1. Module 15: Railway Structures
April 2006
Module 15: Railway Structures
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2. Objectives Bridge Types Substructure Superstructure
April 2006
Bridge Types
Substructure
Superstructure
Loading – E80 vs. HS-20
Special Design Consideration
AREMA MRE
Inspection & Maintenance
Other Structures
In this module we will provide a brief overview of railway structures. At the conclusion you will not have all the skills to run out and design a 60-ft deck-plate girder span, but you will be provided the information and resources to identify bridge types, loading requirements, and design criteria specific to railroad structures. In addition, you will become more familiar with the AREMA Manual for Railway Engineering (AREMA MRE) and it’s provisions applicable to structures. We will also introduce the need for inspection and maintenance, including safety regulations from the FRA. Our discussion here will focus primarily on bridges but there will also be consideration of other railroad structures such as tunnels, sheds, unloading pits and retaining walls.
We will look at bridges from the bottom up, just as they are built. First we’ll demonstrate common substructure materials and construction and then move to the superstructure.
For those of you familiar with bridge design this serve as a familiar review, for others you will become familiar with the proper bridge terminology, as well as some of the challenges and considerations we bridge engineer’s face when presented with a project.
Much of this module, in fact, many of the graphics come straight from the PGRE, which should serve as an excellent resource upon completion of this course.
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3. SUBSTRUCTURE Timber Cast-in-Place Concrete Precast Concrete Steel
April 2006
Timber
Cast-in-Place Concrete
Precast Concrete
Steel
Stone Masonry
Substructure materials will depend on a number of factors, but will primarily be based on superstructure type, span length, and environment.
Five common substructure materials include, timber, concrete (cast-in-place and precast), steel, and to some degree stone masonry.
The photo above represents a steel deck arch truss where the substructure and superstructure are both made of steel.
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4. Timber Substructure Limited to shorter spans Prone to fire and decay
April 2006
Limited to shorter spans
Prone to fire and decay
Quick and cheap!
Timber structures are more common to railroad structures than highway bridges, and exist throughout North America.
Timber presents unique challenges such as decay / rot, insect infestation, and FIRE!
Timber substructure units are commonly referred to as “BENTS” or “TRESTLES” and are typically used for shorter spans, in the 12 – 20-ft. range.
Such bridge were common construction due to the readily available supply of large stands of timber, it is not uncommon to see such structures actually built from their surrounding forest. Although not many are constructed today, it is still important to understand the limitations and pitfalls of such structures and how they affect the system as a whole.
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5. Timber Substructure April 2006
Here you can see a bridge which combines timber substructure and superstructure as well as a steel deck plate girder span.
This a a common arrangement and use of timber and steel.
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6. Concrete Substructure (CIP)
April 2006
Common material
Readily available
Reasonable cost
Long-term service
Low-maintenance
Probably the most common form of substructure construction in both highway and railway bridges is cast-in-place, steel reinforced concrete.
Generally the same load requirements and considerations exist in highway and railway design, in fact they have very much the same form: footing, shaft (or wall) and pier cap.
Critical to the design of any substructure unit is the foundation and founding materials. It is imperative that a proper investigation of such materials be performed prior to design. Such foundations may take on many forms, such as piles (steel, concrete, or timber), spread footing on rock, or drilled shafts, also known as caissons.
The time limitations of this presentations do not permit a lengthy discussion of such foundations, but the following slide demonstrates a few key examples.
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7. Foundations
April 2006
On the left is the installation of steel H-Piles, you can see the hammer and leads attached to the crane boom.
On the right is an example of what can happen when a foundation fails. Here the pile supports have been compromised by scour action caused by a high water event with high velocities.
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8. Precast Substructure April 2006
Bridge construction which employs repetitive installation lends itself to precast construction. Here you can see precast piles which act as the columns of these bents. The bent caps here are cast-in-place, however in some cases precast elemanets may used for this application as well.
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9. Steel Substructure April 2006
Again you can see the combination of a number of elements, but most notable is the steel pile bent used to support both the truss and the adjacent deck-plate girder spans.
