Bridge Abutments and their ten typesArticles > Bridge Abutments and their ten types
An abutment is an essential part of a bridge, which vertically supports the structure of the bridge by means of distributing the weight of the bridge. The span of a bridge is connected with embankments by means of abutments.
Bridge abutments are installed on the ground surface. They connect the ground and deck of the bridge. The load/weight of the bridge deck is supported by the abutments. Abutments are vertically placed within the water or obstacle on a wide and heavy foundation. The abutments survive the pressure exerted by water flow and soil due to their heavyweight. In short bridges, abutments are installed only under the two opposite sides of the bridge at each embankment, while the longer bridges include additional abutments at a particular distance under the long span of the bridges. The number of abutments depends upon the length of the span of the bridge. An abutment has several structural components, which are as under:
1. Bridge Seat: The top of the abutment, which has a wider span than the filament of the abutment, where the deck of the bridge is seated, is called Bridge Seat.
2. Wing Wall: This component of the abutment is only included in the abutments which connect the bridge with an embankment. These are short retaining walls that prevent the embankment from erosion.
3. Back Wall: It is also only included in embankment walls. Back walls are vertically constructed at ends of the most bridges. The back-walls support the expansion joints of the bridge span/deck.
4. Pile of the Abutment: Pile is the filament of the abutment which connects the ground foundation of the abutment with the bridge's seat. The length of the pile depends on the height of the bridge and the depth of the obstacle (stream, river, and canal).
5. Footing of the Abutment: The footing of the abutment is also called the foundation of the bridge. The footing connects the pile with the ground. The footing is wider and very heavy. The purpose of footing is to prevent the abutment to sink into the ground surface.
Types of Abutments:
Most fundamental types of abutments are as under:
(1). Gravity Abutments:
It resists the horizontal earth and water pressure with its dead weight. The foundation of such abutments is very wide and heavy. As the name mentions, the structure of the abutments is simply placed on the ground, the gravitational pull of earth makes the abutment to sustain.
(2). U-shaped Gravity Abutments:
This type of bridge abutment has wings perpendicular to the face which acts as counter-forts. These are very stable types of abutments. The wing walls of the abutment are at a 90-degree angle (perpendicular) to the seat of the bridge. A U-shaped Abutment has a pair of piles, which are at a distance equal to the width of the bridges. These types of abutments are constructed with reinforced cement concrete. At the bottom, both the piles of the abutment are connected with each other by means of foundation. Both the piles have a common foot.
(3). Cantilever Wall Abutments:
There are two purposes of a Cantilever Abutment, one is to retain the soil behind the ends of the bridge, second is to support the bridge superstructure. Some wall abutments are referred to as stub abutments. These abutments are made as short as possible and are installed at the top of fill embankments. Stub abutments usually only retain soils that are slightly higher than the superstructure thickness. Stub abutments can be very economical; however, they tend to increase the length of the end spans. Other wall abutments can be much taller and are often constructed to the full height of the crossing. Full height abutments are more difficult to build; however, they tend to reduce the length of the end spans.
(4). Full Height Abutments:
It is a tall height abutment which is constructed at the lower level roadway and should support the entire embankment. This abutment is costly and is generally used in congested urban and metropolitan areas where structure depth is critical.
(5). Stub Abutments:
Usually supported on piles, they are short abutments installed at the top of an embankment or slope of the embankment. They are very and not visible from above ground level.
(6). Semi-stub Abutments:
The height of a semi-stub abutment is between the heights of full-height and stub-abutment. Unlike the stub, abutments are constructed on the top or somewhere near the top of the embankment, and the full-height abutment is constructed at the bottom of the embankment, the Semi-stub abutments are constructed somewhere between the top and bottom of the embankments. As they are taller than the stub-abutments and shorter than the full-height abutment, therefore they are known as Semi-Stub Abutment.
(7). Counterfort Abutments:
It is similar to the counterfort retaining wall. In counterfort abutment, a thin wall called counterfort connects the breast wall to the footing. These counterforts are built at spaces of regular intervals so that the breast wall is designed as a supported slab rather than as a cantilever.
(8). Spill-Through Abutments:
The intent of spill-through abutments is to reduce the amount of soil pressure on the abutment by installing large voids in the stem. Spill-through abutments are similar to piers except for the majority of the structure is below grade. Information in Section 3.1 can be used for prefabricated bridges with spill-through abutments.
(9). MSE System:
MSE stands for Mechanically Stabilized Earth. MSE true abutments (no piles) are more cost-effective than mixed abutments (piles under the bridge seat). Both are economical compared to conventional concrete abutments and dramatically economical in place of concrete abutments on piles.
(10). Pile Bent Abutments:
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.
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