcharge loading such as fill or bank widening material stored on the slope or train loads. Landslides occur either as a result of reduction in soil strength or an increase in the loading on the slope.

Reductions in soil strength can occur as the result of: An increase in porewater pressure, reducing the available shear strength of the soil. This is a subject that is constantly being reviewed on a regular basis within the regulatory bodies of government and it is therefore always important to review local requirements to guide the engineer through the design process.

Even though one method of analysis may be appropriate to use in an area one feels comfortable in, it may not be appropriate in another location. A good rule of thumb is to contact the local highway department as a starting point and continue your investigation to local authorities. The engineer needs to be aware that one has to maintain existing drainage patterns and not increase headwaters upstream or downstream. Adjacent property owners, whether they are farmers or city dwellers, have certain rights and are protected under common law concerning storm water conveyance and elevation as it relates to property damage.

These conveyance features are typically designed to a particular storm event or storm frequency. In other words, a storm water conveyance feature is going to be associated with a certain amount of risk with respect to failure.

Existing culverts always seem to be a problem and should be looked at carefully. Examples of potential problems include excessive ditch scouring and constant ponding of water along a ditch system. Railway ditches are typically very flat and do not drain well. However, the designer should always review the situation as if there is a solution. If it is economically feasible to remedy the situation, then the area should be regraded and repaired to what is recognized as common engineering practice.

Below is a recommended approach to an existing consistent drainage study: Plot existing and proposed railway right-of-way. Identify floodplain and floodway boundaries. 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.

Dense, high-strength plain or reinforced concrete and heavy shells must be used. Reinforcement is generally used for cast-in-place piles subject to lateral forces.

Where this is done, the reinforcement should be fabricated and accurately placed as a unit, in order that the pile actually conforms to the design. The earliest designs of bridge piers and abutments included outer walls of masonry, usually limestone or granite, with the inner core filled with old rubble. The design and location of the abutments and piers are dependent on the general design of the structure as a whole.

Local conditions such as the natural features at the point of crossing, the type of traffic train consist to which the structure will be subjected, and legal requirements and property rights will govern the design. The rights of adjacent property owners, the requirements of public travel, water-borne traffic and the jurisdiction of public regulatory bodies must receive due consideration in advance of the completion of the design and certainly before construction begins. If the bridge crosses a navigable stream within the United States or a wetland is impacted, the U.

Army Corps of Engineers, the United States Coast Guard in some cases and numerous state and local regulatory agencies have jurisdiction and the proper permits must be secured. Abutments The three primary types of abutments are the "wing," the "U" and the "T. All types possess one characteristic feature, the body or face portion, commonly called the breast, which supports the bridge seat.

The "wing" abutment is the type most widely used where the embankment is not a high fill. It consists of a simple breast wall, flanked by wings. The wings may be turned backwards at an angle of approximately 30 degrees or more with the face of the breast, when required by local conditions. The upper surface of the wings is sloped to conform to the natural slope of the surcharge that it is retaining.

The counterfort and buttress types of abutments are modifications of the "wing" abutment. This type is sometimes modified into the so-called "pulpit" abutment, where the wing length is long enough only to keep the bridge seat clean of the surcharge material behind the abutment.

The "T" abutment is similar to the breast type with the addition of a stem, which extends backwards from the center of the rear face to the top of the embankment slope, and is used to stabilize the breast and to bridge the slope of the embankment. The "breast" type of abutment is a modification of the "wing" abutment in which the wings are eliminated and square ends are provided.

It is commonly used at locations where the embankment is relatively low and water flow is negligible. The "buried" abutment has an opening through the wall, where the surcharge spills around the ends and through the wall opening. This construction is desirable when the approach fill is very high because the continuous fill through the wall results in a material reduction of pressure behind an otherwise solid wall.

The "arch" abutment may be considered a modification of the "U" abutment, where the parallel sidewalls consist of one or more arches. This type is adapted to locations where embankments are so high that "wing" and "U" abutments would be uneconomical. The number and size of the arches are dependent upon the height of the bridge and the type of superstructure. The "hollow" or "box" abutment was a type frequently adopted in grade separation work, at points where city streets are carried beneath railway tracks.

