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Skip to main content. Log In Sign Up. T5S 1G3 drichards hsmm. T he following section is not intended to instruct any person through the process of designing a railway bridge. It is intended to merely guide the engineer in the peculiarities relating to the design of railway bridges and structures as they relate to the design enngineering set forth by AREMA and general railway practice.
It does in fact assume a base level of knowledge pertaining to the design of structures and bridge systems.
As most of the bridge design in North America is generally related to roadways, the majority of comparisons drawn relate to roadway bridges to provide a sense of scope. Chapters within this volume are divided by the three primary materials in use for railway structures including timber, steel and concrete masonry being included in the latter. Other Manual chapters relate additional information, including seismic loading and bridge bearings. However, there remains some structural related information in other chapters, including utility protection and metal pipe loading in Chapter 1, and structural design of overhead catenary systems in Chapter The engineer, prior to the design of railway structures, must understand several considerations relating to material within the AREMA Manual for Railway Engineering.
Such information should be gathered prior to design. Secondly, while there are some common design elements and considerations relating to the application of loading of railway structures across the three major design areas timber, concrete and steelspecific application and the magnitude of these loads does vary from chapter to chapter. The purpose is to inform engineers of design considerations for railway structures that are different from their non-railway counterparts.
Due to variations in design standards between the different railways, consult the controlling railway for their governing standard before starting design.
Common examples of track carrying structures are bridges, trestles, viaducts, culverts, scales, inspection pits, unloading pits and similar construction.
Examples of common ancillary structures are drainage structures, retaining walls, tunnels, snow sheds, repair shops, loading docks, passenger stations and platforms, fueling facilities, towers, catenary frames and the like.
Accordingly, this chapter will focus primarily on track carrying structures.
When designing railway structures, the various sources of their loads must be considered, as they would be with any other similar, non- railway structure. In addition to the dead load of the structure itself, there are the usual live loads from the carried traffic. To these are added the dynamic components of the traffic such as impact, centrifugal, lateral and Figure Typical Railway Bridge – Courtesy of Metra longitudinal forces.
Then there are the environmental considerations such as wind, snow and ice, thermal, seismic and stream flow loads. Once the designer has established the first pass at the load environment for the subject structure, the primary difference between a highway structure and a railway structure should become obvious.
In the typical railway structure, the live load dominates all of the other design considerations. For the engineer accustomed to highway bridge design, where the dead load of the structure itself tends to drive the design considerations, this marks a substantial divergence from the norm. Specifically, the unacceptability of high deflections in railway structures, maintenance concerns and fatigue considerations render many aspects of bridge design common to the highway industry unacceptable in the railway environment.
Chief among these are welded connections and continuous spans. In addition to the types of construction, the engineer must also choose from among the available material alternatives. Generally, these are limited to timber, concrete and steel, or a combination of the three. Exotic materials can also be considered, but they are beyond the scope of this book.
Each material has its specific advantages. Timber is economical, but has strength and life limitations. Structural timber of the size and grade traditionally used for railway structures is getting more difficult to obtain at a price competitive with concrete or steel.
Concrete is also economical, but its strength to weight ratio is poor. Steel has a good strength to weight ratio, but is expensive. The material chosen for the spans will generally determine the designation of the bridge. For instance, steel beam spans on timber piles will be considered a steel bridge. The point where one form of construction with a certain type of material becomes advantageous over another is a matter of site conditions, span length, tonnage carried and railway preference.
While initial cost of construction is a major point in the decision process, the engineer must keep in mind such additional factors as construction under traffic and the long-term maintainability of the final design. The substructure transmits to the underlying soil the forces comprising the dead load of the superstructure and substructure, the live load effect of passing traffic, and the forces from wind, water, etc. Investigate Underlying Soil and Geologic Conditions Before proceeding with the design of a railway bridge, a careful investigation of the underlying soils should be made.
Analysis of information obtained by borings may reveal the necessity or advisability of driving test piles. If conditions demand the use of pile foundations, the relative merits of treated timber, concrete and steel piles should be carefully weighed.
The stability of the substructure is obviously essential to that of the structure as a whole. Its condition should be under observation at all times, and special inspections should be performed during and after freshets, ice gorges, cloudbursts and other unusual happenings, which could have the potential of seriously impacting the safety of the structure. Immediately following such an occurrence, bridge piers and abutments should be examined carefully for evidence of scour or other adverse condition.
Piling Today, most railway bridge foundations begin with driven piles or caissons. Piles may support some other footing component such as piers or tower legs or they may continue to become part of the bent as in trestle construction. While new construction typically favors either precast concrete or steel H-piles, timber piles still have a use in the repair of existing structures and for temporary construction.
