Bridge Abutment Means
Bridge Abutment is an element of a bridge, that gives vertical support to the bridge superstructure at the bridge ends, adjoins the bridge with the approach roadway, and maintains the roadway base materials from the bridge spans.
Several types of abutments can be sufficiently used for a particular bridge site. Economics is usually the major factor in selecting the type of abutment to be utilized.
What Is a Bridge Abutment?
An abutment is an important part of a bridge, which vertically supports the structure of the bridge utilizing for circulating the weight of the bridge. The span of a bridge is associated with embankments by means of abutments.
Abutments Bridge are established on the ground surface. They adjoin the ground and deck of the bridge. The load or weight of the bridge deck is supported by the abutments.
Abutments are vertically positioned within the water or obstacle on a broad and heavy foundation. The abutments withstand the pressure exerted by water flow and soil due to their heavyweight.
Read More: Bridge Components and Their Function
Bridge Abutment Types
The different types of abutments are as follows,
- Gravity Abutment
- U-shaped Gravity Abutment
- Cantilever Wall Abutment
- Full Height Abutment
- Stub Abutment
- Semi-stub Abutment
- Counterfort Abutment
- Spill-Through Abutment
- MSE System
- Pile Bent Abutments
1. Gravity Abutment
Gravity abutment has the function to prevent the horizontal earth and water pressure with its dead weight. The foundation of such abutments is extremely wide and heavy.
As the word mentions, the structure of the abutments is entirely seated on the ground, the gravitational pull of earth makes the abutment sustain.
2. U-shaped Gravity Abutment
This bridge abutment type has wings perpendicular to the face which performs as counter-forts. These are relatively stable types of abutments. The wing walls of an abutment are at a 90° degree angle (perpendicular) to the seat of the bridge.
A U-shaped abutment has a set of piles, which are at a distance similar to the width of the bridges. These kinds of abutments are constructed with reinforced cement concrete.
At the bottom, both the piles of the abutment are attached with each other utilizing the foundation. Both the piles have a common foot.
3. Cantilever Wall Abutments
There are two objectives of a cantilever wall abutment, first is to retain the soil behind the edges of the bridge, second is to support the bridge superstructure. One of the most popular types of abutment structures is a retaining wall. although.
A retaining wall is utilized to hold back an earth embankment or water and to retain a sudden elevation change. The abutment serves the following functions
Allocates the loads from bridge ends to the ground resists any loads that are directly imposed on it gives vehicular and pedestrian access to the bridge. In the case of the retaining wall type Abutment bearing capacity and sliding resistance of the foundation materials and overturning stability must be examined.
4. Full Height Abutments
It is a big height abutment that is constructed at the lower level roadway and should support the whole embankment. This abutment is expensive and is normally utilized in congested urban and metropolitan areas where structure depth is critical.
Full height abutments are more complicated to build; however, they tend to decrease the length of the end spans.
5. Stub Abutment
Usually supported on piles, they are short abutments established at the top of an embankment or slope of the embankment. They are relatively and not visible from above ground level.
Several wall abutments are referred to as stub abutments. These abutments are created as short as feasible and are established at the top of fill embankments. Stub abutments usually just retain soils that are slightly greater than the superstructure thickness.
Stub abutments can be extremely economical; however, they tend to enhance the length of the end spans. Additional wall abutments can be extensively taller and are often constructed to the full height of the crossing.
6. Semi-Stub Abutments
The height of the semi-stub abutment is between the heights of full-height and stub-abutment. Unlike stub abutments, others are built on the top or nearby the top of the embankment, also the full-height abutment is formed at the bottom of the embankment.
These abutments are built in between the top and bottom of the embankments. As they are bigger than the stub abutments and shorter than the full-height abutment, therefore they are remembered as Semi-Stub abutments.
7. Counterfort Abutment
Counterfort Abutments are the same as the counterfort retaining wall. In the counterfort abutment, a thin wall called counterfort attaches the breast wall to the footing.
These counterforts are created at spaces of regular intervals so that the breast wall is composed as a supported slab rather than as a cantilever.
8. Spill-Through Abutment
The main purpose of the spill-through abutments is to decrease the amount of soil pressure on the abutment by establishing huge voids in the stem.
Piers and Spill-through abutments have equality within them instead the majority of the structure is below grade.
9. MSE(Mechanically Stabilized Earth) System
MSE stands for Mechanically Stabilized Earth. MSE real abutments (no piles) are better cost-effective than different abutments (piles under the bridge seat).
Both are inexpensive compared to conventional concrete abutments and dramatically reasonable in place of concrete abutments on piles.
10. Pile Bent Abutment
The pile bent abutment is a variant of a spill-through abutment that restores the wall-like supports with a string of piles, or columns, to carry the support beam.
Uses Of Bridge Abutment
- To transfer the loads from a superstructure to its foundation components.
- To resist or transmit self-weight, lateral loads (such as the earth pressure) and wind loads.
- To support one edge of an approach slab.
- To protect a balance between the vertical and horizontal force elements of an arch bridge.