Such a unit consists of a concrete box provided with a solid rear wall, floor and top. The front is usually open and is composed of two or more columns, or an arch. This type of abutment bridges the sidewalk and supports the ends of the railway span. Design of Abutments Abutments must be stable against overturning in front of the footing or in the face of the wall, and must be safe against crushing, sliding on the foundation or on any horizontal section through the structure.

Abutments may be of the gravity wall design, where the abutment is so proportioned such that no reinforcement steel other than temperature steel is required; or they may be of the semi-gravity style, where the unit is so proportioned that some steel reinforcement is required along the back and along the lower side of the toe. The resultant force on the base of a wall or abutment should be considered to fall within the middle third of the structure if it is founded on soil and within the middle half of the structure if founded on rock, masonry or piling.

The vertical loads to be carried are the live loads except for impact , dead loads from the weight of the span and weight of the abutment and part of the earth on the footing, depending on the design of the abutment. The lateral forces parallel to the axis of the bridge are the train-produced longitudinal forces and the surcharge pressure from the earth due to both its weight and live load.

Piers Piers constitute the intermediate supports for multiple-span bridges. They should rest on stable, unyielding foundations with their bases well below frost line, and also below the elevation of any possible scouring action. Most of the older piers are of the mass type, either solid or cellular, and are built of stone masonry, concrete or reinforced concrete.

They require for their construction, the use of cofferdams or caissons conforming to the relative size of each pier and, in depth, to the elevation of suitable bearing strata. Cofferdams generally are rectangular in shape and are built to expose the earth strata below the ground surface or the excavation within the enclosed area.

They are watertight to the extent required and need strength to resist pressures from the outside. The cofferdam should be designed such that the combined cost of construction, maintenance and pumping is held to a minimum.

Those of relatively small size and depth are sheeted with single or double-row sheeting, while steel sheet piling are commonly used for larger and deeper cofferdams.

Today, use of the mass-type piers in new construction has given way to more suitable and less costly types of pier. These include: As a structural element, it is the portion of the bridge spanning the opening. The superstructure consists of arches, slabs, beams, girders, trusses or troughs, and such floor systems and bracing as may be required.

Superstructures may be divided into two general classes: 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 Figure Open Deck Structure - Courtesy of Canadian decks, the rails are anchored directly to National 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 Figure Open Deck Bridge - Courtesy of Metra 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. Individual railway companies have established policies relating to the use of concrete ties on or around bridge structures that should be reviewed prior to design.

The tie spacing is typically 4 inches between ties for open deck bridges and the usual track tie spacing for ballast deck bridges. It must be recognized that the tie functions as a beam and it must withstand bending and shear stresses, hold the rail to gage and transfer the rail load to the supporting members of the floor system. Open deck bridge ties typically utilize a softwood species of timber.

Superelevation on Decks The superelevation of curved track on a bridge is obtained by: Framing the floor system involves significant detailing and fabrication and is not often performed. The other methods are commonly employed. High speeds in all classes of train service greatly intensify the problems connected with superelevation and alignment on curves. The eccentricity between the curve alignment and that of the bridge structure produces differences in stress in similar members of a floor system, dependent upon their location.

Careful analysis must be done to insure that none of these members are overstressed. Bridge Tie Framing Bridge ties sometimes are dapped where they contact the supporting steel as an aid in maintaining good track alignment over the bridge. This necessitates adzing the tie bottom at each flange edge, which may result in undesirable horizontal shear cracks extending inward from the bottom of the dap. Where dapping is practiced, the depth should be held to the very minimum required and careful check should be made to determine that the remaining depth of the tie is ample to carry the loads.

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 Figure Ballast Decked Bridge.

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. The ballast pan must have sufficient capacity to carry the heavy dead load of the floor and the ballast, and also to properly distribute the live and dead loads from various types of bridge floors to the supporting superstructure. The arrangement of the members in a floor system supporting a bridge floor is different from the arrangement of longitudinal stringers and transverse floorbeams, which make up the floor system of many open- deck spans.