Concrete piles are usually used for large, heavy structures and are very durable, but are difficult to splice. Steel H-piles are easier to drive and splice, and work well in end bearing when driven into rock to resist settlement.
Concrete-filled pipe piles have greater bearing capacity, but are more difficult to drive and splice. Pipe piles possess large moment of inertia; therefore, they are suitable for resisting lateral forces. Alternatively, caissons are large diameter reinforced concrete shafts, usually steel lined and installed by drilling.
They are capable of supporting very large loads with minimum settlement. Piling may be placed in two general classifications: A sheet pile, which forms a continuous interlocking line of timber, concrete or steel piles, is driven close together to form a wall designed to resist the lateral pressure of water, earth or other materials.
Timber and concrete sheet piles are tongued-and-grooved, while steel sheet piles are usually interlocking. Piles are further distinguished by terminology describing their purpose. For example, batter or spur piles are driven at an angle to resist a combination of vertical and lateral forces. Guide or anchor piles are used to provide lateral support for timbers and walers.
Fender piles are used to protect masonry structures, such as piers. Piles are usually driven by diesel hammers, with or without the use of water jets, or by driving in pre-bored holes, or in some cases by the use of hydrostatic pressure.
Certain types of concrete piles are cast in place. Timber Piles Timber piles, when large enough and properly braced, can safely sustain loads ranging from 15 tons to 20 tons each. Consideration must be given to the imposition of bending moments from imposed lateral forces, which may be necessary for a pile to resist. The relative straightness of the pile also must be considered, since a vertical force on a crooked pile produces eccentric loading with accompanying bending stresses.
Embedded in moist ground or submerged in water, timber piles are relatively immune to decay. Timber piles exposed to the air without treatment or other special protection will decay within a few years. Treated or untreated timber pile, however, is susceptible to certain marine organisms found in warm waters.
The wood selected for piling should be of a nature that it will not tend to disintegrate under the impact of the driving hammer, and which will offer the maximum resistance to decay. White oak, cypress and long-leaf yellow pine are particularly suited to this purpose.
Spruce and hemlock are also adaptable, and tamarack is extensively used with satisfactory results in the western section of North America. The general requirements for First-Class timber piles suitable for railway bridge applications are noted in Chapter 7, Section 1.
Each pile consists of the trunk or bole of an individual tree, and the ordinary range of length is from 20 to 60 ft. In special cases, local conditions may make it necessary to penetrate to exceptional depth to obtain footing on a sound-bearing stratum. The above lengths may be exceeded, either by single piles or by splicing two or more ordinary length piles. Piles up to ft in length have been produced. Often, wooden cleats are used for splices to secure longer lengths when needed.
Timber piling above ground or water level is subject to decay, even when treated. One or two defective piles can be spliced into the bent without re-framing the entire bent. However, a number of criteria should be entertained before doing so, including: Steel Piles Steel piles may be divided in two general classifications: Steel piling, whether on dry land or in submerged locations, may be driven to form bents and encased in concrete to form a pier, thus enhancing the strength and providing protection for the steel.
H-Beam Sections H-beam piles Figure are rolled metal sections, possessing wide flanges, and are de- signed especially for pile loading.
They are well adapted to deep penetration because of their relatively small point area. Their volume displacement is also small. This type of pile is very well adapted to serve as a bearing pile at locations where the soil strata above suitable bearing material such as rock or hard pan is shallow and affords little frictional resistance.
Disadvantages include susceptibility to corrosion and under stray current conditions, electrolytic action. Used in friction bearing alone, H- beam piles will generally require a greater length of penetration than a displacement type of pile of the same load-bearing capacity.
Tubular Steel Sections Tubular steel pile, filled with plain or reinforced concrete, is frequently used for special types of bridge pier construction. Other types of tubular steel pile use a cold-rolled fluted section, which also may be tapered. Concrete Piles Concrete piles are relatively immune to the ordinary forces of deterioration and decay in the atmosphere and to the attacks of marine borers in the water.
AREMA: Manual for Railway Engineering – Civil Engineering Community
They also have a greater bearing capacity for the individual pile in comparison with timber piles. A concrete pile can be designed to suit the actual conditions under which it is to be used. The use of large dimension concrete piling sometimes will permit a reduction in the dimension of the foundation to accommodate restricted space.
Concrete piles are also guidw satisfactorily for trestle bents and sheet piling. Concrete piles are capable of supporting loads up to 40 or raailway tons each. Ordinary diameters range from 10 inches to 24 inches with lengths from 20 ft to 60 ft.