Bridge abutments are essential structural elements that support the ends of a bridge, transferring the load from the superstructure to the foundation. There are two primary types of bridge abutments used in construction: monolithic and seat type abutments. The type of abutment selected depends largely on the span of the bridge and its seismic vulnerability. In this article, we will explore these two types of abutments, the seismic vulnerabilities they face, and the various retrofitting techniques employed to enhance their earthquake resistance.
Types of Bridge Abutments
Monolithic Bridge Abutment
Monolithic bridge abutments are typically used in short-span bridges. These abutments are designed as a single, continuous structure that is connected directly to the bridge deck. While simple and cost-effective for short spans, monolithic abutments tend to have higher seismic vulnerability. The lack of a backwall fusion system makes them more susceptible to severe seismic damage. During an earthquake, the movement can directly affect the bridge piles, increasing the risk of total collapse if the seismic forces are too great. The absence of a mechanism to absorb and redirect seismic forces makes these abutments particularly vulnerable.
Seat Type Bridge Abutment
In contrast, seat type abutments are primarily used in long-span bridges, where the forces acting on the structure are more significant. A seat type abutment is designed with a seat or platform to support the bridge superstructure, which can accommodate horizontal movement during an earthquake. The backwall in these abutments is engineered to function as a fuse, allowing it to absorb some of the seismic energy and prevent extreme damage to the bridge piles. This design makes seat type abutments more effective at mitigating seismic risk compared to their monolithic counterparts.
Seismic Vulnerabilities of Bridge Abutments
While both types of abutments play vital roles in supporting a bridge, each comes with its own set of seismic vulnerabilities.
Issues with Monolithic Abutments
- Vulnerable End Diaphragm: The end diaphragm of a monolithic abutment is susceptible to significant damage during an earthquake due to the lack of seismic protection features.
- Severe Seismic Loads: The movement of the bridge structure during seismic events directly impacts the monolithic abutment, leading to potential failure in the abutment piles and the superstructure.
Issues with Seat-Type Abutments
- Inadequate Seat Length: If the seat length of the abutment is too short, it may not be able to accommodate the required movement of the superstructure during an earthquake.
- Large Gap Between Backwall and End Diaphragm: A large gap between the backwall and the end diaphragm can create a weak point in the bridge’s ability to transfer seismic loads efficiently.
- Insufficient Shear Strength: Inadequate transverse and longitudinal shear strength can cause the abutment to fail during an earthquake, leading to severe damage.
General Seismic Concerns
In addition to the issues mentioned above, both types of abutments may exhibit general vulnerabilities, including insufficient shear strength, a weak end diaphragm in monolithic abutments, and gaps that can lead to catastrophic failure. These vulnerabilities necessitate retrofitting to improve the abutment’s ability to withstand seismic loads.
Methods of Seismic Retrofitting of Bridge Abutments
To enhance the seismic performance of bridge abutments, various retrofitting techniques are employed. These techniques are designed to address the specific vulnerabilities of both monolithic and seat-type abutments.
Seat Extenders and Catchers
Seat extenders are typically constructed from concrete or steel and are attached to the face of an abutment or cap beam. They are designed to reduce the possibility of the bridge girder unseating during an earthquake. The seat extender functions similarly to a corbel design and must be capable of handling shear friction, vertical loads, and tensile forces generated when the bridge girder moves longitudinally. Additionally, seat catchers are used to restrict superstructure settling after bearing failure. When the superstructure settles more than 150mm, a seat catcher is introduced to reduce this movement to 50mm, ensuring that the bridge remains intact and safe.
Filling the Gap Between Backwall and End Diaphragm
One common retrofit technique involves filling large gaps between the backwall and the end diaphragm with concrete, steel, or timber. This technique reduces the vulnerability of the bridge by ensuring that the backwall and backfill material help absorb and dissipate seismic energy. While filling the gap, engineers must account for potential thermal movements to prevent damage from expansion and contraction during temperature changes.
L Bracket on Superstructure Soffit
The addition of L-shaped steel brackets to the flanges of steel I-girder bridges can help transfer seismic loads from the superstructure to the abutment and eventually to the soil. These brackets act as a bumper to absorb longitudinal seismic forces, preventing excessive movement of the superstructure during an earthquake.
Shear Keys, Large CIDH Piles, Anchor Slabs, and Vertical Pipes
For short-span bridges, seismic loads are often redirected from the bridge columns to the abutments. Techniques such as shear keys, anchor slabs, large CIDH (cast-in-drilled-hole) piles, and vertical pipes can be used to anchor and strengthen the abutments, preventing excessive movement and ensuring that seismic forces are properly distributed. In bridges that have complex geometries, such as curved or skewed designs, anchor piles and vertical pipes are especially important. These modifications can withstand rotation and other movements that occur during an earthquake.
Detailed Explanation of Retrofit Techniques
Seat Extenders and Catchers
Seat extenders can be made from concrete or steel and are typically designed to fit onto the existing structure. Concrete seat extenders (as shown in Figure 3) and steel bracket extenders (Figure 4) are designed to prevent the bridge girder from unseating during an earthquake. The extender design must account for vertical loads, shear friction, and tensile forces. Seat catchers (Figure 6) are used when the superstructure settles more than 150mm, to minimize further settling.
Filling Gaps Between Backwall and End Diaphragm
In some cases, large gaps are present between the backwall and the end diaphragm of the bridge. To address this, concrete, steel, or timber is used to fill the gap (Figure 8), ensuring that the backwall and the backfill work together to reduce seismic damage.
L Bracket on Superstructure Soffit
L-shaped steel brackets (Figure 9) are added to the superstructure to help transfer seismic loads from the girder to the abutment. These brackets are a cost-effective way to reduce the risk of bridge failure during an earthquake.
Shear Keys, Large CIDH Piles, Anchor Slabs, and Vertical Pipes
Techniques such as shear keys, CIDH piles, and anchor slabs are used to strengthen abutments and distribute seismic loads more effectively. These methods are particularly important for short-span bridges, as they help prevent excessive movement and failure during earthquakes.
Conclusion
Bridge abutments, whether monolithic or seat type, play a crucial role in supporting a bridge and ensuring its safety. However, both types are vulnerable to seismic forces, which can cause significant damage if not properly addressed. Seismic retrofitting techniques, including seat extenders, catchers, gap fillings, L brackets, and various anchoring methods, are essential for enhancing the earthquake resistance of bridge abutments. By utilizing these techniques, engineers can ensure that bridges remain functional and safe during seismic events, thereby safeguarding infrastructure and human lives.