A bridge floor for a ballasted deck may conform to one of several types including: The result is a floor sloped for drainage in both directions, and the bases of which are supported by wide flange beam sets or structural plates, which bear on transverse I- beams supported on deck girders.

Trough Floors Figure Ballast Pan on Stringers - Courtesy of Metra The steel-trough bridge floor has been used in the past primarily for ballasted deck structures over city streets, particularly in connection with track elevation work.

Longitudinal troughs are used at locations where crossings intersect at approximately right angles and where columns are permitted at the center of the street. Such troughs are supported on cross girders framed to the columns, while the outside legs of the two outside troughs are extended upward to form the ballast stops.

After erection, the down-troughs are filled with concrete, which also covers the entire area to a depth of about 3 inches above the tops of the troughs at the end of the bridge, and about 4 inches above the troughs at the center of the bridge.

Along the sides, the concrete filling is flared up against the ballast stops for varying distances above the top of the rail. The concrete filling is sloped for drainage in such a manner as to permit delivery of the water to drain pipes located below the bridge seat level. Suitable reinforcement should be provided immediately below the top of the concrete filling, particularly in the area above the cross girders; otherwise, deflection under live loads will cause transverse cracks in the concrete.

The use of trough floors at locations, where the intersecting angle with the street is acute, or where roadways of unusual width are required, necessitates placing the troughs transversely to the track and framing them to through girders or to through trusses. The design details of these floors are essentially the same as for longitudinal troughs, the exception being the necessity for drain holes through the floors to avoid long, flat slopes for drainage, which in turn, requires the installation of a drainage system to dispose of the accumulated water.

Drainage The primary requisites for bridge floors are economy, minimum weight and water tightness, together with strength and shallow depth. Comparisons of economy should include cost of materials, fabrication and erection. Bridge floors not only catch water but also retain it. As the track must be removed prior, replacement and maintenance of the bridge decks can be difficult and expensive. Every precaution should be taken to insure long life, which requires that all bridge floors be protected by waterproofing.

Water falling on the track percolates through the ballast to the waterproofing where it remains, unless some suitable means for quick runoff has been provided. Quick runoff of precipitation is dependent upon clean ballast and a well-designed drainage system delivering water to outlets through the floor or to drain pipes located at the back of the abutments.

Open Deck vs. Ballast Deck In addition to the obvious weight and construction costs, each of the span alternatives has their unique safety, environmental and maintenance concerns. In some instances, these intangible factors can carry more weight than the resulting cost implications. Only the governing railway can provide guidance as to the importance of these and related issues.

They are typically precast or prestressed sections placed on the structure after steel erection. This means they cannot be considered part of a composite structure and offer no structural benefit, as would a similar concrete deck in the highway counterpart. There are also operating disadvantages to the use of open deck bridges that may not be readily apparent.

Bridge maintenance must often be performed under contractual agreements by bridge and building department forces. Thus, any operation involving an open bridge deck, e. As the adjacent track is also affected by anything affecting the elevation of the rails running across the bridge deck, track department forces must also be involved. Most railways have severely reduced their bridge gang rosters.

Thus, it becomes a real logistics problem to have both groups present at the same time. On ballast deck structures, the ballasted trackage is considered track department work.

Thus, surfacing operations and tie change-out can proceed unhindered. Anchorage of Bridge Ties Bridge ties on open-deck spans are held in position by bolts through the ties in line with the edge of supporting members i. Usually two hook bolts are used on every third, fourth or fifth tie.

The rail may or may not incorporate rail anchors. Anchoring rail on longer open deck structures can create alignment problems resulting from the thermal expansion of the rail. Most traditional mechanisms for fixing the bridge ties to the bridge cannot effectively transfer longitudinal forces. The servicing railroad guidelines pertaining to the anchorage of rail over both ballast and open deck structures should be consulted for guidance in this area.

Guard Timbers Bridge ties are held to a uniform spacing by longitudinal timbers, called "guard timbers," placed outside of the track rails and fastened to the ties by bolts or lag screws. Inner Guard Rails In addition to the guard timbers, two lines of inner guard rails Figure are often used on each track on open and ballasted-deck bridges of such length as individual railways require.

