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Bridge Abutment: Design, Types & Examples

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  • 0:00 Definition & Function…
  • 0:41 Components of an Abutment
  • 2:20 Four Types of Bridge Abutments
  • 3:40 Additional Abutment…
  • 5:48 Lesson Summary
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Lesson Transcript
Instructor: Sunday Moulton

Sunday recently earned a PhD in Anthropology and has taught college courses in Anthropology, English, and high school ACT/SAT Prep.

A bridge's abutment is a support structure used to distribute weight and connect it to an embankment. This lesson introduces the component parts, the four primary types, and six additional classifications used to further define the abutments.

Definition & Function of Abutment

Bridge abutments connect the deck, or surface of the bridge, to the ground and help support its weight both horizontally and vertically. On short bridges, one abutment is placed at either end of the bridge and connected to the embankment, sometimes including a retaining wall.

Longer bridges include additional abutments placed along the length, or span, of the bridge to provide necessary support at regular intervals. While these will not include a retaining wall, they still provide vertical support. The choice of abutment depends on the available space, geologic features of the bridge site, and available funding for the project.

Components of an Abutment

Abutments placed at either end of a bridge typically include the following five components, while those placed along the bridge span will lack both the wing walls and the backwall.

Diagram of Bridge Abutment
Diagram of bridge abutment and image of wing walls

Bridge Seat: This horizontal shelf supports the bridge deck and is near, or on, the top of an abutment. On the embankment, these will hold the end of a span, while those placed along a span will provide support to reduce extended stress. In the diagram, the shelf is represented in the diagram by the letter A.

Wing Walls: These are only included in abutments that connect to the embankment. They are short retaining walls to prevent erosion and add stability. Wing walls are visible in the right half of the image and are not included in the diagram.

Backwall: Also only included in embankment abutments, backwalls rise above the seat to provide a horizontal brace from the bridge deck and reduce erosion at the connecting point. The backwall is represented in the diagram by the letter B.

Pile: An abutment's pile can either be a vertical wall connecting the seat with the foot or a row of columns that serve the same purpose. On an embankment, the vertical wall structure provides a retaining wall, while mid-span abutments use more cost-effective columns. The pile of an embankment abutment is represented by the letter C in the diagram.

Footing: This part of an abutment connects the vertical, load-bearing portion of the abutment to the ground and is usually buried beneath the earth. It consists of a horizontal surface, sometimes wider than the bridge, to distribute the weight. Finally, the foot is represented by the letter D in the diagram.

Four Types of Bridge Abutments

Let's take a few moments to look at the four types of bridge abutments.

1. Typical Gravity Abutment

The typical gravity abutment is the most common type of abutment, and it includes the bridge seat, backwall, footing, and wing walls to hold the bridge's deck while including a retaining wall for the embankment. Usually, the wing walls are placed parallel to the bridge seat or at a slight, backward angle into the embankment.

2. U-Abutment

This U-abutment is a form of abutment that differs from a gravity abutment in the angle of the wing walls. These are placed at a perpendicular angle to the bridge seat, extending toward the embankment and away from the bridge.

3. Spill-Through Abutment

Spill-through abutments usually support the bridge periodically through its span. While they do not include wing walls or a backwall to retain the embankment, the deck rests on a beam supported by columns or a short wall. Water or roadways are able to pass between the supports. The length of the support structure varies to accommodate the distance between the level bridge and the changing terrain below.

4. Pile Bent Abutment

The pile bent abutment is a variant on a spill-through abutment that replaces the wall-like supports with a series of piles, or columns, to hold the support beam.

Additional Abutment Classifications

Engineers also classify abutments based on how they connect to the ground and the embankment. The following types are also classified by the preceding four categories as a combined description, such as semi-stub pile bent abutment. Choosing from these options depends on the cost of the project and the geological factors affecting bridge stability at the site.

Full Height Abutment: This type of abutment supports the entire embankment, beginning at the lower level of a roadway. It is usually used for overpasses. While more expensive than other abutments, it allows for maximum structural depth with minimum width, critical in urban areas.

Semi-Stub Abutment: This abutment helps balance construction costs when more space is available. While not supporting the entire embankment, it begins somewhere between the lower level of a road and the bridge surface.

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