BRIDGE NOMENCLATURE

					                                        CHAPTER 5

                           BRIDGE NOMENCLATURE

General

To enable clear communication and understanding between bridge inspectors and those
performing maintenance and repairs, it is important that uniform terminology and methods for
identifying and locating the various components of a structure be used. It is particularly
important when discussing repairs, materials, defects, or emergencies by telephone to be able to
give a clear, concise, and correct description that will avoid confusion or misunderstandings.

Bridges are the broadest and most widely varied category of structures encountered on railroad
systems. In addition to railroad bridges, there are highway bridges, grade separations, signal
bridges, portions of track scales, turntables, and unloading structures that would fall within the
broad category of bridges. While differing in size and details of design, bridges of the same
basic type and construction materials will share similar component identification terminology,
regardless of the intended use of the bridge. This section will present the types of bridges
commonly found on railroad systems, along with diagrams to provide for clarity and future
reference.

Fundamentally, a bridge is a structure that provides a means to cross or span an obstruction. The
nature of the obstruction can be a waterway, a land feature such as a valley or ravine, another
railroad or a highway, or any other obstacle. The essential parts of a bridge are:

      Substructure. The abutments, piers, or other structures built to support the spans of a
       bridge.

      Superstructure. The entire portion of a bridge structure which primarily receives and
       supports rail, highway, or other traffic loads or facilities, and in turn transfers these loads
       to the bridge substructure. The superstructure may consist of beam, girder, truss, trestle,
       and other types of construction. (See Figure 5-1)

      Deck. The track, roadway, and attachments and incidental parts designed to directly
       support and transmit traffic and facility loads to the superstructure.

Substructure Components and Terminology

Foundation

The foundation is the portion of the structure which transmits the entire load of both the
superstructure and substructure to the underlying soils. Depending on soil conditions at any
particular location, the foundation may consist of spread footings or deep foundations.
Spread footings are typically utilized where rock or hard soils are found at a relatively shallow
elevation.

Deep foundations may consist of driven piles, drilled shafts, caissons, or variations of same.
Deep foundations develop their load carrying capacity either by end bearing (transmitting the
load to a hard layer at the bottom of the element), side friction along the entire length of the
element, or a combination of these.

Abutment

An abutment is a substructure unit composed of stone, concrete, brick, or timber that supports the
end of a single span, or the extreme end of a multi-span superstructure, and in general retains or
supports the approach embankment. (See Figure 5-2) The following are the components of a
typical abutment:

      Bridge Seat.    The top horizontal surface upon which the superstructure is placed and
       supported.

      Backwall.      The topmost vertical portion of an abutment above the bridge seat,
       functioning primarily as a retaining wall for the approach embankment. It may also serve
       as a support for one or more track ties at the end of the bridge.

      Wingwall. A retaining wall extension of an abutment that is intended to restrain and
       hold in place the side slope material of the approach embankment. It may also serve to
       deflect stream water and floating debris into the waterway and prevent embankment
       erosion.

      Breast Wall or Stem. The portion of the abutment between the wings and beneath the
       bridge seat that transmits the bridge loads to the foundation.

      Footing. The enlarged lower portion at the bottom of the abutment. In soft or unstable
       soil conditions, piles may be driven, or other deep foundations utilized, to support the
       footing.

Pier

A pier is a substructure unit composed of stone, concrete, brick, steel, or timber and is built in
shaft or block-like form to support the intermediate ends of the spans of a multi-span structure.
Examples of bridge piers are shown in Figure 5-3. The following are the components of piers
that would generally be encountered on railroads:

      Bridge Seat.    The top horizontal surface upon which the superstructure is placed and
       supported.

      Cap. The topmost horizontal portion of the pier. On a solid shaft pier, the cap, if
       present, is of slightly larger horizontal dimensions than the balance of the pier shaft.
      Pier Wall or Stem. The portion of the pier between the footing and the pier cap.

      Footing. The enlarged lower portion at the bottom of the pier. In soft or unstable soil
       conditions, piles may be driven, or other deep foundations utilized, to support the footing.

Bent

A bent is a supporting unit of a trestle or a viaduct-type structure made up of two or more
column or column-like (post) members connected at their topmost ends by a cap, strut, or other
member that holds them in their correct position. When piles are used as the column element,
the entire construction is designated as a “pile bent”. Alternatively, when the column elements
are constructed of sized timbers supported by a sill, the assemblage is termed a “framed bent”.
When fabricated from steel shape, the assemblage is termed a “steel bent”. Examples of bents
are shown in Figure 5-3. Important elements of bents are as follows:

      Cap. The topmost horizontal member serving to distribute the loads upon the columns.

      Pile Cap. The topmost horizontal member of a pile bent serving to distribute the loads
       upon the piles and to hold them in their proper relative positions.