The two types commonly used are structural angles with a backing timber found often on branch lines and T-rails. On new installations, T-rails are generally used, even to the extent of replacing the angle guards when their renewals are necessary.

Each rail is placed on the inside of the running rail, often without the use of tie plates. Guard rails should be spiked to every tie and spliced at every joint.

They should extend beyond the bridge ends in the direction of approaching traffic. The ends should terminate in a frog point or be joined and securely fastened so that a derailed truck will be straightened in direction and guided into the space between the Figure Inside Tee Guard Rails - Courtesy of BNSF running rail and the guard rail, thereby minimizing the damage that otherwise might result. While the advent of economical steel construction has more or less eliminated timber from new mainline structures of any size, the lower initial cost and ease of construction still makes timber construction attractive for many light density lines.

Additionally, because of the relative ease of repair, many significant older timber structures remain in service today. In all of North America, timber trestles are the preponderant type of structure still found on branch lines, short lines and at temporary crossings. The timber used for timber trestles should be of a firm, close texture, which will afford strong structural members and offer maximum resistance to decay. The timber selected should be sound, free from knots, pitch pockets and other imperfections that might impair its strength or durability.

There is seldom justification for using untreated timber. Terminology The trestle supports are designated as "bents. When the lower ends of the supporting posts are driven into the ground, the structure is known as a "pile trestle. The outside inclined posts, known as "batter posts," the tops being tilted toward the center of the bent and serving the purpose of giving increased stability, are installed adjacent to the plumb posts.

Sway bracing provides additional lateral stability by the use of planks extending diagonally across the bent, through bolted to the ends of the cap and sill and also to the posts or piles. A similar brace, but placed with the opposite direction in slope, is installed on the opposite side of the bent such that the two braces cross in the middle. See Figure For trestles higher than 30 feet, a second bent is added to the top of the existing bent.

Successive stories are added, not exceeding 20 to 30 feet in height, until the required elevation is reached. The bottom panel may be either pile or a frame bent; the upper stories are framed bents, each attached to the top of the lower panel. Each story has its own sway bracing. Shorter bents may utilize a transverse horizontal brace on each side of the bent in lieu of the diagonal bracing where sufficient height does not exist to install conventional sway bracing.

For high timber trestles, the piles are often cut off at the ground line and the sill of the bottom story is framed on the pile tops. Attachment of the longitudinal girts and other bracing is done by through bolting the members. Caps Caps are typically inches in section width and thickness and extend the width of the bent, commonly feet for single tracks.

Bent caps transfer the loads from the stringers to the pile or frame posts. False caps of varying thickness are used to shim up the height of the deck structure when required. Sills, the bottom transverse frame bent member atop the pile, are caps of the same dimensions, but may be longer in length. Stringers The stringers are structural members extending parallel to the rail and spanning the openings between the bents.

See Figure Depending on individual railway standards, they will range in size from 7 to 10 inches wide by 14 to 18 inches deep and one or two spans in length depending on their location. The maximum span for the Figure Timber Stringers and Cap - Courtesy of Metra timber spans commonly in use today is 13 to 15 feet.

On open deck bridges, the stringers are chorded into a minimum of three and generally four or more beams with each adjacent stringer joint offset by one span length from its adjacent neighbor stringers for three span or longer structures. Each group of stringers is centered under the rail in order that load distribution is symmetrical.

On ballast decked bridges, spaced stringers with planking form the pan for a ballast deck. The spacing of stringers facilitates load distribution from the deck and inspection and stringer change-out. The longitudinal stringers should be spaced not less than 7 to 8 inches apart, as this will permit the insertion of suitable reinforcing timbers, if needed. They consist of metal rings, plates or grids, which when embedded partly in the faces of overlapping members, transmits loads from one structural member to another.

Certain types, such as the split ring and the flanged shear plate, fit into precut grooves or daps. Other types, such as the toothed ring and the spike grid, are embedded in the timbers by means of pressure. The action of the connector in the joint is to increase the shear area, which actually carries the load. In timber joints, it is in the section of the timber nearest the contacting faces that the greatest shear stresses are developed.

