Abutment Types

Abutment Types:

Open-End and Closed-End Abutments

From the view of the relation between the bridge abutment and roadway or water flow that the bridge over crosses, bridge abutments can be divided into two categories: open-end abutment and closed-end abutment, as shown in Figure 4.1. For the open-end abutment, there are slopes between the bridge abutment face and the edge of the roadway or river canal that the bridge over crosses.

Those slopes provide a wide-open area for the traffic flows or water flows under the bridge. It imposes much less impact on the environment and the traffic flows under the bridge than a closed-end abutment.

Also, the future widening of the roadway or water flow canal under the bridge by adjusting the slope ratios is easier. However, the existence of slopes usually requires longer bridge spans and some extra earthwork. This may result in an increase in the bridge construction cost.
The closed-end abutment is usually constructed close to the edge of the roadways or water canals.

Because of the vertical clearance requirements and the restrictions of construction right of way, there are no slopes allowed to be constructed between the bridge abutment face and the edge of roadways or water canals, and high abutment walls must be constructed.

Since there is no room or only a little room between the abutment and the edge of traffic or water flow, it is very difficult to do the future widening to the roadways and water flow under the bridge. Also, the high abutment walls and larger backfill volume often result in higher abutment construction costs and more settlement of road approaches than for the open-end abutment.

Generally, the open-end abutments are more economical, adaptable, and attractive than the closed-end abutments. However, bridges with closed-end abutments have been widely constructed in urban areas and for rail transportation systems because of the right-of-way restriction and the large scale of the live load for trains, which usually results in shorter bridge spans.

Monolithic and Seat-Type Abutments

Based on the connections between the abutment stem and the bridge superstructure, the abutments also can be grouped in two categories: the monolithic or end diaphragm abutment and the seat type abutment, as shown in Figure 4.1. The monolithic abutment is monolithically constructed with the bridge superstructure. There is no relative displacement allowed between the bridge superstructure and abutment. All the superstructure forces at the bridge ends are transferred to the abutment stem and then to the abutment backfill soil and footings.

The advantages of this type of abutment are its initial lower construction cost and its immediate engagement of backfill soil that absorbs the energy when the bridge is subjected to transitional movement. However, the passive soil pressure induced by the backfill soil could result in a difficult-to-design abutment stem, and higher maintenance costs might be expected. In practice, this type of abutment is mainly constructed for short bridges.

The seat-type abutment is constructed separately from the bridge superstructure. The bridge superstructure seats on the abutment stem through bearing pads, rock bearings, or other devices. This type of abutment allows the bridge designer to control the superstructure forces that are to be transferred to the abutment stem and backfill soil. By adjusting the devices between the bridge superstructure and abutment, the bridge displacement can be controlled.

This type of abutment may have a short stem or high stem, as shown in Figure 4.1. For a short-stem abutment, the abutment stiffness usually is much larger than the connection devices between the superstructure and the abutment. Therefore, those devices can be treated as boundary conditions in the bridge analysis. Comparatively, the high stem abutment may be subject to significant displacement under relatively less force.

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The stiffness of the high stem abutment and the response of the surrounding soil may have to be considered in the bridge analysis. The availability of the displacement of connected devices, the allowance of the superstructure shrinkage, and concrete shortening make this type of abutment widely selected for long bridge constructions, especially for prestressed concrete bridges and steel bridges.

However, bridge design practice shows that the relatively weak connection devices between the superstructure and the abutment usually require the adjacent columns to be specially designed. Although the seat-type abutment has a relatively higher initial construction cost then the monolithic abutment, its maintenance cost is relatively lower

Also Read: Classification Survey