      Column, Post, Pile. An element situated in a vertical or nearly vertical manner,
       generally having considerable length in comparison to its transverse dimensions.

      Sill. A base piece or member of a bent serving to distribute the column or post loads to
       the foundation or mud sills.

      Mud Sill. A piece of timber, or unit composed of two or more timbers placed upon a soil
       foundation as a support for a framed bent, or other similar member of a structure.

      Transverse or Sway Bracing. Members connecting the columns, sill, and/or caps, in
       order to give rigidity to the complete assemblage in the plane located transverse to the
       bridge alignment.

      Sash Bracing. Horizontal bracing struts between sway bracing panels located transverse
       to the bridge alignment.

   Tower

   A tower is a four-sided substructure framework (two bents braced together longitudinally) in
   a viaduct-type structure that supports the ends of two adjacent spans, or one complete span
   (i.e., tower span) and the ends of two adjacent spans. The column members are braced and
   strutted in tiers, and the planes of either two or four sides may be battered. This term may
   also be used to designate the end supports of suspension spans, vertical lift spans, etc.
   Superstructure Types, Components, and Terminology

   Slab Bridge

   This is the simplest type of superstructure, with the slab carrying the loads directly to the
   abutments or piers. Construction of slabs varies from the natural stone roof used on the
   earliest stone boxes to the reinforced and prestressed concrete designs of today. A typical
   example of a slab bridge is illustrated in Figure 5-4.

   Masonry Arch

   The arch is another type of superstructure that was widely used to bridge larger streams,
   roadways, and other obstructions in the early construction of railroads. The majority of these
   were built of stone that was readily available at or near the construction site. Bricks were
   also used, and in later years, cast in-place concrete arches were constructed. Many of the
   stone arch structures built in the mid-1800’s are still in use today, carrying loads much
   greater than originally anticipated. Examples are shown in Figure 5-5. Important elements
   of a typical arch are included.

          Span. Horizontal distance between spring lines of abutments or piers.
          Rise. Vertical distance between the spring lines and crown.
          Spring Line. The inner edge of the surface or joint upon which the bottom end of the
           arch rests.
          Arch Ring. The entire arch between spring lines.
          Crown. The highest part of the arch ring.
          Keystone. The highest wedge-shaped block or stone at the center of the arch ring.
          Spandrel. A wall or column resting on the arch ribs and supporting the deck.

Timber Trestle

Timber trestles are another type of structure that have been widely used because of availability of
material at or near the site when early railroad construction was at its peak. Many timber trestles
are still in service, having been renewed several times since originally constructed. Trestles are
constructed with either driven pile bents (Figure 5-6) or framed bents (Figure 5-7) as was briefly
discussed in the preceding section on substructures. Figure 5-8 illustrates one example of a
timber trestle (an open deck trestle) and the associated terminology.

Beam Span Bridge

These bridges consist of rolled steel I-shaped or H-shaped members, or reinforced concrete
member, two or more of which support the track or deck and carry the loads directly to the
abutments or piers. These structures may be open deck, where the ties rest directly on the top
flanges of the beams, or ballasted deck, where a concrete slab, steel plate or timber deck supports
the track and roadbed. Ballasted deck bridges utilize ballast retainers or parapet walls to hold the
ballast along the sides. Figure 5-9 shows a typical beam span bridge, and illustrates both open
and ballasted deck configurations.
Deck Plate Girder (DPG) Bridges

Deck plate girder bridges consist of two or more large beam-like members (i.e., girders)
fabricated by riveting, bolting, or welding plates and angles (or just plates) together. The deck
rests upon the top flange of the girders and may either be open (i.e., with the ties resting directly
on the girders), or solid. For solid decks, the deck may consist of a concrete slab, steel plate, or
timber deck that supports the track and roadbed, with ballast retainers or parapet walls to hold
the ballast along the sides. The load capacity of the girders is significantly increased through the
use of bearing and intermediate stiffeners, which serve to reinforce the web. These stiffeners
form rectangular “panels” in the web of the girder. The girders are tied together with bracing
that consists of top laterals, bottom laterals, and cross frames. The bracing is usually made up of
angles, channels and/or tee sections that are connected to gusset plates, which in turn are
attached to the girders at panel points. A typical deck plate girder bridge and its associated
terminology are illustrated in Figure 5-10.

Through Plate Girder (TPG) Bridges

Like deck plate girder bridges, through plate girder bridges consist of two or more large girders
fabricated by riveting, bolting, or welding plates and angles (or just plates) together. Web
stiffeners reinforce the girders and a system of lateral and cross braces tie the girders together
and provide stability. In this case, however, the track or deck is supported by a floor system that
is located below the top flange of the girders. Knee braces are used to provide support to the top
flange against buckling. The floor system itself is usually made up of floor beams and stringers.
The deck rests on the stringers that are framed into the floor beams. The floor beams are
attached to the girders at panel points, and transmit the track and deck load from the stringers to
the girders. The deck may be open (i.e., with ties resting directly on the floor system) or solid
(using a concrete slab, steel plate, or timber deck to support the track and ballast). Figure 5-11
shows a typical through plate girder bridge.