Steel piles are used much like timber piles but provided a much greater load capacity, and are therefore used for longer spans. Steel pile bents offer a longer service life than timber.
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10. Stone Masonry Substructure
April 2006
Constant Maintenance
Antiquated Method
Variable Materials
Here you can see an antiquated construction material and technique which is still common in railroad structures.
Stone blocks were placed on horizontal layers of sand, the open joints were then sealed with cement grout.
This grout requires occasional “repointing” which is what you see here.
Stone masonry was a very labor intensive construction method and still requires labor intensive maintenance.
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11. SUPERSTRUCTURE Timber Steel Cast-in-Place Concrete Precast Concrete
April 2006
Timber
Steel
Cast-in-Place Concrete
Precast Concrete
Stone Masonry
There are many types of superstructures used in railroad bridges, some more common than others. Each span type has it’s own useful purpose but is largely dependent on span length, bridge geometry and environment.
There are many materials which are commonly employed in bridge spans including; timber, steel, concrete and stone masonry. Here we will consider some typical uses and arrangements for each.
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12. Deck Type Open Deck Ballast Deck April 2006
Regardless of the span type it will have one of two types of decks, open or ballasted.
OPEN DECK: An open deck consists of placing bridge ties directly on top of the main load carrying member such as girders or stringers. The ties are typically larger than normal ties and may be up to 14-inches thick depending on the girder spacing. The ties are connected with a hook bolt, typically on every fourth tie. Use of a open deck is particularly difficult on a bridge within a curve or spiral. Such geometry creates a situation where each tie must be dapped to accommodate any superelevation.
Open deck bridges 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 approaches 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.
Open Deck
Ballast Deck
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13. Deck Type Ballast Deck Open Deck April 2006
Here a photos which demonstrate both open and ballasted deck spans.
Open Deck
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14. Timber Superstructure
April 2006
You have seen this photo recently…here it is intended to show the timber superstructure. Such spans are limitied in length, usually 12 – 20-ft.
As many railroads are faced with ever-increasing carloads timber spans create limited capacity on their bridges. There has an initiative to replace these spans or increase their capacity with use of modern materials.
Research is currently underway to explore the use of innovative (and inexpensive) methods to increase bridge loading. Such methods include fiber wrap and parallel strand lumber.
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15. Concrete Superstructure
April 2006
Here you can see the installation of precast box girders, which are being placed on elastomeric bearing pads.
You can probably tell by the age of the truck in the background (AND THE LACK OF FALL PROTECTION) that this method of construction bridges has been around for some time.
The top of the concrete box acts as the ballast trough. Waterproofing and then ballast are placed directly onto the box and hen the track is installed.
It is also common to see AASHTO type girders employed at a very close spacing for spans in the 50 to 60-ft. range.
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16. Steel: Deck-Plate Girder
April 2006
Steel deck-plate girder spans are probably the most common type of span used on railroad bridges. Even larger structures, such as truss spans, typically have deck-plate girder approach spans.
Some of the basic components of such spans are demonstrated above. Some components, such as bearings, are common to all spans. The basic different is the location of the span in relation to the track.
As a rule of thumb, the span length to girder depth ratio typically ranges from 12 to 15. For example, an 80-ft. span might have a 6-ft. deep girder.
The most common girder spacing is 6’-6”.
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17. Deck-Plate Girder April 2006
This photo demonstrates a deck-plate girder span, similar to the previous slide.
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18. Steel: Through-Plate Girder
April 2006
The difference of the Through-Plate girder is the location of the track relative to the girder itself. The track passes between or “THROUGH” the two girders.
LOAD PATH
The load path is similar to residential construction (stringers and joists): Rail to Tie to Stringer to Floorbeam to Girder to Bearing.
The knee brace is added to support the compression (top) flange of the girder.
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19. Through-Plate Girder April 2006
The photos above demonstrate through-plate girder spans with and without the deck.
