4.1.9 Bridges, Abutments and Piers

Prototype Information

Definitions.

Bridge: a structure carrying a road, path, railroad, or canal across a river, ravine, road, railroad, or other obstacle.

Abutment: the part of a structure (such as an arch or a bridge) that directly receives thrust or pressure. Further discussion of abutments will be found at 4.1.9a Abutments

Pier: an intermediate support for the adjacent ends of two bridge spans. Further discussion of piers will be found at 4.1.9b Piers

Reasons for bridges. There are 4 reasons for bridges on a railroad:

  • To conserve ground space. In areas where land is valuable, a bridge may be provided to permit the space under the railroad right of way to be used, space which would be otherwise denied.
  • To span gaps. In valleys and other areas of depression where the slope or nature of the ground precludes the normal procedure of using fill.
  • To transfer or handle cars or locomotives. For the railroad modeler, there is the turntable (used in engine servicing facilities), the transfer bridge (used on barge slips) and transfer tables (used in multiple track car and/or locomotive shops.
  • For economy. A bridge may be provided when time or the expense of other forms of construction such as a fill, or a tunnel, or a change in direction of roadway, is greater than the cost of the bridge.

Type of Bridges. There are two categories of bridges, fixed and moveable, with several types
being used in each of these categories.

Fixed bridges. There are numerous types of fixed bridges. In order of complexity:

  • Beam.  Beam bridges are defined as a simple bridge where the load carrying (horizontal) members of are timber, rolled steel shapes (such as I or H beams) reinforced concrete beam or slab. Beam bridges are often short in nature. 
  • Trestle. Trestles are a single bridge which require one or more intermediate supports. Timber trestles were common in the early days of railroading. The intermediate supports were either piles, tree trunks driven into the soil to provide a better bearing and then fastened together with various size timbers, or timber frame, sawmill cut lumber supported on a timber or concrete foundation. More modern trestles trestles are built of steel from a variety of "I", "WF" and "T" shaped steel. 
  • Girder. Plate steel riveted or welded to angle steel formed the sides of these girder type bridges. These sides are fastened to each other by angles and other shapes such as "I", "WF", "T" and channel shaped steel.These fabricated beam types can either carry the deck on top or have the deck pass between the sides. 
  • Arch. The circular structural member which spans the obstacle and supports the bridge deck. Early arch bridges were made from stone, the viaducts being examples of this type bridge. Some later arch bridges were made from brick in the same fashion as the stone ones. Concrete arches are used to span small streams or drainage ditches. See the Winona, Minnesota John A Latsch Wagon Bridge for an example of a concrete arch bridge.
  • Truss. Structural members which form a rigid framework to support bridge decks. Trusses can be constructed to allow the deck to pass through or above. These types have been built with both wood and steel, and are sub-designated by the type of engineered framework used. Those sub-types include: Howe, Warren, Baltimore, Pennsylvania, "K"  Pratt,  Parker or Camelback.
  • Suspension. The deck is suspended from a series of vertical cables connected to a main cable which is anchored at each end. The cable is run over two or more towers and each section is suspended from the main cable.
  • Cantilever. The deck is carried by a through truss bridge which is anchored at one end only.

Movable Bridges. There are four types of movable bridges:

  • Bascule, of which there are several types. All rotate around a pinion with the bridge(leaves) being counterbalanced by a weight. this configuration takes little power to move them. The trunnion type rotates around the pinion with simple bearings. These are often operated with hydraulic cylinders.
  • Scherzer. This bridge type has a gear affixed to the pinion shaft. As this pinion is pulled back along a fixed rack the pinion rotates and opens the bridge leaf. When the pinion is pushed along the fixed rack it rotates back to the closed position.

  • Retractile. This type of bridge is mounted in a skewed position on trucks or carriages on one side of a body of water. To open the channel to waterway traffic, the bridge is "retracted" or withdrawn to shore. Bridges of this design are considered obsolete because a relatively large piece of land has to be condemned - that is, left unusable - for their operation. However, such bridges are fairly simple to operate because of the relatively small amount of electrical and mechanical equipment needed to run them. 
  • Swing Span. Typically, these are bridges supported on a center pier in the middle of a waterway. They are opened by rotating horizontally on wheels riding on a circular track. In this way, two channels are provided on either side of the bridge. Bridges of this type require waterways of considerable width; in addition, they are slow to operate and restrict the width of the channel. Because of this, they are rarely built today.
  • Vertical Lift. A vertical lift bridge is a movable bridge which carries roadways or walkways, and which can be raised and lowered like an elevator in a building. Vertical lift bridges are raised and lowered using supporting end cables that are attached to rotating drums in the towers on each side of the bridge.

Load Carrying Capacity

Theodore Cooper was an American civil engineer. Cooper's designs included a broad variety of structures, but his most memorable contributions were in the area of bridge design. Between 1885 and 1902, Cooper published several important works on railroad and highway bridge design. His theories strongly influenced the adoption of wheel-load analysis for railroad bridges. 

The Cooper E rating is a measure of the load carrying capacity of a railroad bridge. It s based on the axle load and spacing for locomotives and cars. Cooper's loading system was based on a standard of E10, meaning a pair of 2-8-0 type steam locomotives, pulling an infinite number of rail cars. Each locomotive was given an axle loading of 10,000 pounds for the driving axles, 5,000 pounds for the leading truck, and 6,500 pounds for the tender trucks. Each trailing rail car was given an axle loading of 1,000 pounds per foot of track. The railroad bridge was then engineered to a standard based on the axle loading of the trains using the bridge. (For example, engines with a axle loading of 20,000 pounds needed a E-20 minimum designed bridge).

  • Cooper E-20  1880's standard
  • Cooper E-30  Light service only
  • Cooper E-40  Lightest normally built in 1910
  • Cooper E-50  Common loading 1919
  • Cooper E-60  Common main line 1924
  • Cooper E-70  Heavy main line 1924
  • Cooper E-72  AREA recommendation mid-1990s (concrete)
  • Cooper E-80  AREA recommendation mid-1990s (steel) 

The Cooper System is still in use today for diesel locomotives.

The WWSL

The WWSL does not have a bridge engineer on the staff; all bridge engineering is based on designs obtained from Class 1 railroads during the period, most notably the Union Pacific and the Southern Pacific. Standardized designs are used for shorter spans to economize on design and procurement and to make maintenance and repair quick and efficient.

The WWSL watches the used equipment market and has purchased steel bridges retired from Class 1 railroads and sold for reuse or scrap.

Bridges located on the WWSL

The WWSL has three types of railroad bridges and one type of highway bridge. This information is compiled from the WWSL bridge book. 

Location

Type
Abutment

Pier

N1

Beam

AN1

NA

O1

Pile Trestle

AO1

NA

N2

Deck Plate Girder

AN2

NA

N3

Thru Plate Girder

AN3

NA

N4

Pile Trestle, Deck Plate Girder

AN4

NA

N5

Deck Plate Girder

AN5a, AN5b

PN5a, PN5b

N6

Beam

AN4

NA

N7

Pile Trestle

AN4

NA

S1

Beam

AS1

NA

S2

Deck Steel Trestle

AS2

PS2

S3

Concrete

AS3

PS3

W1

Warren Truss Highway

AW1

W1, W2

S5

Thru Girder, Thru Truss, Thru Girder

AS5

PS5a, PS5b


Resources

Daryl Kruse, Union Pacific Geneva Sub, Whipple bridge construction 

 

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