Truss Bridges

These bridges utilize trusses as the primary members of the superstructure. Trusses are open-
web frames that consist of jointed members so arranged that the frame is divided into a series of
triangular shapes. Due to its inherent strength and stability characteristics, the triangle is the
fundamental element in truss design. Figure 5-12 illustrates the various truss arrangements
typically used in railroad bridges. As this figure shows, a bridge may be either a through truss
(where the trains travel through the structure) or a deck truss (where trains travel over the top of
the structure).

The majority of truss railroad bridges are constructed of steel, but a few timber trusses still
remain in limited service for rail traffic. Timber trusses are also in use on some overhead
highway bridges, and generally have relatively low load limits. Therefore, the remainder of this
discussion of truss bridges will be primarily applicable to trusses constructed of steel.
The individual components of trusses may be solid rods, eye bars, pipe, tubing, rolled sections,
sections built up from plates and angles, or various combinations thereof. Rivets, bolts, welds,
pins, or a combination of these may be used to connect the truss components. Although the size
and shape of trusses will vary widely, many essential components will be common to all. The
majority of truss bridges carrying railroad traffic will be of the through truss type, divided
primarily between the riveted and pin-connected variety. Figure 5-13 illustrates the typical
component arrangement and terminology for through truss bridges.

Referring to Figure 5-13, the perimeter members of a truss consist of a top chord, bottom chord,
and end posts. The interior members of a truss that complete the triangular construction consist
of diagonals, intermediate posts, and hangers. These members are connected to gusset plates,
which form the panel points of the truss. The track is supported by a floor system, usually made
up of floor beams and stringers. The track rests on the stringers, and the stringers are framed into
the floor beams. The floor beams are attached to the trusses at panel points. The truss is
laterally braced by sway bracing, top laterals, and bottom laterals.

Movable Bridges

A movable bridge is a bridge of any type having one or more spans capable of being raised,
turned, lifted, or slid from its normal traffic service location to provide for the passage of
navigation.

Swing Bridge

A swing bridge is a span, usually of truss or plate girder construction, designed to be supported
solely on a pier at its center when its end supports have been withdrawn or released. It is
equipped to be turned in a horizontal plane once it is released from its end supports in order to
open the navigable waterway. When closed in the normal traffic position, the span is supported
at the center pier and at two outer rest piers or abutments. Swing bridges may be of center-
bearing or rim-bearing construction. See the Movable Bridge section of this Handbook for
further details. A typical swing bridge is shown in Figure 5-14. Components or mechanical
systems encountered on swing spans may include the following:

      Center Bearing. This is usually a bronze disc running in oil.

      Rack. Large toothed gear segments that are anchored to the center pier, concentric with
       the center pivot, and are part of the mechanism by which the bridge is rotated.

      Pinion. A small, mechanically driven toothed gear that meshes with the rack and applies
       the rotating force to the span through its shaft bearing attachments to the span.

      Balance Wheels. These run on a circular track on the outer edges of the center pier.

      Live Load Support Wedges. Wedge-shaped bearing blocks (usually mechanically
       driven) that are placed under the outer ends of the bridge to lift and support the ends of
       the span under traffic. Wedges may also be used under the truss or girder at the center
       pier to remove all or part of the traffic load from the center bearing.

      Span Locks or Rail Locks. Mechanical devices that positively engage the swing span to
       the fixed rest pier or approach rails when in the closed or normal traffic position.

Bascule Bridge

A bascule bridge is a span, usually of plate girder or truss construction, which lifts by rotating
vertically about a horizontal axle or trunnion or on a rolling surface. A counterweight is used to
offset the dead load of the leaf overhanging the trunnion, thus minimizing the power required to
open the bridge. A typical bascule bridge is shown in Figure 5-14.

Vertical Lift Span

This bridge consists of a movable span, usually of truss construction, with a fixed tower or
towers at each end. The span is connected to cables that pass over sheaves (pulleys) atop the
towers and connect to counterweights on the other side. The actual lifting is performed (usually
by electric motors) through the turning of the counterweight sheaves, or drums that wind
separate uphaul and downhaul cables. The general arrangement of a vertical lift span is
illustrated in Figure 5-14.

Turntables

Similar in principle to a swing bridge, turntables are usually found at engine houses or servicing
facilities. They are used to turn locomotives or other equipment, and to transfer them from one
to another of multiple tracks that radially extend away from the turntable. Turntables usually
consist of a plate girder span (either deck or through plate girder design) that is rotated on a
center bearing, with the ends supported by trucks (i.e., wheel assemblies) running on a circular
rail. Power for rotation is usually supplied by electric motors that drive one or more of the
wheels traveling over the circular rail. Turntables are usually placed in a concrete pit that has
circular walls and a sloped floor for drainage.