By embedding the connectors in this highly stressed shear area, the efficiency of the joint is strengthened significantly. In between, there is every possible combination of span and tower design. However, regardless of the specific span type, most steel structures are designed with simple spans. This facilitates ease of construction and maintenance under traffic. It also allows spans to be cascaded to different locations as needs arise.

Simple spans are easier to analyze and for the most part, use simple, economical details. Girder Spans For short spans, rolled or welded sections are well suited for most applications. Spans up to seventy feet have been constructed using rolled steel beams. However, fifty feet is generally considered a practical maximum for rolled steel sections exclusive of special situations.

Such structures are easy to fabricate and readily accept open and ballast decks. Additionally, they can be made more compact top of rail to lowest member by using multiple beams spaced with diaphragms. For spans over fifty feet, rolled sections generally do not offer sufficient section modulus to control deflection. For these longer spans, a built up section Figure 8- 13 is more desirable as it produces a more efficient use of the material.

Such built up sections are either welded or bolted plate girders and can achieve spans of to feet. Deck plate girders Figure are typically the preferred design for locations where vertical clearance under the bridge is not critical, i.

The top flange of the deck plate girder can be utilized to support the deck, thus no flooring system is required. See Figure Figure Deck Plate Girder - Courtesy of Metra The elimination of the floor framing system and the need for girder bracing with knee braces required of through plate girders, makes the deck plate girder the more efficient and cost effective design. Deck plate girders are well suited for either open or ballast decks. However, the engineer must consider the presence of cover plates on the top flange for long spans and make the appropriate allowances in the deck Figure Schematic of a Deck Plate Girder - Courtesy of Canadian structure.

This may require National specific dapping of the wood ties in open decks or different ballast pans in concrete ballast decks. The governing railway must be consulted for their standard details in this matter.

Deck plate girders also require a greater total envelope beneath the track structure, thus limiting clearances below. As indicated above, through plate girders are less efficient than deck plate girders of equal length.

This is because the top cannot be directly supported and there is the added weight of the floor system Figure Knee braces are incorporated at each floor beam to girder connection to provide top of girder support. The stringer and floor beam flooring system Figure drives the need for a deeper girder because of the greater depth required of the stringers to carry the imposed loads on the entire panel between floor beams rather than the distributed load spread out to each close-centered floor beam.

Combined, these two factors make for a heavier span than a deck plate girder span of equal length. However, given the opportunity to decrease the depth of construction from the top of rail to lowest member, through plate girder spans are frequently employed in tight clearance situations such as over roadways.

The engineer must pay particular attention to side clearances since the track is effectively inside the structure. Special precautions must be taken when renewing bridge ties on through plate girder bridges utilizing an open deck in CWR — particularly in hot weather or in curves during cool weather.

Often the rail must be cut. In the pony girder, the floor beam connections to the longitudinal girders are made about half way up the girder. This minimizes the need for the knee brace system to support the girder, but it also reduces the vertical clearance under the structure as well, although not to the extent of the deck plate girder.

Truss Spans Steel trusses Figure offer a practical solution for spans over - feet. Trusses are usually of open web design, consisting of top and bottom chord members connected by diagonal and vertical members called hangers. These members may be either of bolted or riveted construction.

A bridge truss has two major structural advantages. Both of these factors lead to economy in material and a reduced dead load. The increased depth also leads to reduced deflections, i.

The advantages are achieved at the expense of increased fabrication, inspection and maintenance costs. A truss is simply a framework for carrying a load. Like the top and bottom flanges of a girder span, the top chord members of a truss are in compression and the bottom chords are in tension. Formerly, trusses were pin connected, which freed the structure of imposed moments. Today connections are bolted, relieving the associated problem of pin wear at the expense of proportioning members for the Figure Through Riveted Truss - Courtesy of BNSF moments created by a fixed connection.

However, there are still significant numbers of pin-connected trusses in service. In this design, the diagonal web members are in compression; the vertical web members are in tension. In the Pratt truss, the vertical web members are in compression, and the diagonal members are in tension. The panel connections were pinned connected.