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20. Steel: Truss Spans Longer Spans Various Types Limited Geometry
April 2006
Longer Spans
Various Types
Through
Deck
Pony
Limited Geometry
Expensive
Of all superstructure types, the truss is by far the most complicated and difficult to design and construct, however, the truss offers the greatest span length.
In this diagram we have labeled the common elements of a truss. Each element has a purpose and is critical to either the load carrying capacity or stability of the span.
There are many configurations to truss spans, they can b either THROUGH, DECK, or less common PONY.
You may hear of many types of trusses, such as the PRATT, PENNSYLVANNIA, HOWE, WHIPPLE, etc, but probably the most common is the WARREN truss, evidenced by it’s W.
Trusses may limit both horizontal and vertical clearances, they are also very expensive to fabricate and erect.
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21. Steel: Truss Pin-Connected Rivet vs. Bolt Tuning Age & Condition
April 2006
Frankly there are VERY FEW railroad trusses built today, in fact many railroads are making a concerted effort to replace every pin-connected truss on their system.
This antiquated constructed technique made use of larger (6”+ dia.) pins to connect multiple pieces at each panel point of the truss. These pieces are commonly referred to as EYEBARS.
As the pins wear with age (as they all do) some of the eyebars become loose. This is a concern because it means that some of the bars will receive less load and others receive more load.
There are individuals who have learned to maintain the structures by literally “tuning” the bridge. The eyebars can be shortened by heating and cooling them, this process is repeated until all the bars indicate a harmony or better yet “frequency.” This process takes several iterations and is quite labor intensive, requiring highly specialized skills.
Pin-Connected
Rivet vs. Bolt
Tuning
Age & Condition
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22. Steel: Movable Spans Bascule Span Lift Span Swing Span April 2006
Movable spans are typically used for long spans over navigable waterways.
There are three basic types of movable spans, as shown above; Swing, Lift, and Bascule.
Again, the time limitations of this presentation do not permit a lengthy discussion about movable spans, but you should be familiar with such configurations and understand that these bridges present many challenges, especially with respect to electrical and mechanical systems as well as the rail interface on the structure.
Most of them require extensive routine maintenance as well as a LABOR force to move the span on demand.
Some railroads are moving towards remote operations performed by a dispatcher. In the post 9 / 11 world some key bridges may even include SCADA (system control and data acquisition) which are tied into security systems, including cameras and toxic fume detectors.
Swing Span
Lift Span
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23. One E-80 (Locomotive) Axle = Highway Truck (HS-20)!
Loading
April 2006
Cooper E-80 Loading
The one component singularly unique to railway structures is the vehicle loading. The standard loading scheme incorporated by AREMA is the Cooper E-Series loading. An E-80 loading is eight-times heavier than an E-10 load.The specifics of the E-series loading are not entirely unlike many of the loadings produced by modern equipment despite the fact that the vehicles are very different.
Heavy double stack cars with axle loads of 78,750 lb per axle and 125-ton capacity 4-axle cars (315,000 lb gross weight) are operated on an ever-increasing basis. These cars produce nearly the equivalent of E-80 on shorter spans. The alternate 4-axle load, introduced in 1995, 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.
The Alternate Live Load has 4 axle loads of 100-kips, spacing similar to those found in typical 4-axle coupled cars. For shorter span lengths, the alternate load produces approximately a 25% higher bending moment than the Cooper's E-80 Loading. Short spans, stringers and floorbeams (< Approx. 54 ft) will be governed by Alt. Live Load.
NOTE: a single drive axle of the Cooper E-80 is greater that the entire HS-20 load!
Alternate Live Load
100,000
One E-80 (Locomotive) Axle = Highway Truck (HS-20)!
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24. Design Issues Fatigue Impact Fracture Critical Members (FCM)
April 2006
Fatigue
Impact
Fracture Critical Members (FCM)
Deflections
FATIGUE failure occurs when a material is subjected to repeated stress even if the stress level never exceeds the ultimate strength. As an example pick up the PAPER CLIP we’ve provided. We’ll quickly demonstrate how fatigue creates stress and failure.