Transfer Tables

Transfer tables are found in shop or maintenance facilities and are used to move rolling stock or
track mounted equipment from one track to another. Transfer tables typically consist of a steel
beam or deck plate girder structure mounted on wheels and rails in a pit that allow the structure
to move transversly along the length of the pit and line-up the rails on the transfer table with the
approach rails on any track on either side of the pit. Drive mechanisms are similar to turntables.

Identifying Structures and Components

It is very important to establish a uniform system of identifying a structure as well as the
components that make up that structure. The system should be consistent from structure to
structure and should be known and understood by not only the inspectors, but also any other
individual who may need to read, interpret, or further evaluate the information contained in the
report. Following are some examples of items to be addressed in a standard identification:

      Bridge Number – Typically the bridge milepost, a unique number. Any sequential
       numbering system can be used, but mileposts make the structure location easy to identify
       and if a new structure is added, the new number is easily fit into the system.

      Abutment, Pier, & Span Numbers – These numbers typically increase in the direction of
       ascending mileposts on the railroad. Starting at the lower milepost end of the bridge
       would be Span 1, Span 2, Span 3…..and so on. In like fashion, Abutment 1, Pier 2, Pier
       3, Pier 4…..Abutment 10 or Bent 1, Bent 2, Bent 3, etc.

      Component Numbers – Components numbered along the length of a bridge or span are
       typically perpendicular to the track and numbered once again with increasing mileposts.
       For instance, an intermediate floor beam of a span in the middle of a multi-span bridge
       might be Floor Beam 5 of Span 3. Members that run parallel to the track such as
       stringers or girders can be numbered from left to right, once again facing ascending
       mileposts.

      Compass directions may be used, but there are pitfalls to be aware of:

           o Bridges are seldom oriented in a true north-south or east-west direction so
             compass directions are of questionable value.

           o The railroad often has an established direction of say “railroad east” with the east
             always being the lower milepost, whether it’s actually east or not. Using a
             railroad direction is manageable, but decisions have been made to change railroad
             directions, and it makes it very difficult to compare current reports with historical
             data.

           o There is a tendency to use identifiers such as “northwest bearing”. This can be
             very confusing if the railroad east end of the bridge is not the compass east end. It
             would be clearer to say west end, north bearing which would require anyone
             reading the report to establish the railroad west end (whether it’s actually compass
             west or not) and then find the north bearing.

           o Directions can often be used very successfully in conjunction with numbering
             systems to get down to smaller details, such as “east bottom flange of #5 floor
             beam”.

It is recommended that a written standard identification system be created and provided to
anyone preparing or reading an inspection report. It is also recommended that a key be placed on
the report itself, explaining the numbering system so that each report can stand alone.

Certain inventory information should be provided for each structure.        As a minimum, the
following information should be provided:
      The railroad’s name, division and/or subdivision.

      The nature of the crossing (river, roadway, etc.) and name if available.

      A unique identification number as approved by the owner, the age of structure, if it is an
       open deck or ballast deck bridge, and the total length, maximum height and number of
       panels (spans).

      Name of inspector and members of inspection party, and date of inspection.

Condition Ratings

In addition to the detailed notes describing specific conditions or deficiencies, it is common
practice to assign a condition rating to each condition. Condition rating systems can be very
complex or very simple. An example of a relatively straightforward system would be:

       P1 – Requires immediate attention
       P2 – Poor condition, keep under observation until repaired
       P3 – Fair condition, should be monitored
       P4 – Item noted, but of no concern

The rating system should be tailored to the needs of the individual railroad. The rating system
should be clearly defined and provided with each inspection report.




                                     Figure 5-1 Page 1 of 2
Figure 5-1 Page 2 of 2
Figure 5-2 Typical Bridge Abutment
Figure 5-3 Pier Types Sheet
     Figure 5-4 Concrete Slab




Figure 5-5 Examples of Arch Bridges
 Figure 5-6 Pile Trestle




Figure 5-7 Framed Trestle
Figure 5-8
Figure 5-9 Typical Beam Span Bridge
      Figure 5-10 Typical Deck Girder Bridge
(Figure 15-7-3 from AREMA Manual for Railway Engineering)
    Figure 5-11 Typical Through Girder Bridge
(Figure 15-7-2 from AREMA Manual for Railway Engineering)
Figure 5-12 Page 1 of 2
Figure 5-12 Page 2 of 2
     Figure 5-13 Typical Through Truss Bridge
(Figure 15-7-1 from AREMA Manual for Railway Engineering)
Figure 5-14

				
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