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  • Published on: 7th September 2019
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What are railroad bridges?

Railway structures include a wide range of construction planned to support Track and house railway operations. Common examples of track carrying structures are bridges, trestles, culverts, viaducts, scales, inspection pit and unloading pit.

Railroad bridges are useful for serving to get trains from one place to another and to allow roads to pass underneath unobstructed by the passing of frequent trains. However, they also add to the decor, history and advancement of the railroad.

History of railroad bridges

Before 1840 most bridges in this country were relatively weak such as wood and masonry structures. During this time engineers designed some quite stunning, and perhaps somewhat traumatic, wooden bridges that appeared as if they might not hold a person let alone a heavy train. After the arrival of the railroad, engineers were forced to make much stronger bridges because of the heavy equipment that they were required to accommodate. Wood was mostly used through the 1870’s. After that, wood gave way to iron and steel, which then led to arches and cantilevers. In the 1890’s we started seeing lift drawbridges. Around the same time, concrete, instead of stone, started being used for abutments and pier supports. However, after iron and steel began to be widely working in the latter 19th century new bridge designs were employed.

In the 1930’s use of rivet was changed to bolts and by the 1950’s, the steel construction was mostly welded. “E” number is used to indicate how much of a load each locomotive axle can support. E-30 would indicate light service with a load capacity of 30,000 pounds per axle. E-60 load capacity was common in the 1920’s. An E-70 bridge should be able to support 70,000 pounds per axle, required for modern mainline service at high speed.

Types of bridges

The following is an outline of different types of railroad bridges and how they are used-

• Deck bridges are those which have supporting structure (girders or trusses or arches) located below the track, so we can easily see the trains crossing the structure.

• Through bridges are those which have supporting structure above the track.

• Beams, Girders and Trusses are the different supporting elements used for a deck or through bridge. Beams are usually 12-36” in depth, girders are 36 -170”, and trusses are over 170”.

• A Trestle is a type of bridge having several short spans of track that are supported along the length by “trestles” (pilings, or bents) of timber or steel. If the “trestle” is made up of stone, it is usually called a viaduct.

• Arch bridges are used for covering large distances. It requires strong supports at each end and the progressively smaller strutted supports curve upward to the middle creating an arch.

• A Continuous truss bridge also covers long distances using single long truss supported by multiple piers. If the continuous bridge has only one pier support in the middle, it’s called a two span bridge. If it has two piers, it’s called a three span bridge and so on.

• A Cantilever bridge is basically two halves of a bridge with each half having only one center support, and the two bridge sections connected in the middle and hinged in such a way as to avoid transfer of stresses from one side to the other.

• Suspension Bridges can span several thousand feet using a series of cables running over two towers with the floor of the bridge hanging from the main wire cables by suspender cable.

Components of bridge

Each material has its specific advantages. Timber is economical, but has low strength and durability.

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.

(1) Bridge Deck

The bridge deck is that portion of a railway bridge that supplies a means of carrying the track rails. 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 the rails are anchored directly to timber bridge ties supported directly on the floor system of the superstructure. On ballasted bridge

decks, the rails are anchored directly to timber track ties supported in the ballast section. The ballasted bridge decks require a floor to support the ballast section and such floors are designated by their types, such as timber floors, structural plate floors, buckle plate floors or concrete slab floors, all of which transfer loads directly to the superstructure.

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 , but their use over streets and highways requires additional measures such as canopies, plates or wooden flooring to protect highway traffic from falling objects, water or other materials during the movement of trains.

Open-deck construction establishes a permanent elevation for the rails. Normal

surfacing and lining operations, particularly in curves, eventually result in line swings

leading into the fixed bridge. The grade frequently is raised to the extent that the

bridge eventually becomes low. The bridge dumps are of a different modulus than the

rigid deck. Thus, it becomes difficult to maintain surface off of the bridge as well.

This equates to extensive maintenance costs that shortly will surpass the first cost

savings gained by installing an open deck bridge over a ballast deck bridge. In welded

rail, tight rail conditions can occur at the fixed ends of an open deck bridge, thus

requiring an increased level of surveillance in hot weather.

Requirements for Ties

For ballast deck structures, bridge ties are no different than those found in traditional track construction. However, in track constructed with concrete ties, the track is often times transitioned to timber ties before crossing the structure. Some railway companies and agencies have had difficulty with fouled ballast, track alignment and deck surface damage resulting from the use of concrete ties on bridges. Individual railway fastened with metal straps to the bottoms of bridge ties to bring all ties to the required surface. Procedures for dapping and/or shimming ties for super elevation a

(2) Ballasted Decks

A ballasted deck provides a better riding track. The track modulus is consistent on the dumps of the bridge as well as across the bridge. Thus, one is unlikely to have surface runoff problems on the bridge dumps. Surfacing and lining operations can continue across the bridge unimpeded. However, care must be exercised to maintain a permanent grade line in the vicinity of and over a ballasted deck bridge to be certain that excessive quantities of ballast are not accumulated on the bridge structure through track raises during successive re-ballasting operations. Ballasted decks 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.

(3) 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.

Structural Design

Considerations

8.5.1 Introduction

With the exception of larger bridges, most year service life often begins to reach their practical service life at about 30 years of age. 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

100 years old.

8.5.2 Bridge Loading

In the design of any structure, the designer must consider several different load types,

including, but not limited to, dead load, live load, wind, weather (snow, ice, etc.),

earthquake or any combination there of. Like other governing codes and design

organizations including ACI, AISC and AASHTO, AREMA sets forth guidelines for

both allowable stress for steel (Chapter 15) and timber (Chapter 7) and load factor

design guidelines for concrete (Chapter 8) to be used in the design of structures subject

to railway loading. 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 150 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.g. a

composite structure with steel stringers and a concrete deck), both Chapters 8 and 15

must be referenced throughout the design process. Some other chapters of the

AREMA Manual for Railway Engineering may reference one of the structural chapters

when addressing structural issues.

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.) supported by

the structure. The weight of track material (running rails, guard rails, tie plates, spikes

and rail clips) is taken as 200 pounds per lineal track foot. Ballast is assumed to be 120

lbs per cubic foot. Treated timber is assumed to be 60 lbs per cubic foot.

Waterproofing weight is the actual weight. The designer should allow for additional

ballast depth for future grade or surfacing raises (generally 8. . 12.). 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.

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