The Whipple truss in turn modified the Pratt truss. It uses a double system of web members, each diagonal extending over two panels. This permitted longer span lengths than achievable with the Pratt truss. The Pennsylvania truss was another refinement of the Pratt truss. It uses sub-divided panels and curved top chords for through trusses and curved bottom chords for deck trusses.

This type of truss is used for long spans, where simple Pratt or Warren trusses cannot obtain economical construction. The connections at the panel points were made by pins, but today are bolted. In the original Warren truss, all of the web members were inclined, being alternately subject to compression and tension. This type was rarely used for pin-connected bridges. The loading and unloading of the panel continual reversal of axial force in the web members created pin wear.

However, this truss, modified by the introduction of vertical members for the support of the Figure Truss Schematic - Courtesy of Canadian National panel load and with riveted or bolted connections at the panel points, is now the truss of choice for short spans. It is also widely used for longer spans by subdividing the panels. In Figure , the dotted lines and in Figure , the light diagonal lines are called counters. With only the dead load of the structure, the adjacent diagonals act only as tension members.

However, when a live load is introduced on the adjacent span, the formerly tensile load becomes compressive and the member may undergo critical buckling. Counters offset the applied reversal in loading. Most through trusses include overhead bracing. Thus, interior vertical clearances must be considered in addition to side clearances. Similar to plate girders, deck trusses Figure are typically more efficient than through trusses for all the same reasons specified earlier in the discussion regarding deck plate girders versus through plate girders.

They may be composed of bents supported by suitable foundations, e. Viaducts Figure H-pile Bent - Courtesy of Metra A viaduct Figure is any series of spans, whether arches or steel girders, that is supported on high steel towers.

Typically, railway viaducts are of steel construction and are distinguished by unusual height and significant length.

The spans are often alternating long and short girders, usually deck plate girders. The short or tower spans are commonly 30 ft to 50 ft in length, while the long, or intermediate spans Figure Railway Viaduct - Courtesy of Canadian Pacific Railway are 40 ft to ft long.

American Railway Engineering and Maintenance-of-Way Association

Keeping the short span over the tower top ensures that the spans will remain as simple spans. Sometimes a bent, instead of a tower, is used adjacent to the abutments. This bent supports the adjoining ends of two long spans, the second one terminating on the first tower. The length of the spans is dependent upon the height and length of the structure, as well as on the loads to be carried. Consideration is given to the proper balance between the costs of the substructure and the superstructure.

Generally, the longest spans are in the highest structures. Although many large and costly stone arches are still in service, reinforced concrete is used exclusively for the erection of modern masonry bridges. Arches Stone masonry arches and boxes came into use early in the life of railways in North America. They were constructed in single and multiple spans, and a large number are still in service on important main lines after more than a century of continuous service.

Structures of this character are built of stone masonry or of concrete. Bridges of this type are built either in single or multiple spans with the bearings for the upright supports either fixed or hinged, although hinged bearings are generally preferred.

The construction material is typically concrete or steel, which may be formed for either curved or a flat soffit. Such structures when built of concrete are slab bridges in which the horizontal member is solid; or ribbed bridges in which the horizontal member consists of ribs or girders supporting a slab floor. When built of steel, they consist of frames supporting a concrete slab floor.

The frames are spaced to facilitate attachment of bracing between them. The outside frames should be encased in concrete integral with the slab floor.

Like arches, rigid-frame structures do not tolerate foundation settlement. Rigid-frame structures permit the use of quite long spans with relatively shallow construction depth.

Arema Manual Railway

For this reason, they were frequently used in connection with grade-separation projects. They lend themselves very readily to pleasing designs and are sometimes found more economical than simple spans under certain conditions. In some instances where construction depth is limited, the track rails are attached directly to the slabs by suitable bearing plates and fastenings direct fixation.

The length of span is limited by the construction depth available and the construction cost as compared to other types of construction. Slab bridges were very common at one time with a number still remaining in service today. There are much more economical ways of spanning small openings available to the designer today.

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