IMPACT is an increase in loading due to a sudden load application. Impact can be created by a number of factors, but is typically caused by a flat wheel, a drastic change in track modulus (e.g. bridge approach), and rocking affect of a loaded train.
FRACTURE CRITICAL MEMBERS are also more common to railroad structures. These are members that are subject to tension loads and whose failure would mean catastrophic failure of the entire span. These elements require special design, detailing, and welding practices, as well as additional inspection. Some common DCM include:
Web and bottom flange of girders in a two-girder, simple span.
Certain Tension members of trusses, such as bottom chords and some diagonals.
Hanger components of pin and hanger assemblies.
Unlike highway structures where stress typical control girder design, it is not uncommon for DEFLECTIONS to control railroad girder design. Meaning that high strength steels is unwarranted because it is the moment of inertia (I) not the section modulus (S) that will drive the design.
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25. Work without Interruptions
April 2006
Bridges are critical links which require a lot of time and attention, in the form of maintenance and money.
As there are ever-increasing demands on railroad infrastructure it becomes even more imperative that work on structures be performed without interruption to train operations.
It has come to the point where available tracktime (above all other considerations) will dictate design. Available tracktime windows may vary from a matter if a few hours to a few days, during an holiday outage.
Railroads and / or Contractors are willing to spend additional money in the form of labor or materials to prevent or minimize interruptions to traffic.
Construction
Inspection
Maintenance
Design Parameters
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26. FRA Bridge Worker Safety
April 2006
SAFETY FIRST!
Recovery Plan & Equipment
Potential Fines / Penalties
Federal law (FRA) 46CFR 214 Part B Workplace Safety requires that any bridge worker working at a height of 12’ or greater above the surface below must be either equipped with a fall arrest system capable of limiting free fall distance to a maximum fall of 6’ or utilize a fall prevention system that does not permit him/her to reach an edge where a fall could occur, or must remain inside hand rails protecting the edge of the structure. Workers who remain exclusively between the running rails of the bridge are exempted from wearing fall arrest or prevention equipment regardless of the presence of hand rails so long as the worker remains at least 6’ from a hole large enough to fall through. Any worker utilizing fall protection equipment must first be trained on its proper use. All workers employing fall protection equipment must receive a daily job briefing. A full rescue plan must be discussed in the briefing and a means of performing a rescue must be present at the job site. Further information can be found at the FRA web site FRA.DOT.GOV.
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27. AREMA MRE Navigation Chap. 7 – Timber Structures
April 2006
The AREMA Manual for Railway Engineering (MRE) is a four volume set of recommend practice for railroads and others concerned with engineering, design and construction of railroad facilities.
Each of the chapters is authored by a committee comprised of technical experts from within the industry and includes representatives if railroads, consultants, and contractors, and suppliers.
Those chapters related to structures, specifically, 7, 8, and 15 are located in Volume 2. Because these chapters are authored by committee there are differences that exist. For example the equation used to calculate the effects of impact are considerably different from Chap. 8 to Chap. 15.
Chap. 7 – Timber Structures
Chap. 8 – Concrete Structures
Chap. 15 – Steel Structures
They’re ALL Different!
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28. Additional Structures
April 2006
Culverts
Retaining Walls
Tunnels
Unloading Pits
Buildings / Sheds
Platforms
We have dedicated a lot of time here for discussions about bridges but there are other types of structures on the railroad, some of which are listed above.
The design of these structures is highly specialized and beyond the scope of this presentation., but we would like to offer a few photographic examples.
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29. Retaining Structures Drilled Shaft April 2006
Retaining walls can take may shapes and make use of many types of materials.
Here we demonstrate a several.
Drilled Shaft
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30. Retaining Structures
April 2006
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31. Questions? Authors: Don McCammon, P.E. Dallas R. Richards, P.E.
April 2006
Authors:
Don McCammon, P.E.
HDR Engineering, Inc.
(406)
Dallas R. Richards, P.E.
Arcadis, Inc.  (703)
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32. Revision History