Components Of Bridge Abutment
An abutment has various structural components, abutments placed at either end of a bridge typically include the following five components, which are as under:-
- Bridge Seat
- Wing Wall
- Back Wall
- Pile Of The Abutment
- Footing Of The Abutment
1. Bridge Seat
The top of the abutment, which includes a broader span than the filament of the abutment, where the deck of the bridge is placed, is called Bridge Seat.
2. Wing Wall
This component of the abutment is just comprised of the abutments which adjoin the bridge with an embankment. These are short retaining walls that prohibit the embankment from erosion.
3. Back Wall
It is also simply contained in embankment walls. Back walls are vertically established at ends of the most bridges. The back walls support the expansion joints of the bridge span or deck.
4. Pile Of The Abutment
The Pile of the abutment is the filament that is attached to the foundation of the abutment using the bridge’s seat. The length of the pile relies on the height of the bridge and the depth of the obstruction (canal, river, and stream).
5. Footing Of The Abutment
The footing of the abutment is similarly called the foundation of the bridge. The footing attaches the pile with the ground. The footing is broader and extremely heavy. The purpose of footing is to avoid the abutment to sink into the ground surface.
Selection Of Abutment
The process of choosing the most appropriate type of abutments can be based on the following consideration,
- Construction and maintenance expenses.
- Cut or fill earthwork situation.
- Traffic maintenance during the construction.
- Construction period.
- Protection of construction workers.
- Availability and expenditure of backfill material.
- Superstructure depth.
- Size of an abutment.
- Horizontal and vertical alignment differences.
- Area of excavation.
- Aesthetics and similarity to adjoining structures.
- Previous experience with the classification of the abutment.
- Ease of access for assessment and maintenance.
- Expected life, loading condition, and acceptability of deformations.
Forces On Bridge Abutment
Earth pressures exerted on an abutment can be evaluated according to the way and the magnitude of the abutment movement. The forces on abutment are as follows:-
- At-rest Earth Pressure
- Active Earth Pressure
- Passive Earth Pressure
1. At-rest Earth Pressure
When the wall is stabilized rigidly and does not change positions, the pressure exerted by the soil on the wall is called the at-rest earth pressure.
2. Active Earth Pressure
When a wall runs away from the backfill, the earth pressure reduces (active pressure).
3.Passive Earth Pressure
When it runs toward the backfill, the earth pressure rises (passive pressure).
Bridge Abutment Design Consideration
The following consideration is taken while designing the bridge abutment,
- Differential settling is prohibited of the approach slab (landing) and the deck of the bridge which is liable to generate a trip hazard and the aesthetics of the road.
- Prevent shear failures in the approach slab (landing) from inappropriate subsurface support due to poor soils, fill from the excavation for the bridge abutment, or different factors.
- Approach slab length shall be suitable to the width of the bridge deck and abutment height as specified by a licensed experienced structural engineer.
- Combine the approach slab into the bridge abutment by setting the approach slab into a suitably sized notch in the bridge abutment. Attach with anchors as required.
- Subsurface elements below the approach slab shall contain suitably compacted soil, aggregate, and flowable fill.
- Enlarge subsurface configuration to the road subsurface as required to prevent the area newly excavated for the construction of the bridge abutment.
- Long-term smooth change from the approach slab to the bridge deck and avoidance of shear failure in the approach slab may be used. An alternate technique must be designed by a licensed engineer.
Types Of Abutment Bridges Support
Supports of abutments are classified according to their structure and functions. An explanation of each type of abutment supports are as follows:-
- Piles Or Drilled Shafts
- Spread Footings
1. Piles Or Drilled Shafts
Most of the abutments are supported on piles to avoid abutment settlement. Bridge approach embankments are usually built of fill material that can experience settlement over many years.
This settlement may be the result of the category of embankment material or the actual foundation material under the embankment. By driving piles through the embankment and into the actual ground, abutments usually do not settle with the embankment.
A settling embankment may be withstood by the abutment piles through friction between the piles and fill material. The enhanced load to friction piles and the want for pre-boring should be evaluated.
Boring of Piles Or Drilled Shafts
It is mainly not essential to pre-bore non-displacement piles for any fill depths, and it is not essential to pre-bore displacement piles for fill depths smaller than 15 feet below the bottom of the footing.
But, for some difficult soils, this may not apply. Recognize the soils report to specify if pre-boring is required. If needed, the Special Provisions must be written with pre-boring guidelines.
The lateral resistance on a pile may be greatly dependent on the material into which the pile is driven than on the pile category.
Read More: What Is Culvert | Types of Culvert With Uses
2. Spread Footings
Abutments on the spread footings are normally utilized simply in cut sections where the actual soil can sustain reasonable pressures without unnecessary settlement. The bearing resistance is defined by the geotechnical section or the geotechnical adviser.
With improved techniques and decent control of embankment construction, spread footings can be utilized successfully on fill material.
Construction must be measured to allow the foundation material to consolidate before the spread footings are established. A benefit of spread footings is that the different types of settlement between approach fills and abutments are minimized.
Use of Spread Footing
The usage of spread footings is allotted greater consideration for simple-span bridges than for continuous-span bridges.
But, under particular conditions, continuous-span bridges can be constructed for lesser amounts of settlement. Drainage for abutments on spread footings can be extremely critical. For these explanations, pile footings are usually chosen.
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