4.1.9A Bridges, Abutments and Piers (Abutments)

Prototype Information

An abutment is the substructure of a bridge designed for the end support of the bridge span(s). It transfers to the underlying soil the forces produced by the structure. These forces include: 1) the dead load of the substructure and superstructure,  2) the live load or effect of passing traffic, and 3) the pressure of wind and water, etc.

Types of abutments

The arrangement of the abutment is necessarily dependent upon the general design of the structure, and that is governed by the local conditions. The two primary characteristics of every abutment are the breast - which directly supports the span substructure, and the back wall - which prevents the subroadbed or roadbed to be eroded at the point the bridge begins, or encroachment of any retained material upon the bridge seat (single track abutment) or shelf apron (multiple track abutment). A third characteristic important for prototype abutments, but not for railroad modeling, is that most of every abutment is constructed below ground.

Abutments have been built in many shapes and materials. Those shapes generally fall into three general categories: 1) straight, 2) wing,  or 3) U-shaped. Specialty abutment shapes are based on unique needs for retaining the adjacent fill and encroachment of that fill. 

Double track straight abutment

Straight Abutments. .The straight abutment looks like a very simple rectangle (breast) with a step (bridge seat or shelf if it is a double track abutment) with a back wall behind the bridge seat. 

In this example note that the bottom of the abutment is sitting on wood piles driven into the earth base. This provides the necessary stability for the structure - otherwise the structure would sink into the ground from all its weight.

Wing Abutment - Parallel wings

Wing Abutments. The wing abutment is the type most widely used by the railroads. It embodies the basic form of the straight embankment, flanked by wings. These wings are either: 1) parallel to the length of the abutment, or 2) turned forward or backward depending on the right of way terrain. The upward wing surface is sloped to conform to the natural slope of the retained material (normally an embankment).

U-Shaped. The U-shaped abutment is characterized by two wings which extend backward from the breast/back wall and at right angles to its face.The length of the wings are determined by the necessity to prevent encroachment of retained material on the bridge seat. The area behind the back wall and between the wings is filled with retained material.

 

 

 

 

Timber Bulkhead

Timber Bulkheads. Abutments for wood trestles (both pile and frame) can be of any type listed above. However, the great majority use timber bulkheads (for economy and speed of construction). The terminology is a bit different but the general engineering concept are the same. 

The bulkhead retains the soil at the land side end of the trestle and provides support for the first row of trestle deck panels. The timber bulkhead consists of: 1) dump boards (holding back the ballast and retained material (think back wall), 2) and a low bent (in height) of identical structure as the rest of the bridge (think bridge seat). The timber bulkhead can have timber wings (of similar construction)  parallel, forward, or backward.

The WWSL

The WWSL has thirteen bridges along along its right-of-way. Each bridge has a specifically engineered abutment. Those abutments types are (by location): 

Location   Abutment Type

N1             Steel bulkhead
O1             Pile bulkhead
N2             Timber wing (new construction is a concrete wing in parallel)
N3             Timber wing
N4             Concrete straight
N5             Concrete U-shaped
N6             Pile bulkhead
N7            Timber bulkhead (skewed)
S1             Timber bulkhead (on piles)
S2              Concrete straight
S3              Concrete straight
W1            Concrete straight (retaining wall)
S5             Masonry straight

References

Walter Mason Camp, Notes on Track, Google books Chapter V.

Mallery, Paul, Bridge & Trestle Handbook, Boynton & Associates

Train Shed Cyclopedia, Bridges and Trestles, Newton K. Gregg, Publisher