Bridge construction employs two types of abutments: monolithic and seat type. Monolithic abutments are used for short span bridges, while seat type abutments are utilized for long span bridges. The backwall in seat type abutments can function as a fuser and restrict bridge pile earthquake damage, reducing the likelihood of severe seismic damage compared to monolithic abutments. Although backwall damage due to seismic loads may occur, it is tolerable and will not result in the total collapse of the bridge, preventing extreme seismic damage to the bridge pile. During earthquakes, bridge abutments may exhibit various issues such as inadequate seat length, a large gap between the end diaphragm of the bridge superstructure and backwall, insufficient transverse and/or longitudinal shear strength, and a vulnerable end diaphragm in the case of monolithic abutments. To eliminate these seismic vulnerabilities, it is necessary to retrofit the bridge. Various seismic retrofit techniques are discussed in the following sections.
Methods of Seismic Retrofitting of Bridge Abutments
The task at hand involves the use of seat extenders and catchers to fill the gap between the backwall and end diaphragm of a bridge superstructure. To accomplish this, we have the option to use materials such as concrete, steel, or timber.
In addition, we need to install an L bracket on the superstructure soffit to provide support. This will help ensure that the seat extenders and catchers are securely in place and can effectively bear the weight of the bridge.
Other essential components required for the project include shear keys, large CIDH piles, anchor slabs, and vertical pipes. These elements are crucial in ensuring that the bridge structure is stable and can withstand the forces and stresses that it may encounter.
Overall, the use of these materials and components is critical to the successful completion of the bridge construction project. The choice of materials and design of the structure will depend on various factors, such as the location of the bridge, the anticipated weight and traffic loads, and the environmental conditions.
Seat Extenders and Catchers for Retrofitting of Bridge Abutments
Seat extenders are frequently utilized in bridge construction to enhance the stability of abutments or capped beams during seismic events. These seat extenders are typically constructed using either concrete, as depicted in Figure-3, or steel brackets, as shown in Figure-4. These extenders are attached to the existing face of abutments or capped beams, serving as an additional layer of reinforcement to prevent the unseating of bridge girders during earthquakes. By using seat extenders made of concrete or steel brackets, the risk of girder unseating during seismic activity is reduced, improving the overall safety and stability of the bridge structure.
Fig.3: Details of Concrete Seat Extender
Fig.4: Details of Steel Bracket Extender
Fig.5: Seat Extender
The design of a seat extender is similar to a corbel design, and there are various forces that need to be considered when designing the steel used to connect the seat extender to the face of a cap beam or abutment. These forces include shear friction for vertical loads, tensile forces generated by shear forces when the bridge girder moves longitudinally and drives the extender away from the abutment or cap beam face, compression strut forces, and bearing loads under the bridge girder. To ensure a satisfactory bond between the existing concrete and the seat extender, it is recommended to deliberately rough up the abutment or cap beam concrete face.
In cases where the bridge superstructure settles more than 150mm after bearing failure, a seat catcher should be introduced. The purpose of the seat catcher is to limit the superstructure settling to a minimum of 50mm, while also increasing the seat width.
Fig.6: Catcher Provided for a Vulnerable Bearing In Missouri, USA
Fig.7: Catcher Beam used for Steel Girder
The seat catcher is positioned on the upper part of the abutment of the cap beam, and its design closely resembles that of the seat extender. It is important to ensure that enough space is provided to allow for bearing inspection and replacement when considering the installation of the seat catcher. Proper consideration should also be given to leaving adequate space for easy access to inspect and replace the bearings when incorporating a seat catcher into the design.
Use of Concrete, Steel or Timber to Fill the Gap between Backwall and End Diaphragm of Bridge Superstructure
Bridges are vital infrastructure for transportation and connectivity, and it is crucial to ensure their safety and resilience. However, during earthquakes, bridges may suffer damage due to various factors, such as large gaps between the backwall and end diaphragm. This gap can be a weak point for the bridge as the column needs to deflect significantly to activate the back-fill soil.
To address this issue, one possible solution is to fill the gap with a suitable material such as concrete, steel, or timber. This technique is known as a seismic retrofit, and it aims to reduce seismic damages by involving the backwall and the backfill material in contributing to the bridge’s stability.
While filling the gap with a suitable material can improve the bridge’s seismic performance, it is essential to pay attention to thermal movements. Thermal expansion and contraction can cause stresses that may lead to cracking or other types of damage. Therefore, it is crucial to use appropriate materials that can accommodate the thermal movements while also providing the necessary structural support to the bridge.
Fig.8: Abutment Blocking
L Bracket on Superstructure Soffit
The addition of L-shaped steel to the flanges of a steel I-girder bridge serves as a bumper, enabling the transfer of longitudinal reactions from the superstructure to the abutment and eventually to the soil. The L-shaped steel provides added support to the bridge’s flanges, enhancing its overall strength and stability. This additional support allows the bridge to better withstand external forces, such as heavy loads or strong winds, which may cause the structure to vibrate or sway. By redirecting these reactions to the abutment and soil, the L-shaped steel helps to maintain the structural integrity of the bridge, preventing any potential damage or failure. Ultimately, the use of L-shaped steel is an important measure to ensure the safety and durability of steel I-girder bridges.
Fig.9: Bracket at Bridge Abutment
Shear Keys, Large CIDH Piles, Anchor Slabs and Vertical Pipes
To retrofit a short span bridge, the most suitable strategy is to drive seismic loads away from the columns and footing towards the abutments. This can be accomplished by strengthening or anchoring the abutment, which restricts its movement during earthquakes and causes most of the loads to affect the abutments. Techniques such as seismic anchor slabs, anchor piles, vertical pipes, or shear keys can be employed for bridge abutment modification or anchoring. For bridges that are substantially curved or skewed, it is recommended to use anchor piles or vertical pipes due to the complex geometry of the bridge. Anchor piles at cute corners of the abutment are significantly important as they resist movement caused by the bridge rotating away from the abutment, for which there would be no force to withstand. To learn more about related topics, one can refer to Methods of Bridge Column Casing, which covers the properties, details, and uses of different methods, as well as Factors Affecting Bond Strength of Overlay Concrete on Bridge Decks.